'91 3.1 won't run with gas cap on...?

[khmer75]

I removed the cap the other day and heard a "whoosh" as the tank sucked in some air. It runs okay with the cap loose, but occasionally barfs gas out of the filler, which is of course both expensive and dangerous. Apparently the emissions control system is not permitting air to return to the tank as fuel is used. Any ideas where I should look for the problem? Thanks,
Eric

I own a 90' Regal GS. My problem was solved with a new vented gas cap.If your not getting any CIS lights coming on.
Get one from an automotive parts store.Have them look it up on their computers to make sure you get a correct match.If your getting a CIS light,have it scanned.Most places like Autozone will do it for free.
Trying the cheaper first might help.

[khmer75]

Thanks. That's a great idea. I did not think such a thing existed. Do you have a model or part number?
I'm thinking the problem might be a plugged carbon canister. Any thoughts on that?

I don't have a part number right off hand.I you ask a parts store they can look it up for you.Tell them you want a vented fuel cap.I would try that first then got to the next step.