Pressure cap

A special cap for the radiator, it holds a predetermined amount of pressure on the water in the cooling system, this enables the water to run hotter without boiling. In this way, the engine can operate at a higher and a more efficient temperature without danger of overheating. The cap also has a vacuum valve that lets air into the cooling system if the pressure falls too low to prevent outside air pressure from causing a partial collapse of the radiator. See radiator pressure cap, safety pressure cap