The radiator cap on a modern cooling system

Correct answer to question 79 (Auto & Shop Information): holds pressure in the system to raise the coolant's boiling point

Method

Raising the pressure raises the temperature at which coolant boils, giving a margin above normal running temperature. The cap also has a relief valve that vents to the overflow bottle, which is why removing it from a hot engine is dangerous. Controlling flow is the thermostat's job.