Radiator Pressure Caps

Pressure caps are necessary because they allow the coolant system to maintain the proper pressure by venting air or fluid. To do this, pressure caps have a pressure valve and a vacuum valve.

The pressure valve allows the system to maintain the proper pressure as well as to vent off any excess pressure that builds up in the system. The vacuum valves purpose is to allow radiator fluid to flow back into the radiator from the reservoir bottle when the engine cools. This is important, because as the temperature rises the coolant volume expands and as it cools, it contracts.

Never assume that because the coolant level in the reservoir is at the proper level, the coolant is also at the proper level in the radiator. When removing a pressurized cap, it is mandatory to allow the engine to cool or the cap may fly high in the air with hot coolant coming out like a geyser—causing severe burns and injury.

On most radiators, the cap is spring-loaded. It is pressed on and turned about 90° clockwise until hitting the stops on the radiator neck. Some coolant systems do not use spring-loaded radiator caps. Instead, these systems will have a pressurized recovery system, using a threaded on version of a pressure cap.

In general, for every pound of pressure increase in the coolant system, the boiling point of the coolant increases by two to three degrees Fahrenheit. At higher altitudes, atmospheric pressure decreases and that causes boiling points to decrease. For every 1,100 feet in altitude, the boiling point of water will decrease by 2°F. Because it is important for the engine to reach normal operating temperature without the coolant boiling, it is especially important that the cooling system is in good working order when taking trips that involve significant changes in altitude.

Pressurized caps are rated in pounds per square inch. The rating varies from manufacturer to manufacturer and is typically in the range of 13 PSI to 16 PSI. You can always find the correct rating stamped on the pressure cap. Caps must be tested to ensure that they will maintain the proper pressure.

Because loss of pressure reduces the boiling point as well as the circulation of the coolant, cavitation may occur, which will damage the water pump. Prior to replacing the pressure cap, view our cooling system diagnostic page of our website to ensure that you have ruled out any other cause of insufficient pressure.

The following step-by-step procedures will guide you through the proper techniques for safely removing the cap and avoiding injury.