Fuel Cap


A collapsing fuel tank usually indicates that the fuel tank cap is defective. A defective cap will cause the tank to collapse because as the fuel is removed during normal driving, no air can get into the tank to replace the volume of the fuel.

To remedy the problem, remove the filler cap and replace it with a new one.

There are several types of fuel caps.

Filler tube caps are nonventing and usually have some type of pressure-vacuum relief valve arrangement.

Pressure-vacuum gasoline filler cap. Courtesy of DaimlerChrysler Corporation.
  • Under normal operating conditions the valve is closed, but whenever pressure or vacuum is more than the calibration of the cap, the valve opens.
  • Once the pressure or vacuum has been relieved, the valve closes. Most pressure caps have four antisurge tangs that lock onto the filler neck to prevent the delivery system's pressure from pushing fuel out of the tank.
  • By turning such a cap one-half turn, the pressure in the tank will not be released all at once.
  • Then, with another quarter turn, the cap can be removed.

A threaded filler cap may be threaded into the upper end of the filler pipe.

  • These caps require several turns counterclockwise in order to be removed.
  • The long threaded area on the cap is designed to allow any remaining fuel tank pressure to escape during cap removal.
  • The cap and filler neck has a ratchet-type torque-limiting design to prevent overtightening.
  • When the cap is installed, tighten the cap until a clicking noise is heard.
  • This noise indicates the cap is properly tightened and fully seated.
When a gasoline tank filler cap is replaced, the replacement cap must be exactly the same as the original cap or a malfunction may occur in the filling and venting system, resulting in higher emission levels and the escape of dangerous hydrocarbons.