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, do the following steps:
- Slowly rotate the cap counterclockwise ¼ turn and then stop. Do not press down.
- Allow any residual pressure, indicated by a hissing sound, to be relieved.
- After the hissing stops, continue to rotate the cap counterclockwise until the cap is removed.
- To retighten the cap, use hand tight pressure only.
Caution: With a pressurized cooling system, the coolant temperature in the radiator can be considerably higher than the boiling point of the solution at atmospheric pressure. Removal of the surge tank cap, while the cooling system is hot and under high pressure, causes the solution to boil instantaneously with explosive force. This will cause the solution to spew out over the engine, the fenders, and the person removing the cap. Serious bodily injury may result.
