A typical 4 cylinder vehicle cruising along the highway at around 50 miles per hour, will produce 4000 controlled explosions per minute inside the engine as the spark plugs ignite the fuel in each cylinder to propel the vehicle down the road. Obviously, these explosions produce an enormous amount of heat and, if not controlled, will destroy an engine in a matter of minutes. Controlling these high temperatures is the job of the cooling system.
The modern cooling system has not changed much from the cooling systems in the model T back in the '20s. Oh sure, it has become infinitely more reliable and efficient at doing it's job, but the basic cooling system still consists of liquid coolant being circulated through the engine, then out to the radiator to be cooled by the air stream coming through the front grill of the vehicle.
Today's cooling system must maintain the engine at a constant temperature whether the outside air temperature is 110 degrees Fahrenheit or 10 below zero. If the engine temperature is too low, fuel economy will suffer and emissions will rise. If the temperature is allowed to get too hot for too long, the engine will self destruct.
Actually, there are two types of cooling systems found on motor vehicles: Liquid cooled and Air cooled. Air cooled engines are found on a few older cars, like the original Volkswagen Beetle, the Chevrolet Corvair and a few others. Many modern motorcycles still use air cooling, but for the most part, automobiles and trucks use liquid cooled systems and that is what this article will concentrate on.
The cooling system is made up of the passages inside the engine block and heads, a water pump to circulate the coolant, a thermostat to control the temperature of the coolant, a radiator to cool the coolant, a radiator cap to control the pressure in the system, and some plumbing consisting of interconnecting hoses to transfer the coolant from the engine to radiator and also to the car's heater system where hot coolant is used to warm up the vehicle's interior on a cold day.
A cooling system works by sending a liquid coolant through passages in the engine block and heads. As the coolant flows through these passages, it picks up heat from the engine. The heated fluid then makes its way through a rubber hose to the radiator in the front of the car. As it flows through the thin tubes in the radiator, the hot liquid is cooled by the air stream entering the engine compartment from the grill in front of the car. Once the fluid is cooled, it returns to the engine to absorb more heat. The water pump has the job of keeping the fluid moving through this system of plumbing and hidden passages.
A thermostat is placed between the engine and the radiator to make sure that the coolant stays above a certain preset temperature. If the coolant temperature falls below this temperature, the thermostat blocks the coolant flow to the radiator, forcing the fluid instead through a bypass directly back to the engine. The coolant will continue to circulate like this until it reaches the design temperature, at which point, the thermostat will open a valve and allow the coolant back through the radiator.
In order to prevent the coolant from boiling, the cooling system is designed to be pressurized. Under pressure, the boiling point of the coolant is raised considerably. However, too much pressure will cause hoses and other parts to burst, so a system is needed to relieve pressure if it exceeds a certain point. The job of maintaining the pressure in the cooling system belongs to the radiator cap. The cap is designed to release pressure if it reaches the specified upper limit that the system was designed to handle. Prior to the '70s, the cap would release this extra pressure to the pavement. Since then, a system was added to capture any released fluid and store it temporarily in a reserve tank. This fluid would then return to the cooling system after the engine cooled down. This is what is called a closed cooling system.
Circulation
The
coolant follows a path that takes it from the water pump, through
passages inside the engine block where it collects the heat
produced by the cylinders. It then flows up to the cylinder
head (or heads in a V type engine) where it collects more heat from
the combustion chambers. It then flows out past the
thermostat (if the thermostat is opened to allow the fluid to
pass), through the upper radiator hose and into the radiator.
The coolant flows through the thin flattened tubes that make up the
core of the radiator and is cooled by the air flow through the
radiator. From there, it flows out of the radiator, through
the lower radiator hose and back to the water pump. By this
time, the coolant is cooled off and ready to collect more heat from
the engine.
The capacity of the system is engineered for the type and size of the engine and the work load that it is expected to undergo. Obviously, the cooling system for a larger, more powerful V8 engine in a heavy vehicle will need considerably more capacity then a compact car with a small 4 cylinder engine. On a large vehicle, the radiator is larger with many more tubes for the coolant to flow through. The radiator is also wider and taller to capture more air flow entering the vehicle from the grill in front.
Antifreeze
The coolant that courses through the engine
and associated plumbing must be able to withstand temperatures well
below zero without freezing. It must also be able to handle
engine temperatures in excess of 250 degrees without boiling.
A tall order for any fluid, but that is not all. The fluid
must also contain rust inhibiters and a lubricant.
The coolant in today's vehicles is a mixture of ethylene glycol (antifreeze) and water. The recommended ratio is fifty-fifty. In other words, one part antifreeze and one part water. This is the minimum recommended for use in automobile engines. Less antifreeze and the boiling point would be too low. In certain climates where the temperatures can go well below zero, it is permissible to have as much as 75% antifreeze and 25% water, but no more than that. Pure antifreeze will not work properly and can cause a boil over.
Antifreeze is poisonous and should be kept away from people and animals, especially dogs and cats, who are attracted by the sweet taste. Ethylene Glycol, if ingested, will form calcium oxalate crystals in the kidneys which can cause acute renal failure and death.
The
Radiator
The radiator core is usually made of
flattened aluminum tubes with aluminum strips that zigzag between
the tubes. These fins transfer the heat in the tubes into the
air stream to be carried away from the vehicle. On each end
of the radiator core is a tank, usually made of plastic that covers
the ends of the radiator,
On most modern radiators, the tubes run horizontally with the plastic tank on either side. On other cars, the tubes run vertically with the tank on the top and bottom. On older vehicles, the core was made of copper and the tanks were brass. The new aluminum-plastic system is much more efficient, not to mention cheaper to produce. On radiators with plastic end caps, there are gaskets between the aluminum core and the plastic tanks to seal the system and keep the fluid from leaking out. On older copper and brass radiators, the tanks were brazed (a form of welding) in order to seal the radiator.
