Radiator Cap: Description and Operation

Figure 7-Pressure Cap And Bottle:

The Coolant bottle is equipped with a pressure cap which releases pressure at some point within a range of 110 kPa ± 14 kPa (16 psi ± 2 psi).

The system will operate at higher than atmospheric pressure which raises the coolant boiling point, thus allowing increased radiator cooling capacity.
There is a spring-loaded vent valve in the center of the cap that allows the system to pressurize and depressurize without creating a vacuum. If the valve is stuck open, coolant will escape to the overflow hose. There is also a gasket in the cap to seal to the top of the filler neck.

CAUTION: Use only the pressure cap specified for this vehicle. Use of other pressure caps can lead to coolant loss and overheating.