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 pumped by a mechanical water pump 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 Components of a
Cooling System
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:
a)
Combustion gases could leak into the coolant passages
causing excessive pressure in the cooling system.
b)
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.
c) 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.
