Radiator Cap: Description and Operation
CAP - RADIATOR PRESSUREDESCRIPTION
All cooling systems are equipped with a pressure cap. For 5.9L engines, the pressure cap is located on top of the radiator outlet tank. For all engines, the pressure cap is located on top of the coolant degas container. The cap releases pressure at some point within a range of 97-to-124 kPa (14-to-18 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 97-to-124 kPa (14-to-18 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 the cooling system. This causes the vacuum valve to open and coolant in the reserve/overflow container to be drawn through the recovery hose connecting the filler neck and reserve/ overflow container. If the vacuum valve is stuck shut, or the recovery hose is kinked, radiator hoses will collapse on cool down.