The surge tank uses a pressure-vent cap to allow a buildup of 103 kPa (15 psi) in the cooling system. The cap allows the system to operate at a higher-than-atmospheric pressure. This pressure raises the boiling point of the coolant to about 125°C (262°F) at sea level.

The pressure-type cap contains a blowoff (pressure) valve (2) and a vacuum (atmospheric) valve (1). The cap protects the radiator by relieving any pressure that exceeds 103 kPa (15 psi).

In order to prevent radiator collapse, caused by the 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 new threaded design to allow for easier and safer removal. In order to safely remove the cap from a cooling system refer to Cooling System Draining and Filling in Draining and Filling Cooling System.