The tanks, whether plastic or brass, each have a large hose connection, one mounted towards the top of the radiator to let the coolant in, the other mounted at the bottom of the radiator on the other tank to let the coolant back out. On the top of the radiator is an additional opening that is capped off by the radiator cap. More on this later.
Another component in the radiator for vehicles with an automatic transmission is a separate tank mounted inside one of the tanks. Fittings connect this inner tank through steel tubes to the automatic transmission. Transmission fluid is piped through this tank inside a tank to be cooled by the coolant flowing past it before returning the the transmission.
Radiator
Fans
Mounted on the back of the radiator on
the side closest to the engine is one or two electric fans inside a
housing that is designed to protect fingers and to direct the air
flow. 
These fans are there to keep the air flow
going through the radiator while the vehicle is going slow or is
stopped with the engine running. If these fans stopped
working, every time you came to a stop, the engine temperature
would begin rising. On older systems, the fan was connected
to the front of the water pump and would spin whenever the engine
was running because it was driven by a fan belt instead of an
electric motor. In these cases, if a driver would notice the
engine begin to run hot in stop and go driving, the driver might
put the car in neutral and rev the engine to turn the fan faster
which helped cool the engine. Racing the engine on a car with
a malfunctioning electric fan would only make things worse because
you are producing more heat in the radiator with no fan to cool it
off.
The electric fans are controlled by the vehicle's computer. A temperature sensor monitors engine temperature and sends this information to the computer. The computer determines if the fan should be turned on and actuates the fan relay if additional air flow through the radiator is necessary.
If the car has air conditioning, there is an additional radiator mounted in front of the normal radiator. This "radiator" is called the air conditioner condenser, which also needs to be cooled by the air flow entering the engine compartment. You can find out more about the air conditioning condenser by going to our article onAutomotive Air Conditioning. As long as the air conditioning is turned on, the system will keep the fan running, even if the engine is not running hot. This is because if there is no air flow through the air conditioning condenser, the air conditioner will not be able to cool the air entering the interior.
Pressure cap and reserve
tank
As
coolant gets hot, it expands. Since the cooling system is
sealed, this expansion causes an increase in pressure in the
cooling system, which is normal and part of the design. When
coolant is under pressure, the temperature where the liquid begins
to boil is considerably higher. This pressure, coupled with
the higher boiling point of ethylene glycol, allows the coolant to
safely reach temperatures in excess of 250 degrees.
The radiator pressure cap is a simple device that will maintain pressure in the cooling system up to a certain point. If the pressure builds up higher than the set pressure point, there is a spring loaded valve, calibrated to the correct Pounds per Square Inch (psi), to release the pressure.
When the
cooling system pressure reaches the point where the cap needs to
release this excess pressure, a small amount of coolant is bled
off. It could happen during stop and go traffic on an
extremely hot day, or if the cooling system is
malfunctioning. If it does release pressure under these
conditions, there is a system in place to capture the released
coolant and store it in a plastic tank that is usually not
pressurized. Since there is now less coolant in the system,
as the engine cools down a partial vacuum is formed. The
radiator cap on these closed systems has a secondary valve to allow
the vacuum in the cooling system to draw the coolant back into the
radiator from the reserve tank (like pulling the plunger back on a
hypodermic needle) There are usually markings on the side of
the plastic tank marked Full-Cold, and Full Hot. When the
engine is at normal operating temperature, the coolant in the
translucent reserve tank should be up to the Full-Hot line.
After the engine has been sitting for several hours and is cold to
the touch, the coolant should be at the Full-Cold line.
Water
Pump
A water pump is a simple device that will
keep the coolant moving as long as the engine is running. It
is usually mounted on the front of the engine and turns whenever
the engine is running. The water pump is driven by the engine
through one of the following:
A fan
belt that will also be responsible for driving an additional
component like an alternator or power steering pumpThe water pump is made up of a housing, usually made of cast iron or cast aluminum and an impeller mounted on a spinning shaft with a pulley attached to the shaft on the outside of the pump body. A seal keeps fluid from leaking out of the pump housing past the spinning shaft. The impeller uses centrifugal force to draw the coolant in from the lower radiator hose and send it under pressure into the engine block. There is a gasket to seal the water pump to the engine block and prevent the flowing coolant from leaking out where the pump is attached to the block..
Thermostat
The thermostat is simply a valve that measures the
temperature of the coolant and, if it is hot enough, opens to allow
the coolant to flow through the radiator. If the coolant is
not hot enough, the flow to the radiator is blocked and fluid is
directed to a bypass system that allows the coolant to return
directly back to the engine. The bypass system allows the
coolant to keep moving through the engine to balance the
temperature and avoid hot spots. Because flow to the radiator
is blocked, the engine will reach operating temperature sooner and,
on a cold day, will allow the heater to begin supplying hot air to
the interior more quickly.
Since the
1970s, thermostats have been calibrated to keep the temperature of
the coolant above 192 to 195 degrees. Prior to that, 180
degree thermostats were the norm. It was found that if the
engine is allowed to run at these hotter temperatures, emissions
are reduced, moisture condensation inside the engine is quickly
burned off extending engine life, and combustion is more complete
which improves fuel economy.
The heart of a thermostat is a sealed copper cup that contains wax and a metal pellet. As the thermostat heats up, the hot wax expands, pushing a piston against spring pressure to open the valve and allow coolant to circulate.
The thermostat is usually located in the front, top part of the engine in a water outlet housing that also serves as the connection point for the upper radiator hose. The thermostat housing attaches to the engine, usually with two bolts and a gasket to seal it against leaks. The gasket is usually made of a heavy paper or a rubber O ring is used. In some applications, there is no gasket or rubber seal. Instead, a thin bead of special silicone sealer is squeezed from a tube to form a seal.
There is a mistaken belief by some people that if they remove the thermostat, they will be able to solve hard to find overheating problems. This couldn't be further from the truth. Removing the thermostat will allow uncontrolled circulation of the coolant throughout the system. It is possible for the coolant to move so fast, that it will not be properly cooled as it races through the radiator, so the engine can run even hotter than before under certain conditions. Other times, the engine will never reach its operating temperature. On computer controlled vehicles, the computer monitors engine temperatures and regulates fuel usage based on that temperature. If the engine never reaches operating temperatures, fuel economy and performance will suffer considerably.
