Pressure Cap Description

Caution:
- Under some conditions, the ethylene glycol engine coolant Is combustible. In order to help avoid being burned, do not spill the antifreeze or the coolant on the exhaust system or the hot engine parts.
- As long as there is pressure in the cooling system, the temperature can be considerably higher than the boiling temperature of the solution In the radiator, without causing the solution to boll. Removal of the radiator cap while the engine Is hot and the pressure is high will cause the solution to boil Instantaneously and possibly with explosive force, spewing the solution over the engine, fenders, and person removing the cap. Under some conditions, the engine coolant is combustible.

The plastic pressure-vent cap allows a build up of 100 kPa (14.5 psi) in the cooling system. This pressure raises the boiling point of the engine coolant to about 125° C (257° F) at sea level. The pressure-type cap contains a blow-off (pressure) valve and a vacuum (atmospheric) valve. A spring holds the pressure valve against the valve seat and relieves any cooling system pressure exceeding 100 kPa (14.5 psi).
The vacuum valve opens as the cooling system pressure drops below the atmospheric pressure in order to prevent the radiator, the coolant surge tank, or the hoses from collapsing because of the vacuum caused during engine cooling. As the valve opens, air is drawn into the coolant surge tank, equalizing the system pressure with that of the atmosphere.
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 surge tank cap is removed. There is no specific orientation of the lettering on the cap versus the cap being tight. Hand tighten the surge tank cap to about 5 Nm (44 inch lbs.).