Description - Diesel

WARNING: This page is about a different car, the 2015 Jeep Grand Cherokee. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Radiator Cap

Courtesy of CHRYSLER GROUP, LLC

All cooling systems are equipped with a pressure cap. For 2013 diesel engines, the pressure cap is located on top of the pressurized coolant bottle. The cap releases pressure at some point within a range of 145 kPa (21 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 145 kPa (21 psi).
Two O-ring gasket seals the pressurized coolant bottle. This is done to maintain vacuum during coolant cool-down and to prevent leakage when system is under pressure.