Engine Cooling

The Car Cooling System

by: Kevin Schappell
The function of the engine's cooling system is to remove excess heat from the engine, to keep the engine operating at the most efficient temperature, and to allow the engine to reach its ideal operation temperature in the shortest time possible. In an ideal world, the cooling system keeps the engine running at its most efficient temperature no matter what the operating conditions are.

As petrol 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 petrol, 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:

  1. the engine's water jacket
  2. thermostat
  3. water pump
  4. radiator and radiator cap
  5. cooling fan (electric or belt-driven)
  6. hoses
  7. heater core
  8. expansion (overflow) tank

Burning of fuel in an engines produce very large amounts of heat and pressure; temperatures can reach up to 4,000 degrees F in the engine when the air/petrol mix is in certain proportions correctly. Under normal operating conditions the temperature is about 2,000 degrees F. The cooling system removes around 1/3 of all the heat that is produced in the engine combustion chamber.

The exhaust system also 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 will burn away and thus fail to protect it. This lack of lubrication can quickly destroy a car engine.

On the other side, if an engine runs at too low a temperature, it loses efficiency, the oil starts to get dirty (adding wear and reducing power output), deposits form, and fuel mileage is poor-- not to mention poor exhaust emissions! For these reasons, the cooling system only comes into action when the engine has heated up to its optimal temperature.

Type of Car Engine Cooling System

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

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 Engine Cooling Problems:

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

Broken tubing. Hoses and tubing wear out and leak coolant fluid. Once the coolant has left the system it can no longer cool the engine and therefore the engine will over heat.

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

Damaged 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 good, stable system. If your cap does not hold enough pressure, then the car engine could overheat on hot days since the system never becomes pressurised.

Water pump failure. Most commonly you will hear a horrible screeching noise and will be able to see engine coolant leaking from the front of the pump or underneath the car. Often there are early signs of trouble with small spots of coolant under the car after being parked overnight and a strong coolant odor whilst you are driving.

Head gasket. Do you have large amounts of white smoke flowing out of your exhaust? Then you could have a problem with your 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 overheated in the past. When very hot, the cylinder head can warp and prevent the engine head gasket from sealing properly.

Preventive Maintenance on Engine Cooling Systems

  1. Check all belts and hoses regularly. (at oil change is a good time)
  2. Look out for coolant leaks underneath the car, they could be signs of trouble to come.
  3. Change your coolant every 2 - 3 years depending on the manufactorers recommendations.
  4. Inspect your radiator cap for deterioration of the rubber seal. Replace if you think it is worn. $5 - $10 is cheap insurance.
  5. 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 car 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 engine radiator when the engine is hot. The pressure in the cooling system can cause hot coolant to splash out and burn you

Due to the number of comments on this page/topic we have moved further discussion to the forum.

"Very,very clear and instructive! Thank you a lot!"


"very useful. I had an overheating problem in the morning and had to park by the road and look for water. I Intend to use your tips to sort out the problem. Thank you."


"My car overheated recently, due to no water/coolant remaining. It drove home ok but overheated every 15miles as there was no way to keep water in the coolant tank (it was pouring out, it seemed into the oil!) I took it to my local garage, believing it was the gasket, he has just told me that it is the manifold? and this must be replaced before he will know if the gasket is also knackered. is this likely?"


"I have a Vauxhall Zafira diesel 51 plate overheating all the time but no loss of coolant or oil,told to replace thermostat valve but do not know were this is ? "


"Very informative. However, there is one important peace of information that could be added: If an engine is overheating while driving and you are not able to stop immediately, open all the windows and put the heating on full. This will help distribute some heat until you reach a point where you can pull over your car."


"i fond this information usefull but my car is still overheating and has none of these problems"


"nice info, very useful, but could you tell me roughly how much liquid (coolant and water mixture) is needed roughly for the whole cooling system as i wanted to flush the cooling system? thanks"


"Yes this item helped... It was nice and easy to read and understand and not to over the top or technical."

alan scott

"than u tjis info was cool"


"having just bought a second hand car 3 days ago, whilst travelling home from a light shopping trip my car blew out smoke from under the bonnet and liquid from underneath. having read your page on engine overheating, i found it very imformative. well done!"


"good article for beginners and more advanced users can benefit."


"spot on"

r reynold

"Very useful, and confired what I thought. My car has a blown head gasket"


"Very useful information, Thank you."


"Brilliant page I found it very informative I've just had a thirty minute journey turn into a two and a half hour journey because the car was overheating. I decided when I got in to look up on the net and see what I came up with and I think it is either the water pump or timing belt near. the place where you put your water in The pipes are warm but not too hot near the front of the car the pipes are absolutely red hot I said I didn't think the water was getting through to the radiator but when I pumped the pipes could hear the water inside ithem When you took the top off the container you seen and heard the water dropping again Now I've read this site I feel more settled... Would definately use it again"

T. Moores

"Than you just what i was looking for informative and easy to understand"


"Informative information and useful."


"nice , informative"


"very nice illustration. some pictures could have made it even better."