A plastic pressure-vent cap is used to allow a buildup of 103 kPa (15 psi) in the cooling system. This pressure raises the boiling point of coolant to approximately 125°C (262°F) at sea level.

The pressure-type surge tank cap contains a blow-off (pressure) valve and a vacuum (atmospheric) valve. The cap protects the radiator by relieving pressure exceeding 103 kPa (15 psi).

In order to prevent radiator collapse caused by vacuum as the engine cools, the vacuum valve prevents excessive pressure differences from occurring between the cooling system and the atmosphere.

The surge tank cap has a threaded design to allow easier and safer removal. In order to safely remove the cap from a cool system, rotate the cap slowly counterclockwise 1/4 turn and then stop. Allow any pressure to relieve. After all hissing stops, continue to rotate counterclockwise until the cap is removed.

There is no specific orientation of the lettering on the cap versus the cap being tight. To retighten the cap, use approximately 5 N·m (44 lb in) of force (hand tight pressure).