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How A Car’s Cooling System Works

Alright, today on Repairs101 I thought I’d go over the basics of how your car’s cooling system works.


So an engine’s primary function is to produce power or motive force and one of the by products it creates as a result is heat. So the system developed to extract heat is fairly simple. A fluid – mostly water – is run through the passages and galleries in the engine. Then when the fluid reaches temperature, it’s released into the radiator; there it exchanges heat with the relative wind from forward momentum or by the air forced over the radiator by a fan.

Water Pump

Now the heart and the brains of the cooling system, respectively, are the water pump and the thermostat. The water pump is usually found on the front of an engine. Belt driven and used to circulate coolant from the engine block to the radiator.


The thermostat is an automatic valve that opens when the water inside the engine hits a predetermined temperature. When it opens the coolant flows until the fluid inside the engine is cool enough to shut the thermostat again. Now the radiator is a fluid – air heat exchanger that has tiny passages in it with large surface areas exposed to the cool air.

You can test your cooling system by pressurizing it cold with a pressure test kit. You can see the fluid escaping as I release the pressure. So never open a hot rad cap because the heat tells you it’s under pressure.

Rad Cap

Now the cap has two functions. One, to seal the system so it will pressurize when it’s heated. And two, to blow off excess fluid due to thermal expansion when it becomes over-pressurized.

In the old days the coolant would just dump out onto the street. Modern designs will catch the fluid from the overflow tube, and redirect it into an expansion tank. Where it will be collected and then drawn back into the system as it cools off.

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