Fuel Filler Cap
WARNING: This page is about a different car, the 2003 Dodge Intrepid, 2003 Chrysler Intrepid, 2003 Chrysler Concorde, and 2003 Chrysler 300M. However, it is still accessible from the selected car via links, so may be relevant.
The plastic fuel tank filler cap is threaded/quarter turn onto the end of the fuel filler tube. Its purpose is to retain vapors and fuel in the fuel tank. The fuel filler cap incorporates a two-way relief valve that is closed to atmosphere during normal operating conditions. The relief valve is calibrated to open when a pressure of 2.5 psi or vacuum of 0.6 in. Hg occurs in the fuel tank. When the pressure or vacuum is relieved, the valve returns to the normally closed condition. Remove filler cap before servicing any fuel system component to relieve tank pressure.