Bypass
System
This is a passage that allows the
coolant to bypass the radiator and return directly back to the
engine. Some engines use a rubber hose, or a fixed steel
tube. In other engines, there is a cast in passage built into
the water pump or front housing. In any case, when the
thermostat is closed, coolant is directed to this bypass and
channeled back to the water pump, which sends the coolant back into
the engine without being cooled by the radiator.
Freeze
Plugs
When an engine block is manufactured, a
special sand is molded to the shape of the coolant passages in the
engine block. This sand sculpture is positioned inside a mold
and molten iron or aluminum is poured to form the engine
block. When the casting is cooled, the sand is loosened and
removed through holes in the engine block casting leaving the
passages that the coolant flows through.
Obviously, if we don't plug up these holes, the coolant will pour
right out.
Plugging
these holes is the job of the freeze-out plug. These plugs
are steel discs or cups that are press fit in the holes in the side
of the engine block and normally last the life of the engine with
no problems. But there is a reason they are called freeze-out
plugs. In the early days, many people used plain water in
their engines, usually after replacing a burst hose or other
cooling system repair. "It is summer and I will replace the
water with antifreeze when the weather starts turning".
Needless to say, people are forgetful and many a motor suffered the fate of the water freezing inside the block. Often, when this happened the pressure of the water freezing and expanding forced the freeze-out plugs to pop out, relieving the pressure and saving the engine block from cracking. (although, just as often the engine cracked anyway). Another reason for these plugs to fail was the fact that they were made of steel and would easily rust through if the vehicle owner was careless about maintaining the cooling system. Antifreeze has rust inhibitors in the formula to prevent this from happening, but those chemicals would lose their effect after 3 years, which is why antifreeze needs to be changed periodically. The fact that some people left plain water in their engines greatly accelerated the rusting of these freeze plugs.
When a freeze plug becomes so rusty that it perforates, you have a coolant leak that must be repaired by replacing the rusted out freeze plug with a new one. This job ranges from fairly easy to extremely difficult depending on the location of the affected freeze plug. Freeze plugs are located on the sides of the engine, usually 3 or 4 per side. There are also freeze plugs on the back of the engine on some models and also on the heads.
As long as you are good about maintaining the cooling system, you need never worry about these plugs failing on modern vehicles
Head Gaskets and
Intake Manifold Gaskets
All internal
combustion engines have an engine block and one or two cylinder
heads. The mating surfaces where the block and head meet are
machined flat for a close, precision fit, but no amount of careful
machining will allow them to be completely water tight or be able
to hold back combustion gases from escaping past the mating
surfaces.
In order to seal the block to the heads, we use a head gasket. The head gasket has several things it needs to seal against. The main thing is the combustion pressure on each cylinder. Oil and coolant must easily flow between block and head and it is the job of the head gasket to keep these fluids from leaking out or into the combustion chamber, or each other for that matter.
A typical head gasket is usually made of soft sheet metal that is stamped with ridges that surround all leak points. When the head is placed on the block, the head gasket is sandwiched between them. Many bolts, called head bolts are screwed in and tightened down causing the head gasket to crush and form a tight seal between the block and head.
Head
gaskets usually fail if the engine overheats for a sustained period
of time causing the cylinder head to warp and release pressure on
the head gasket. This is most common on engines with cast
aluminum heads, which are now on just about all modern
engines.
Once coolant or combustion gases leak past the head gasket, the gasket material is usually damaged to a point where it will no longer hold the seal. This causes leaks in several possible areas. For example:
Some engines are more susceptible to head gasket failure than others. I have seen blown head gaskets on engines that just started to overheat and were running hot for less than 5 minutes. The best advice I can give is, if the engine shows signs of overheating, find a place to pull over and shut the engine off as quickly as possible.
Head gaskets themselves are relatively cheap, but it is the labor that's the killer. A typical head gasket replacement is a several hour job where the top part of the engine must be completely disassembled. These jobs can easily reach $1,000 or more.
On V type engines, there are two heads, meaning two head gaskets. While the labor won't double if both head gaskets need to be replaced, it will probably add a good 30% more labor to replace both. If only one head gasket has failed, it is usually not necessary to replace both, but it could be added insurance to get them both done at once.
A head gasket replacement begins with the diagnosis that the head gasket has failed. There is no way for a technician to know for certain whether there is additional damage to the cylinder head or other components without first disassembling the engine. All he or she knows is that fluid and/or combustion is not being contained.
One way to tell if a head gasket has failed is through a combustion leak test on the radiator. This is a chemical test that determines if there are combustion gases in the engine coolant. Another way is to remove the spark plugs and crank the engine while watching for water spray from one or more spark plug holes. Once the technician has determined that a head gasket must be replaced, an estimate is given for parts and labor. The technician will then explain that there may be additional charges after the engine is opened if more damage is found.
Heater
Core
The hot coolant is also used to provide
heat to the interior of the vehicle when needed. This is a
simple and straight forward system that includes a heater core,
which looks like a small 
version
of a radiator, connected to the cooling system with a pair of
rubber hoses. One hose brings hot coolant from the water pump
to the heater core and the other hose returns the coolant to the
top of the engine. There is usually a heater control valve in
one of the hoses to block the flow of coolant into the heater core
when maximum air conditioning is called for.
A fan, called a blower, draws air through the heater core and directs it through the heater ducts to the interior of the car. Temperature of the heat is regulated by a blend door that mixes cool outside air, or sometimes air conditioned air with the heated air coming through the heater core. This blend door allows you to control the temperature of the air coming into the interior. Other doors allow you to direct the warm air through the ducts on the floor, the defroster ducts at the base of the windshield, and the air conditioning ducts located in the instrument panel.
Hoses
There
are several rubber hoses that make up the plumbing to connect the
components of the cooling system. The main hoses are called
the upper and lower radiator hoses. These two hoses are
approximately 2 inches in diameter and direct coolant between the
engine and the radiator. 
