The Stovebolt Site Forums The Shop Area General Truck Talk Vented Gas Cap, am I crazy?

you'll probably be better off if you don't fill all the way up to the top of the filler neck to allow room for expansion, this applies to all the new cars as well since they are not vented at all, fumes are stored in the charcoal canister and burned later while driving, overfilling causes the canister to become saturated with raw fuel instead of fumes. causes variety of performance problems and check engine lights to come on.