Operation CHARM: Car repair manuals for everyone.

Initial Inspection and Diagnostic Overview

Hold the cap in hand, right side up. The vent valve at the bottom of the cap should open. If the rubber gasket has swollen and prevents the valve from opening, replace the cap.
Hold the cleaned cap in hand upside down. If any light shows between vent valve and rubber gasket, replace cap. Do not use a replacement cap that has a spring to hold the vent shut.
Replacement cap must be of the type designed for coolant reserve system with a completely sealed diaphragm spring, and rubber gasket to seal to filler neck top surface. This design assures coolant return to radiator.