Pressure Cap: Operation
WARNING: This page is about a different car, the 2006 Dodge Viper. However, it is still accessible from the selected car via links, so may be relevant.
The pressure cap allows the cooling system to operate at higher than atmospheric pressure which raises the coolant boiling point. This pressure allows for increased radiator cooling capacity. The pressure cap will release cooling system pressure at some point within a range of 117 kPa +/-7 kPa (17 psi +/-1 psi).
A spring-loaded vent valve in the center of the cap allows the system to pressurize and depressurize without creating a vacuum. If the valve is stuck open, coolant will escape to the overflow hose. A gasket seals the cap to the top of the filler neck.