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 on
Automotive 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 pump
A serpentine belt, which also drives the alternator, power
steering pump and AC compressor among other things.
The timing belt that is also responsible for driving one or more
camshafts.
The 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:
combustion gases could leak into the coolant passages causing
excessive pressure in the cooling system.
coolant could leak into the combustion chamber causing coolant
to escape through the exhaust system, often causing a white cloud of
smoke at the tailpipe.
Other problems such as oil mixing with the coolant or being
burned out the exhaust are also possible.
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:
a visual inspection of all cooling system components, including
belts and hoses
a radiator pressure cap test to check for the recommended system
pressure level
a thermostat check for proper opening and closing
a pressure test to identify any external leaks to the cooling
system parts; including the radiator, water pump, engine coolant
passages, radiator and heater hoses and heater core
an internal leak test to check for combustion gas leakage into
the cooling system
an engine fan test for proper operation
a system power flush and refill with car manufacturer's
recommended concentration of coolant
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.
Notice: The information on this site is
not intended as a substitute for the advice of a professional who is
qualified to examine, diagnose and repair your vehicle.
A
Short Course on
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.
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.
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.
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.
A fan belt that will also be responsible for driving an
additional component like an alternator or power steering pump
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.
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". 
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. 
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. 
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. 
Pressure test to identify any external leaks