On the car you buy, used or new, the engine and coolant system is really a simple boiler Car engines operate on the same principal as a locomotive steam engine boiler. The difference is that instead of heating the water (coolant) with firewood or coal, the hot engine in the car provides the heat because ignition is taking place inside of its cylinders at 2,000oF. This is true of all cars whether or not you make a car comparison between different models before buying one.

Modern car engine coolant systems are operated at about 220o F; which is above the 212oF that water boils at sea level. As the coolant temperate exceeds 212oF, it begins to turn to steam and the pressure rises. That means empty volumes above the coolant level will be filled with pressurized "steam". The hot engine continues to add heat to the coolant until the coolant reaches about 220oF operating temperature. The water pump continually moves the coolant around the engine cylinder block until the coolant temperature reaches this operating temperature.

Now the radiator starts to do its job At the operating temperature a sensitive valve opens in the radiator, allowing the hot coolant to be pumped into the radiator where the air being pushed out of the way by the moving car will hopefully be enough to continually maintain its system operating temperature at 220oF. But remember, the pressure in the entire coolant system now is greater than the outside air we are breathing. That means the coolant wants to get out, and it will, through any tiny crack it can find. Eventually some will get out, and the coolant supply will decrease. If it decreases enough, the engine cannot be cooled enough and engine parts will expand and "seize", ruining the engine.

How to avoid an overheated engine To avoid this problem, the engine system has a clever makeup coolant tank connected to it through a check valve.

What's a check valve? Here's how to make one in your kitchen and see how one works. Take a throwaway plastic cup and make a round hole in its bottom. Take a round marble bigger than the hole and dump a lot of kitchen cleanser around the hole in the cup; then take the marble and rub it around the hole until it really make a nice round seat in the hole.

Now fill the cup half full of water. If you did a good job no water will sneak around the marble and drip out of the bottom of the cup. Hooray!

Now take your finger and slightly lift up the marble from beneath the cup, raising it slightly above the seat you created by scrubbing with cleanser (do it over the sink) and water will leak out on your finger.

You have just made a one-way check valve. The coolant makeup tank has such a check valve in it but also has a spring to keep the ball pressed down on the valve seat. That side is toward the radiator, so that coolant under pressure cannot push back into the coolant makeup tank, which is under atmospheric pressure.

What happens when the car cools down? When the coolant in the engine and radiator cools down to below boiling temperature, the steam that was above the liquid coolant will condense (turn back to water or coolant) and the coolant system will have a pressure that is less than the outside pressure of the air you are breathing..................a vacuum will form in the coolant system. Now it gets exciting.

The cycle reverses Whereas while the engine was running, the hot coolant wanted to get out; but now that it has cooled down, the outside pressure (what you're breathing) is more than the pressure in the coolant system (that includes the entire engine, the radiator and all the connecting hoses) and now the engine and radiator combined want to suck something in, namely more coolant.

When this happens, the check value in the coolant is opened by atmospheric pressure, overcoming the spring on the check valve, and fresh makeup coolant is sucked into the radiator, thereby replenishing coolant to normal.

So every time you stop and car and let the engine cool down, the whole system automatically restores the coolant level to normal without any driver assistance. Magic? You bet.

Maintenance Always keep the coolant makeup reservoir at least half full. It's usually under the hood in a semi-transparent plastic tank, fastened against the inside of one of the front fenders, and the coolant is always colored (in winter) so the level can be easily monitored.

Something to think about: The hot air you arrange to heat your feet, inside the car, is really inside air blowing across a very small radiator, just like the big radiator under the hood. This foot-warmer radiator too goes through the same pressure increases and decreases, as does the whole coolant system, since it is part of it.

So don't ever kick it with a sharp toe if you're mad at it while the engine is running, or you will get a free bath of really, really hot, coolant water sprayed all over your feet.