Two additional hoses, called heater hoses, supply hot coolant
from the engine to the heater core. These hoses are
approximately 1 inch in diameter. One of these hoses may have
a heater control valve mounted in-line to block the hot coolant
from entering the heater core when the air conditioner is set to
max-cool. A fifth hose, called the bypass hose, is used to
circulate the coolant through the engine, bypassing the radiator,
when the thermostat is closed. Some engines do not use a
rubber hose. Instead, they might use a metal tube or have a
built-in passage in the front housing.
These hoses are designed to withstand the pressure inside the cooling system. Because of this, they are subject to wear and tear and eventually may require replacing as part of routine maintenance. If the rubber is beginning to look dry and cracked, or becomes soft and spongy, or you notice some ballooning at the ends, it is time to replace them. The main radiator hoses are usually molded to a shape that is designed to rout the hose around obstacles without kinking. When purchasing replacements, make sure that they are designed to fit the vehicle.
There is a small rubber hose that runs from the radiator neck to the reserve bottle. This allows coolant that is released by the pressure cap to be sent to the reserve tank. This rubber hose is about a quarter inch in diameter and is normally not part of the pressurized system. Once the engine is cool, the coolant is drawn back to the radiator by the same hose.
Cooling System Maintenance and
Repair
An engine that is overheating will quickly self
destruct, so proper maintenance of the cooling system is very
important to the life of the engine and the trouble free operation
of the cooling system in general.
The most important maintenance item is to flush and refill the coolant periodically. The reason for this important service is that anti-freeze has a number of additives that are designed to prevent corrosion in the cooling system. This corrosion tends to accelerate when several different types of metal interact with each other. The corrosion causes scale that eventually builds up and begins to clog the thin flat tubes in the radiator and heater core. causing the engine to eventually overheat. The anti-corrosion chemicals in the antifreeze prevents this, but they have a limited life span.
Newer antifreeze formulations will last for 5 years or 150,000 miles before requiring replacement. These antifreezes are usually red in color and are referred to as "Extended Life" or "Long Life" antifreeze. GM has been using this type of coolant in all their vehicles since 1996. The GM product is called "Dex-Cool".
Most antifreeze used in vehicles however, is green in color and should be replaced every two years or 30,000 miles, which ever comes first. You can convert to the new long life coolant, but only if you completely flush out all of the old antifreeze. If any green coolant is allowed to mix with the red coolant, you must revert to the shorter replacement cycle.
Look for a shop that can reverse-flush the cooling system. This requires special equipment and the removal of the thermostat in order to do the job properly. This type of flush is especially important if the old coolant looks brown or has scale or debris floating around in it.
If you remove the thermostat for a reverse flush, always replace it with a new thermostat of the proper temperature. It is cheap insurance.
The National Automotive Radiator Service Association (NARSA) recommends that motorists have a seven-point preventative cooling system maintenance check at least once every two years. The seven-point program is designed to identify any areas that need attention. It consists of:
Let's take these items one at a time.
Visual
Inspection
What you are looking for is the
condition of the belts and hoses. The radiator hoses and
heater hoses are easily inspected just by opening the hood and
looking. You want to be sure that the hoses have no cracking
or splitting and that there is no bulging or swelling at the
ends. If there is any sign of problems, the hose should be
replaced with the correct part number for the year, make and model
of the vehicle. Never use a universal hose unless it is an
emergency and a proper molded hose is not available.
Heater hoses are usually straight runs and are not molded, so a universal hose is fine to use and often is all that is available. Make sure that you use the proper inside diameter for the hose being replaced. For either the radiator hoses or the heater hoses, make sure that you route the replacement hose in the same way that the original hose was running. Position the hose away from any obstruction that can possibly damage it and always use new hose clamps. After you refill the cooling system with coolant, do a pressure test to make sure that there are no leaks.
On most older vehicles, the water pump is driven by a V belt or serpentine belt on the front of the engine that is also responsible for driving the alternator, power steering pump and air conditioner compressor. These types of belts are easy to inspect and replace if they are worn. You are looking for dry cracking on the inside surface of the belt.
On later vehicles, the water pump is often driven by the timing belt. This belt usually has a specific life expectancy at which time it must be replaced to insure that it does not fail. Since the timing belt is inside the engine and will require partial engine disassembly to inspect, it is very important to replace it at the correct interval. Since the labor to replace this belt can be significant, it is a good idea to replace the water pump at the same time that the belt is replaced. This is because 90 percent of the labor to replace a water pump has already been done to replace the timing belt. It is simply good insurance to replace the pump while everything is apart.
Radiator pressure cap
test
A radiator pressure cap is designed to
maintain pressure in the cooling system at a certain maximum
pressure. If the cooling system exceeds that pressure, a
valve in the cap opens to bleed the excessive pressure into the
reserve tank. Once the engine has cooled off, a negative
pressure begins to develop in the cooling system. When this
happens, a second valve in the cap allows the coolant to be
siphoned back into the radiator from the reserve tank. If the
cap should fail, the engine can easily overheat. A pressure
test of the radiator cap is a quick way to tell if the cap is doing
its job. It should be able to hold its rated pressure for two
minutes. Since radiator caps are quite inexpensive, I would
recommend replacing it every 3 years or 36,000 miles, just for
added insurance. Make absolutely sure that you replace it
with one that is designed for your vehicle.
Thermostat check for
proper opening and closing
This step is only
necessary if you are having problems with the cooling
system.
A thermostat is designed to open at a certain coolant
temperature. To test a thermostat while it is still in the
engine, start the engine and let it come to normal operating
temperature (do not let it overheat). If it takes an
unusually long time for the engine to warm up or for the heater to
begin delivering hot air, the thermostat may be stuck in the open
position. If the engine does warm up, shut it off and look
for the two radiator hoses. These are the two large hoses
that go from the engine to the radiator. Feel them carefully
(they could be very hot). If one hose is hot and the other is
cold, the thermostat may be stuck closed.
