Gas Tank Vent

[Beemer]

Can anyone tell me how to check whether or not the gas tank vent is clogged in a Mark III?

[Mark IV]

Remove the inner fender shield on the right hand side and inspect the vent hose that runs from the tank to the filler. Any kinks will prevent proper tank venting.

[Beemer]

Looking at this afternoon. Thanks.

[ACademic]

FYI, you can inspect this without removing the shield. The vent hose should attach to the top of the tank with one worm clamp. If you don't see any obvious kinks, then remove the hose from the tank end, open up your fill cap, and blow some compressed air out toward the fill cap. That will ensure that you don't have any debris lodged in there and you're blowing it out the car rather than in the tank. Or, better yet, use a shop vac and place the tank end of the hose into the suction. The vent is (I believe) either 5/8" or 3/4" ID, so I'm suspecting you have a bend/kink.

[Beemer]

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We verified the vent wasn't kinked or clogged. My neighbor has an earlier Mark III and we switched the gas caps and the problem went away. An investigation of the caps showed his inside was different than the one on my car. I spoke with Olthoff people and they said it is a common problem with the later model gas caps. They are too tight. The newer caps have raised rings under the gasket causing the problem. They had me remove the gasket and cut 3 equal distant spokes about a 1/32" deep from the center to the edge of the inside edge of the cap across the rings under the gasket. I replaced the gasket and it worked and the pressure in the tank is gone. Thanks all who commented.