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