If you are having problems and suspect the thermostat, remove it and place it in a pot of water. Bring the water to a boil and watch the thermostat. You should see it open when the water reaches a boil. Most thermostats open at about 195 degrees Fahrenheit. An oven thermometer in the water should confirm that the thermostat is working properly.
Pressure
test to identify any external leaks
Pressure
testing the cooling system is a simple process to determine where a
leak is located. This test is only performed after the
cooling system has cooled sufficiently to allow you to safely
remove the pressure cap. Once you are sure that the cooling
system is full of coolant, a cooling system pressure tester is
attached in place of the radiator cap. The tester is than
pumped to build up pressure in the system. There is a gauge
on the tester indicating how much pressure is being pumped.
You should pump it to the pressure indicated on the pressure cap or
to manufacturer's specs.
Once pressure is applied, you can begin to look for leaks. Also watch the gauge on the tester to see if it loses pressure. If the pressure drops more than a couple of pounds in two minutes, there is likely a leak somewhere that may be hidden. It is not always easy to see where a leak is originating from. It is best to have the vehicle up on a lift so you can look over everything with a shop light or flashlight. If the heater core in leaking, it may not be visible since the core is enclosed and not visible without major disassembly, but one sure sign is the unmistakable odor of antifreeze inside the car. You may also notice the windshield steaming up with an oily residue.
Internal leak
test
If you are losing coolant, but there are
no signs of leaks, you could have a blown head gasket. The
best way to test for this problem is with a combustion leak test on
the radiator. This is accomplished using a block
tester. This is a kit that performs a chemical test on the
vapors in the radiator. Blue tester fluid is added to the
plastic container on the tester. If the fluid turns yellow
during the test, then exhaust gasses are present in the
radiator.
The most common causes for exhaust gasses to be present in the radiator is a blown head gasket. Replacing a bad head gasket requires a major disassembly of the engine and can be quite expensive. Other causes include a cracked head or a cracked block, both are even more undesirable than having to replace a head gasket.
When a head gasket
goes bad
The process of replacing a head
gasket begins with completely draining the coolant from the
engine. The top part of the engine is then disassembled along
with much of the front of the engine in order to gain access to the
cylinder heads. The head or heads are then removed and a
thorough inspection for additional damage is done.
Before the engine can be reassembled, the mating surfaces of the head and block are first cleaned to make sure that nothing will interfere with the sealing properties of the gasket. The surface of the cylinder head is also checked for flatness and, in some cases, the block is checked as well. The head gasket is then positioned on the block and aligned using locator pegs that are built into the block. The head is then placed on top of the gasket and a number of bolts, called head-bolts are coated with oil and loosely threaded into the assembly. The bolts are then tightened in a specific order to a specified initial torque using a special wrench called a torque wrench. This is to insure that the head gasket is crushed evenly in order to insure a tight seal. This process is then repeated to a second, tighter torque setting, then finally a third torque setting. At this point, the rest of the engine is reassembled and the cooling system is filled with a mixture of antifreeze and water. Once the engine is filled, the technician will pressure test the cooling system to make sure there are no leaks.
In many engines, coolant also passes between the heads and the intake manifold. There are also gaskets for the intake manifold to keep the coolant from leaking out at that point. Replacing an intake manifold gasket is a much easier job than a head gasket, but can still take a couple of hours or more for that job.
Engine Fan
Test
The radiator cooling fan is an important
part of the cooling system operation. While a fan is not
really needed while a vehicle is traveling down the highway, it is
extremely important when driving slowly or stopped with the engine
running. In the past, the fan was attached to the engine and
was driven by the fan belt. The speed of the fan was directly
proportional to the speed of the engine. This type of system
sometimes caused excessive noise as the car accelerated through the
gears. As the engine sped up, a rushing fan noise could be
heard. To quiet things down and place less of a drag in the
engine, a viscous fan drive was developed in order to disengage the
fan when it was not needed.
When computer controls came into being, these engine driven fans gave way to electric fans that were mounted directly on the radiator. A temperature sensor determined when the engine was beginning to run too hot and turned on the fan to draw air through the radiator to cool the engine. On many cars, there were two fans mounted side by side to make sure that the radiator had a uniform air flow for the width of the unit.
When the car was in motion, the speed of the air entering the grill was sufficient to keep the coolant at the proper temperature, so the fans were shut off. When the vehicle came to a stop, there was no natural air flow, so the fan would come on as soon as the engine reached a certain temperature.
If the air conditioner was turned on, a different circuit would come into play. The reason for this is the air conditioning system always requires a good air flow through the condenser mounted in front of the radiator. If the air flow stopped, the air conditioned air coming through the dash outlets would immediately start warming up. For this reason, when the air conditioner is turned on, the fan circuit would power the fans regardless of engine temperature.
If you notice that the engine temperature begins rising soon after the vehicle comes to a stop, the first thing to check is fan operation. If the fan is not turning when the engine is hot, a simple test is to turn the AC on. If the fan begins to work, suspect the temperature sensor in the fan circuit (you will need a wiring diagram for your vehicle to find it). In order to test the fan motor itself, unplug the two wire connector to the fan and connect a 12 volt source to one terminal and ground the other. (it doesn't matter which is which for this test) If the fan motor begins to turn, the motor is good. If it doesn't turn, the motor is bad and must be replaced.
In order to test the system further, you will need a repair manual for the year, make and model vehicle and follow the troubleshooting charts and diagnostic procedures for your vehicle. On most systems, there will be a fan relay or fan control module that can be a trouble spot. There are a number of different control systems, each requiring a different test procedure. Without the proper repair information, you can easily do more harm than good.
Cooling system power
flush and refill
While you can replace old
coolant by draining it out and replacing it with fresh coolant, the
best way to properly maintain your cooling system is to have the
system power flushed. Power flushing will remove all the old
coolant and pull out any sediment and scale along with it.
