Radiator Pressure Cap: Description
WARNING: This page is about a different car, the 2006 Dodge Magnum, 2006 Dodge Charger, and 2006 Chrysler 300. However, it is still accessible from the selected car via links, so may be relevant.
All cooling systems are equipped with a pressure cap on the radiator. This cap releases pressure at some point within a range of 110-138 kPa (16-20 psi). The pressure relief point (in pounds) is engraved on top of the cap (3).
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 (4). This valve opens when system pressure reaches the release range of 110-138 kPa (16-20 psi).
A rubber gasket (2) seals the radiator filler neck. This is done to maintain vacuum during coolant cool-down and to prevent leakage when system is under pressure.