Pressure/Vacuum Relief Fuel Cap
WARNING: This page is about a different car, the 1995 Ford Bronco. However, it is still accessible from the selected car via links, so may be relevant.
This system consists of a sealed filler cap with an integral pressure/vacuum relief valve. Fuel system vacuum relief is provided after one in. Hg. Pressure relief is provided after 1.8 psi (.13 kg/cm2 ). Under normal conditions, filler cap allows air to enter fuel tank as fuel is used, without allowing fuel vapors to escape.