Power flushing requires a special machine that many auto repair shops have for the purpose. The procedure requires that the thermostat is removed, the lower radiator hose is disconnected, and the flush machine is connected in line. The lower hose is connected to the machine and the other hose from the machine is connected to the radiator where the lower hose was disconnected from.
Water, and sometimes, a cleaning agent is pumped through the cooling system in a reverse path from the normal coolant flow. This allows any scale to be loosened and flow out. Once clear water is coming out of the system, the hose is reconnected and a new thermostat is installed. Then the cooling system is refilled with the appropriate amount of antifreeze to bring the coolant to the proper mixture of antifreeze and water. For most vehicles and most climates, the mixture is 50 percent antifreeze and 50 percent water. In colder climates, more antifreeze is used, but must never exceed 75 percent antifreeze. Check your owner's manual for the proper procedures and recommendations for your vehicle.
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Heat engines generate mechanical power by extracting energy from heat flows, much as a water wheel extracts mechanical power from a flow of mass falling through a distance. Engines are inefficient, so more heat energy enters the engine than comes out as mechanical power; the difference is waste heat which must be removed. Internal combustion engines remove waste heat through cool intake air, hot exhaust gases, and explicit engine cooling.
Engines with higher efficiency have more energy leave as mechanical motion and less as waste heat. Some waste heat is essential: it guides heat through the engine, much as a water wheel works only if there is some exit velocity (energy) in the waste water to carry it away and make room for more water. Thus, all heat engines need cooling to operate.
Cooling is also needed because high temperatures damage engine materials and lubricants. Internal-combustion engines burn fuel hotter than the melting temperature of engine materials, and hot enough to set fire to lubricants. Engine cooling removes energy fast enough to keep temperatures low so the engine can survive.
Some high-efficiency engines run without explicit cooling and with only incidental heat loss, a design called adiabatic. Such engines can achieve high efficiency but compromise power output, duty cycle, engine weight, durability, and emissions.[citation needed]
Most internal combustion engines are fluid cooled using either air (a gaseous fluid) or a liquid coolant run through a heat exchanger (radiator) cooled by air. Marine engines and some stationary engines have ready access to a large volume of water at a suitable temperature. The water may be used directly to cool the engine, but often has sediment, which can clog coolant passages, or chemicals, such as salt, that can chemically damage the engine. Thus, engine coolant may be run through a heat exchanger that is cooled by the body of water.
Most liquid-cooled engines use a mixture of water and chemicals such as antifreeze and rust inhibitors. The industry term for the antifreeze mixture is engine coolant. Some antifreezes use no water at all, instead using a liquid with different properties, such as propylene glycol or a combination of propylene glycol and ethylene glycol. Most "air-cooled" engines use some liquid oil cooling, to maintain acceptable temperatures for both critical engine parts and the oil itself. Most "liquid-cooled" engines use some air cooling, with the intake stroke of air cooling the combustion chamber. An exception is Wankel engines, where some parts of the combustion chamber are never cooled by intake, requiring extra effort for successful operation.
There are many demands on a cooling system. One key requirement is that an engine fails if just one part overheats. Therefore, it is vital that the cooling system keep all parts at suitably low temperatures. Liquid-cooled engines are able to vary the size of their passageways through the engine block so that coolant flow may be tailored to the needs of each area. Locations with either high peak temperatures (narrow islands around the combustion chamber) or high heat flow (around exhaust ports) may require generous cooling. This reduces the occurrence of hot spots, which are more difficult to avoid with air cooling. Air-cooled engines may also vary their cooling capacity by using more closely spaced cooling fins in that area, but this can make their manufacture difficult and expensive.
Only the fixed parts of the engine, such as the block and head, are cooled directly by the main coolant system. Moving parts such as the pistons, and to a lesser extent the crank and rods, must rely on the lubrication oil as a coolant, or to a very limited amount of conduction into the block and thence the main coolant. High performance engines frequently have additional oil, beyond the amount needed for lubrication, sprayed upwards onto the bottom of the piston just for extra cooling. Air-cooled motorcycles often rely heavily on oil-cooling in addition to air-cooling of the cylinder barrels.
Liquid-cooled engines usually have a circulation pump. The first engines relied on thermo-syphon cooling alone, where hot coolant left the top of the engine block and passed to the radiator, where it was cooled before returning to the bottom of the engine. Circulation was powered by convection alone.
Other demands include cost, weight, reliability, and durability of the cooling system itself.
Conductive heat transfer is proportional to the temperature difference between materials. If engine metal is at 250 °C and the air is at 20°C, then there is a 230°C temperature difference for cooling. An air-cooled engine uses all of this difference. In contrast, a liquid-cooled engine might dump heat from the engine to a liquid, heating the liquid to 135°C (Water's standard boiling point of 100°C can be exceeded as the cooling system is both pressurised, and uses a mixture with antifreeze) which is then cooled with 20°C air. In each step, the liquid-cooled engine has half the temperature difference and so at first appears to need twice the cooling area.
However, properties of the coolant (water, oil, or air) also affect cooling. As example, comparing water and oil as coolants, one gram of oil can absorb about 55% of the heat for the same rise in temperature (called the specific heat capacity). Oil has about 90% the density of water, so a given volume of oil can absorb only about 50% of the energy of the same volume of water. The thermal conductivity of water is about 4 times that of oil, which can aid heat transfer. The viscosity of oil can be ten times greater than water, increasing the energy required to pump oil for cooling, and reducing the net power output of the engine.
Comparing air and water, air has vastly lower heat capacity per gram and per volume (4000) and less than a tenth the conductivity, but also much lower viscosity (about 200 times lower: 17.4 × 10−6 Pa·s for air vs 8.94 × 10−4 Pa·s for water). Continuing the calculation from two paragraphs above, air cooling needs ten times of the surface area, therefore the fins, and air needs 2000 times the flow velocity and thus a recirculating air fan needs ten times the power of a recirculating water pump. Moving heat from the cylinder to a large surface area for air cooling can present problems such as difficulties manufacturing the shapes needed for good heat transfer and the space needed for free flow of a large volume of air. Water boils at about the same temperature desired for engine cooling. This has the advantage that it absorbs a great deal of energy with very little rise in temperature (called heat of vaporization), which is good for keeping things cool, especially for passing one stream of coolant over several hot objects and achieving uniform temperature. In contrast, passing air over several hot objects in series warms the air at each step, so the first may be over-cooled and the last under-cooled. However, once water boils, it is an insulator, leading to a sudden loss of cooling where steam bubbles form (for more, see heat transfer). Unfortunately, steam may return to water as it mixes with other coolant, so an engine temperature gauge can indicate an acceptable temperature even though local temperatures are high enough that damage is being done.
