This page last modified
April, 2005

Basic Cooling System Theory

The internal combustion engine is a very inefficient way to power a car. Most of the energy of the fuel is wasted creating heat. This heat, if not removed, will quickly cause the engine to get so hot that the metal parts will melt. The cooling system is there to remove that excess heat. As engine efficiency has improved over the years, cooling systems have gotten smaller and smaller. In the fifties, the average cooling system contained 24 or more liters of coolant, now, the average is somewhere around 8. My Firefly holds less than 4.

The way the whole thing works is really quite simple. The heat is transferred from the hot engine parts to the coolant, which flows to the radiator where the heat gets transferred to the air that is flowing through the fins. Then the coolant flows back into the engine where it picks up more heat and so on . . .

The main problem that occurs is problems with the flow. Flow problems are pretty simple to figure out, usually there is an obstruction such as a stuck thermostat, water pump not turning, collapsed radiator hose, plugged radiator, air bubble in the engine, etc.

Cooling System Components:

Cooling System Components


The radiator is the key to the entire cooling system, it is the part that actually transfers the excess heat produced in the engine to the outside air. It is made up of several rows of small tubes connecting two containers which hold the coolant. Small fins are placed around the tubes to direct air around the outside of the tubes and to help the heat transfer from the tubes to the outside air.

Radiator hoses:

The radiator hoses don't do much except transport the coolant into and out of the radiator. The normal flow is from the lower hose to the water pump, through the cooling passages in the engine, and back through the thermostat to the upper hose and then to the radiator.

Water pump:

The water pump furnishes the force that moves the coolant around the system. (Duh) Most common problem is a leak that will appear at the small hole that you can see near the shaft if you look closely. This leak isn't a problem if you make sure that you top up the system on a regular basis.


The thermostat blocks off the main path for the hot coolant to return to the radiator, causing a large portion of it to be directed through the heater core and back to the engine until it reaches normal operating temperature and the thermostat opens. At this point coolant starts to flow to the radiator through the upper radiator hose. This function makes your heater produce heat faster, as well as helps the engine warm up quicker.

Heater hoses:

Transport coolant to and from the heater core. (duh) Look for leaks at the hose clamps where they connect to the heater core and the engine.

Heater core:

A mini-radiator that is used to heat the air inside the car. The hot coolant flows through the tubes and air is directed through the fins, absorbing the heat and then being directed to the cab or to the windshield defrost vents. When your heater core starts to get plugged up, you will gradually get less and less heat from your heater. A flush might help, but it could also trigger a leak...


In the olden days, cars used plain water for coolant, and it worked fairly well, except when it froze. Modern cars run at a higher temperature and use much smaller cooling systems. The vast majority of cars use a 50-50 mix of ethylene glycol and water for coolant, which also gives anti-freeze and anti-corrosion protection. In extreme cold climates, a mixture of up to 75% antifreeze to 25% water is used. Don't try to run your car on pure anti-freeze! The engine will overheat. If you use distilled water for the mix, you'll have significantly less corrosion.

Radiator cap:

The cap just keeps the coolant from sloshing out of the radiator and it maintains a positive pressure in the cooling system. Since the boiling temperature of the coolant is dependent on the pressure, this feature is necessary for proper functioning of the system.


The thermostat is probably the cheapest part of the cooling system. Mainly for this reason, every customer that comes in the door with an overheating problem is required to say, "It's probably the thermostat." This is a law of nature. The thermostat has one job, that is to block the main coolant return passage to the radiator until the engine heats up to operating temperature. Then the thermostat opens and the coolant circulates. If the top radiator hose gets hot, and coolant flows freely into the top of the radiator, then the thermostat is opening. Still, a new one can't hurt, and most of them are fairly easy to replace, so why not? Be warned though, I've seen brand new thermostats that were defective.

Bypass hose:

The bypass hose allows a small portion of the coolant to circulate when the thermostat is closed, get it? Bypassing the thermostat. Some engines don't have a bypass hose, but rather a bypass passage that is part of the engine cooling jacket. Not much to go wrong, unless the hose develops a leak, which they do with depressing regularity.

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