shortchevy
I'm Awesome
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The old style vented caps is same as what shortchevy is talking about, on the older cars before charcoal canisters. When you remove the cap, it has a few small holes in middle with a diaphragm that moves up and down depending on pressure or suction. When the pressure reaches a certain point it pushes the diaphragm up and past a vent passage in the side of the cap letting air escape; when the suction reaches a certain point, it pulls diaphragm down past another passage that lets air in. True a hole in the cap will also work, but it can also cause gas to spittle out or dust to get in. Best thing to do is just run the old style vent hose that mounts in the gas filler neck, and sticking a little push-in breather or fuel filter into the end of the hose will keep dirt dobbers from clogging it up.
I knew I wasnt going crazy! I have a gas cap from a pre-emissions vw on my Jetta and its fine. I had also heard of people drilling a hole in there current gas cap.