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 manufactorers 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.

WARNING: Never open your radiator when the engine is hot. The
pressure in the system can cause hot coolant to splash out and burn
you.