My
here. I thought the B-bodies with their flat tanks and filler necks behind the license plate used non-vented caps. That is the reason there is a vent line that goes from the tank, up through trunk floor, up wheel well and back out. The vent line forms a gas trap that ensures proper gas path. I recall reading that vented caps could expel fuel from a slug of fuel sloshing against the cap on hard acceleration. I always make sure the vent is clear and have always used non-vented caps. Never had an issue.
I think e-bodies and chargers where the filler neck is far above fuel level used vented caps (depending on ECS requirements).
Anyone have a parts book or manual handy?