An engine needs different temperatures. The inlet including the compressor of a turbo and in the inlet trumpets and the inlet valves need to be as cold as possible. Acountercurrent heat exchange with forced cooling air does the job. The cylinder-walls should not heat up the air before compression, but also not cool down the gas at the combustion. A compromise is a wall temperature of 90°C. The viscosity of the oil is optimized for just this temperature. Any cooling of the exhaust and the turbine of the turbocharger reduces the amount of power available to the turbine, so the exhaust system is often insulated between engine and turbocharger to keep the exhaust gases as hot as possible.
The temperature of the cooling air may range from well below freezing to 50°C. Further, while engines in long-haul boat or rail service may operate at a steady load, road vehicles often see widely varying and quickly varying load. Thus, the cooling system is designed to vary cooling so the engine is neither too hot nor too cold. Cooling system regulation includes adjustable baffles in the air flow (sometimes called 'shutters' and commonly run by a pneumatic 'shutterstat); a fan which operates either independently of the engine, such as an electric fan, or which has an adjustable clutch; a thermostatic valve or just 'thermostat' that can block the coolant flow when too cool. In addition, the motor, coolant, and heat exchanger have some heat capacity which smooths out temperature increase in short sprints. Some engine controls shut down an engine or limit it to half throttle if it overheats. Modern electronic engine controls adjust cooling based on throttle to anticipate a temperature rise, and limit engine power output to compensate for finite cooling.
Finally, other concerns may dominate cooling system design. As example, air is a relatively poor coolant, but air cooling systems are simple, and failure rates typically rise as the square of the number of failure points. Also, cooling capacity is reduced only slightly by small air coolant leaks. Where reliability is of utmost importance, as in aircraft, it may be a good trade-off to give up efficiency, durability (interval between engine rebuilds), and quietness in order to achieve slightly higher reliability — the consequences of a broken airplane engine are so severe, even a slight increase in reliability is worth giving up other good properties to achieve it.
Air-cooled and liquid-cooled engines are both used commonly. Each principle has advantages and disadvantages, and particular applications may favor one over the other. For example, most cars and trucks use liquid-cooled engines, while many small airplane and low-cost engines are air-cooled.
It is difficult to make generalizations about air-cooled and liquid-cooled engines. Air-cooled Deutz diesel engines are known[according to whom?] for reliability even in extreme heat, and are often used in situations where the engine runs unattended for months at a time.[citation needed]
Similarly, it is usually desirable to minimize the number of heat transfer stages in order to maximize the temperature difference at each stage. However, Detroit Diesel 2-stroke cycle engines commonly use oil cooled by water, with the water in turn cooled by air.[citation needed]
The coolant used in many liquid-cooled engines must be renewed periodically, and can freeze at ordinary temperatures thus causing permanent engine damage. Air-cooled engines do not require coolant service, and do not suffer engine damage from freezing, two commonly cited advantages for air-cooled engines. However, coolant based onpropylene glycol is liquid to -55 °C, colder than is encountered by many engines; shrinks slightly when it crystallizes, thus avoiding engine damage; and has a service life over 10,000 hours, essentially the lifetime of many engines.
It is usually more difficult to achieve either low emissions or low noise from an air-cooled engine, two more reasons most road vehicles use liquid-cooled engines. It is also often difficult to build large air-cooled engines, so nearly all air-cooled engines are under 500 kW (670 hp), whereas large liquid-cooled engines exceed 80 MW (107000 hp) (Wärtsilä-Sulzer RTA96-C 14-cylinder diesel).
Cars and trucks using direct air cooling (without an intermediate liquid) were built over a long period from the very beginning and ending with a small and generally unrecognized technical change. Before World War II, water-cooled cars and trucks routinely overheated while climbing mountain roads, creating geysers of boiling cooling water. This was considered normal, and at the time, most noted mountain roads had auto repair shops to minister to overheating engines.
ACS (Auto Club Suisse) maintains historical monuments to that era on the Susten Pass where two radiator refill stations remain (See a picture here). These have instructions on a cast metal plaque and a spherical bottom watering can hanging next to a water spigot. The spherical bottom was intended to keep it from being set down and, therefore, be useless around the house, in spite of which it was stolen, as the picture shows.
During that period, European firms such as Magirus-Detz built air-cooled diesel trucks, Porsche built air-cooled farm tractors,[1] and Volkswagen became famous with air-cooled passenger cars. In the USA, Franklin built air-cooled engines. The Czechoslovakia based company Tatra is known for their big size air-cooled V8 car engines, Tatra engineer Julius Mackerle published a book on it. Air-cooled engines are better adapted to extremely cold and hot environmental weather temperatures, you can see air-cooled engines starting and running in freezing conditions that stuck water-cooled engines and continue working when water-cooled ones start producing steam jets. Also the possibility of working at higher temperatures air-cooled engines have may be an advantage from a thermodynamic point of view. The worst problem met in air-cooled aircraft engines was the so-called "Shock cooling", when the airplane entered in a dive after climbing or levelled flight with throttle opened, with the engine under no-load while the airplane dives generating less heat, and the flow of air that cools the engine is increased, a catastrophic engine failure may result as different parts of engine have different temperatures, and thus different thermal expansions. In such conditions, the engine may get stuck, and any sudden change or imbalance in the relation between heat produced by the engine and heat dissipated by cooling may result in an increased wear of engine, as a consequence also of thermal dilatation differences between parts of engine, liquid cooled engines having more stable and uniform working temperatures.
