Radiator Cap: Description and Operation
RADIATOR PRESSURE CAPDESCRIPTION
All vehicles are equipped with a pressure cap (Fig. 11). This cap releases pressure at some point within a range of 124 - 145 kPa (18 - 21 psi). The pressure relief point (in pounds) is engraved on top of the cap
The cooling system will operate at pressures slightly above atmospheric pressure. This results in a higher coolant boiling point allowing increased radiator cooling capacity. The cap contains a spring-loaded pressure relief valve. This valve opens when system pressure reaches the release range of 124 - 145 kPa (18 - 21 psi).
A rubber gasket seals the radiator filler neck. This is done to maintain vacuum during coolant cool-down and to prevent leakage when system is under pressure.
OPERATION
A vent valve in the center of the cap will remain shut as long as the cooling system is pressurized. As the coolant cools, it contracts and creates a vacuum in cooling system. This causes the vacuum valve to open and coolant in reserve/overflow tank to be drawn through connecting hose into radiator. If the vacuum valve is stuck shut, or overflow hose is kinked, radiator hoses will collapse on cool-down.