Ford Mustang 1989-1993 Repair Guide

Pressure/Vacuum Relief Fuel Cap


The fuel cap contains an integral pressure and vacuum relief valve. The vacuum valve acts to allow air into the fuel tank to replace the fuel as it is used, while preventing vapors from escaping the tank through the atmosphere. The vacuum relief valve opens after a vacuum of -0.5 psi (3.45 kPa). The pressure valve acts as a backup pressure relief valve in the event the normal venting system is overcome by excessive generation of internal pressure or restriction of the normal venting system. The pressure relief range is 1.6-2.1 psi (11.0-14.5 kPa). Fill cap damage or contamination that stops the pressure vacuum valve from working may result in deformation of the fuel tank.


  1. Unscrew the fuel filler cap. The cap has a pre-vent feature that allows the tank to vent for the first 3 / 4 turn before unthreading.
  3. Remove the screw retaining the fuel cap tether and remove the fuel cap.
  5. Installation is the reverse of the removal procedure. When installing the cap, continue to turn clockwise until the ratchet mechanism gives off 3 or more loud clicks.