The purpose of the engine's cooling system is to remove excess
heat from the engine, to keep the engine operating at its most
efficient temperature, and to get the engine up to the correct
temperature as soon as possible after starting. Ideally, the
cooling system keeps the engine running at its most efficient
temperature no matter what the operating conditions are.

As fuel is burned in the engine, about one-third of the energy in
the fuel is converted into power. Another third goes out the
exhaust pipe unused, and the remaining third becomes heat energy.

A cooling system of some kind is necessary in any internal
combustion engine. If no cooling system were provided, parts
would melt from the heat of the burning fuel, and the pistons
would expand so much they could not move in the cylinders (called
"seize").

The cooling system of a water-cooled engine consists of: the
engine's water jacket, a thermostat, a water pump, a radiator and
radiator cap, a cooling fan (electric or belt-driven), hoses, the
heater core, and usually an expansion (overflow) tank.

Fuel burning engines produce enormous amounts of heat;
temperatures can reach up to 4,000 degrees F when the air-fuel
mixture burns. However, normal operating temperature is about
2,000 degrees F. The cooling system removes about one-third of
the heat produced in the combustion chamber.

The exhaust system takes away much of the heat, but parts of the
engine, such as the cylinder walls, pistons, and cylinder head,
absorb large amounts of the heat. If a part of the engine gets
too hot, the oil film fails to protect it. This lack of
lubrication can ruin the engine.

On the other hand, if an engine runs at too low a temperature, it
is inefficient, the oil gets dirty (adding wear and subtracting
horsepower), deposits form, and fuel mileage is poor-- not to
mention exhaust emissions! For these reasons, the cooling system
is designed to stay out of the action until the engine is warmed
up.

There are two types of cooling systems; liquid cooling and air
cooling. Most auto engines are cooled by the liquid type; air
cooling is used more frequently for airplanes, motorcycles and
lawnmowers.

Liquid cooled engines have passages for the liquid, or coolant,
through the cylinder block and head. The coolant has to have
indirect contact with such engine parts as the combustion
chamber, the cylinder walls, and the valve seats and guides.
Running through the passages in the engine heats the coolant (it
absorbs the heat from the engine parts), and going through the
radiator cools it. After getting "cool" again in the radiator,
the coolant comes back through the engine. This business
continues as long as the engine is running, with the coolant
absorbing and removing the engine's heat, and the radiator
cooling the coolant.

A cooling system pressure tester is used to check the pressure in
the cooling system, which allows the mechanic to determine if the
system has any slow leaks. The leak can then be found and fixed
before it causes a major problem.

The above information is directly from the Auto Insight program
which you can buy online from AutoEducation.com.

Common Problems:

Let's look at the common problems cars have with the cooling
system.

Broken hose. Hoses wear out and can leak. Once the coolant has
left the system it can no longer cool the engine and it
overheats.

Broken fan belt. The water pump is driven by the engine through a
belt. If the belt breaks the water pump can not turn and coolant
will not be circulated through the engine. This will also lead to
engine overheating.

Faulty radiator cap. The radiator cap is designed to hold a
certain pressure in the coolant system. Most caps hold 8 - 12
PSI. This pressure raises the point in which the coolant will
boil and maintains a stable system. If your cap does not hold
pressure, then the car could overheat on hot days since the
system never becomes pressurized.

Water pump failure. Most commonly you will hear a screeching
noise and will be able to see coolant leaking from the front of
the pump or under the car. Early signs are small spots of coolant
under the car after being parked overnight and a strong coolant
odor while driving.

Head gasket... have large amounts of white smoke flowing out of
your exhaust? Could be a head gasket. The head gasket seals the
cylinder head to the engine block and also seals the coolant
passages. When this gasket fails coolant can enter the cylinder
and it will be turned to vapor as the engine fires. Head gaskets
most often fail after the engine has experienced an overheating
situation. When super hot, the cylinder head can warp and allow
the gasket to fail.
Preventive Maintenance:

Check all belts and hoses regularly. (at oil change is a good
time)

Look out for coolant leaks underneath the car, they could be
signs of trouble to come.

Change your coolant every 2 - 3 years depending on the
manufacturer's recommendations.

Inspect your radiator cap for deterioration of the rubber seal.
Replace if you think it is worn. $5 - $10 is cheap insurance.

Have your coolant system flushed every 5 years. It gets all the
corrosion which has built up out of the system.
What to discuss with your mechanic:

Let your mechanic know when your overheating problems occur.
Overheating when idling points to a different problem than
overheating at highway speeds.

Ask your mechanic if it's worth changing the timing belt or chain
while he is replacing your water pump. Many times the timing belt
turns the water pump so it has to be removed anyway to access the
water pump.