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Liquid cooling is also employed in maritime vehicles (vessels, ...). For vessels, the seawater itself is mostly used for cooling. In some cases, chemical coolants are also employed (in closed systems) or they are mixed with seawater cooling.
The change of air cooling to liquid cooling occurred at the start of World War II when the US military needed reliable vehicles. The subject of boiling engines was addressed, researched, and a solution found. Previous radiators and engine blocks were properly designed and survived durability tests, but used water pumps with a leaky graphite-lubricated "rope" seal (gland) on the pump shaft. The seal was inherited from steam engines, where water loss is accepted, since steam engines already expend large volumes of water. Because the pump seal leaked mainly when the pump was running and the engine was hot, the water loss evaporated inconspicuously, leaving at best a small rusty trace when the engine stopped and cooled, thereby not revealing significant water loss. Automobile radiators (or heat exchangers) have an outlet that feeds cooled water to the engine and the engine has an outlet that feeds heated water to the top of the radiator. Water circulation is aided by a rotary pump that has only a slight effect, having to work over such a wide range of speeds that its impeller has only a minimal effect as a pump. While running, the leaking pump seal drained cooling water to a level where the pump could no longer return water to the top of the radiator, so water circulation ceased and water in the engine boiled. However, since water loss led to overheat and further water loss from boil-over, the original water loss was hidden.
After isolating the pump problem, cars and trucks built for the war effort (no civilian cars were built during that time) were equipped with carbon-seal water pumps that did not leak and caused no more geysers. Meanwhile, air cooling advanced in memory of boiling engines... even though boil-over was no longer a common problem. Air-cooled engines became popular throughout Europe. After the war, Volkswagen advertised in the USA as not boiling over, even though new water-cooled cars no longer boiled over, but these cars sold well. But as air quality awareness rose in the 1960s, and laws governing exhaust emissions were passed, unleaded gas replaced leaded gas and leaner fuel mixtures became the norm. Subaru chose liquid-cooling for their EA series (flat) engine when it was introduced in 1966.
A special class of experimental prototype internal combustion piston engines have been developed over several decades with the goal of improving efficiency by reducing heat loss These engines are variously called adiabatic engines, due to better approximation of adiabatic expansion, low heat rejection engines, or high temperature engines. They are generally diesel engines with combustion chamber parts lined with ceramic thermal barrier coatings. Some make used of titanium pistons and other titanium parts due to its low thermal conductivity ] and mass. Some designs are able to eliminate the use of a cooling system and associated parasitic losses altogether.[11] Developing lubricants able to withstand the higher temperatures involved has been a major barrier to commercialization.
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A car engine produces a lot of heat when it is running, and must be cooled continuously to avoid engine damage.
Generally this is done by circulating coolant liquid usually water mixed with an antifreeze solution through special cooling passages. Some engines are cooled by air flowing over finned cylinder casings.
A water-cooled engine block and cylinder head have interconnected coolant channels running through them. At the top of the cylinder head all the channels converge to a single outlet.
A pump, driven by a pulley and belt from the crankshaft, drives hot coolant out of the engine to the radiator, which is a form of heat exchanger.
Unwanted heat is passed from the radiator into the air stream, and the cooled liquid then returns to an inlet at the bottom of the block and flows back into the channels again.
Usually the pump sends coolant up through the engine and down through the radiator, taking advantage of the fact that hot water expands, becomes lighter and rises above cool water when heated. Its natural tendency is to flow upwards, and the pump assists circulation.
The radiator is linked to the engine by rubber hoses, and has a top and bottom tank connected by a core a bank of many fine tubes.
The tubes pass through holes in a stack of thin sheet-metal fins, so that the core has a very large surface area and can lose heat rapidly to the cooler air passing through it.
On older cars the tubes run vertically, but modern, low-fronted cars have crossflow radiators with tubes that run from side to side.
In an engine at its ordinary working temperature, the coolant is only just below normal boiling point.
The risk of boiling is avoided by increasing the pressure in the system, which raises the boiling point.
The extra pressure is limited by the radiator cap, which has a pressure valve in it. Excessive pressure opens the valve, and coolant flows out through an overflow pipe.
In a cooling system of this type there is a continual slight loss of coolant if the engine runs very hot. The system needs topping up from time to time.
Later cars have a sealed system in which any overflow goes into an expansion tank, from which it is sucked back into the engine when the remaining liquid cools.
Fins on an air-cooled cylinder are wider at the top, where most heat is generated. Horizontal air-cooled engines have cooling ducts to the fins.
The radiator needs a constant flow of air through its core to cool it adequately. When the car is moving, this happens anyway; but when it is stationary a fan is used to help the airflow.
The fan may be driven by the engine, but unless the engine is working hard, it is not always needed while the car is moving, so the energy used in driving it wastes fuel.
To overcome this, some cars have a viscous coupling a fluidclutch worked by a temperature sensitive valve that uncouples the fan until the coolant temperature reaches a set point.
Other cars have an electric fan, also switched on and off by a temperature sensor.
To let the engine warm up quickly, the radiator is closed off by a thermostat, usually sited above the pump. The thermostat has a valve worked by a chamber filled with wax.
When the engine warms up, the wax melts, expands and pushes the valve open, allowing coolant to flow through the radiator.
When the engine stops and cools, the valve closes again.
Water expands when it freezes, and if the water in an engine freezes it can burst the block or radiator. So antifreeze usually ethylene glycol is added to the water to lower its freezing point to a safe level.
Antifreeze should not be drained each summer; it can normally be left in for two or three years.
In an air-cooled engine, the block and cylinder head are made with deep fins on the outside.
Frequently a duct runs all around the fins, and an engine-driven fan blows air through the duct to take heat away from the fins.
A temperature-sensitive valve controls the amount of air being pushed around by the fan, and keeps the temperature constant even on cold days.
Air-cooled engines and high-performance water-cooled engines may have in addition to a water radiator a small extra radiator, through which engine oil flows to be cooled.
