Pressure Cap: Inspection

Hold the cap in your hand, right side up  (Figure ). The vent valve at the bottom of the cap should be normally closed. The vent valve should open with a slight pull with your finger nail. Replace the pressure cap for any of the following:

• Rubber gasket has swollen, preventing the valve from opening, replace the cap.
• ANy light can be seen between the vent valve and the rubber gasket. Use only a replacement cap that has a spring to hold the valve shut. 
• Gasket on the bottom of the cal shows noticable thinning.
• Cap has been through more then one engine overheat.