Radiator Pressure Cap: Description
WARNING: This page is about a different car, the 2003 Dodge Dakota. However, it is still accessible from the selected car via links, so may be relevant.
All cooling systems are equipped with a pressure cap (Fig 1) in the radiator upper hose. This cap releases pressure at some point within a range of 131-to-158 kPa (19-to-23 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 131-to-158 kPa (19-to-23 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.