So itís not leaking gasoline out of the cap area? You can just smell fumes?
Then I would suspect you have a cracked fuel line/elbow where it comes out of the tank. Do a search here for ďleaking gasĒ and youíll see BMW used cheap fuel tank fittings, a problem that has plagued them for years. They know itís a problem, but choose to save $0.50 and not use steel fittings. Not a big job to repair yourself, as long as the tank itself is not cracked.