Radiator Pressure Cap: Operation

The pressure cap performs two tasks in a closed cooling system. The pressure cap must allow the cooling system to build pressure during engine operation, and allow the cooling system to loose pressure when not operating.

With the engine operating, the pressure cap must allow the cooling system to operate at higher than atmospheric pressure. This increased pressure raises the boiling point of the coolant, thereby allowing the cooling system to operate more efficiently. However, under excessive pressure, 110 kPa +/- 14 kPa (16 psi +/- 2 psi) the cooling system will overcome the spring pressure in the pressure cap and allow engine coolant to flow into the reservoir. This pressure relief will continue until the pressure is reduced and the spring in the pressure cap can return to its normally closed position.

With the engine not operating, the pressure cap must allow the cooling system to loose pressure and recover engine coolant. This recovery is accomplished through the use of a lighter spring in the pressure cap. As the hot engine coolant cools, it creates a vacuum in the cooling system. As this vacuum is created, it pulls the vacuum valve open and allows engine coolant to be pulled back into the engine from the coolant reservoir tank. This continues until the system pressure is equalized, and the vacuum valve is then returned to its normally closed position.