Radiator Cap: Description and Operation

WARNING: Under pressure, the temperature of the solution in the radiator can be considerably higher, without boiling. Removing the radiator cap while the engine is hot (pressure is high), will cause the solution to boil instantaneously, with explosive force. The solution will spew out over the engine, fenders, and the person removing file cap. Serious bodily injury may result Flammable antifreeze, such as alcohol, is not recommended for use at any time. Flammable antifreeze could cause a serious fire.

Description
The pressure-type radiator filler cap contains a blow-off valve and a vacuum valve. The pressure blow-off valve is held against its seat by a spring of predetermined strength. Internal pressure is relieved as it exceeds cooling system design limits, protecting the radiator. The vacuum valve is held against its seat by a light spring. This permits the valve to open, and relieve the vacuum created as the system cools off. The vacuum might otherwise collapse the radiator. The design of the radiator cap discourages inadvertent removal. To safely remove the cap, rotate the cap slowly counterclockwise (do not press downy to the detent. Allow any pressure to vent.

Operation
Press down on the cap and continue to rotate the cap counterclockwise, then lift the cap off. Under the diaphragm spring at the top of the cap is a rubber gasket. Embossed on the cap is a caution against its being opened and arrows indicating the proper closed position. For radiator cap testing, refer to Radiator Leak Testing.