Evolvo What you are saying about that little pumping action is exactly what I believe is happening with the non vented gas cap. Yes, I could vent the cap by drilling a small hole, but then I run into two problems - possible slosh on my new $$ paint and a second fume exit.

Now, the hard part - room for much modification above the tank - not much. I have a standard 'A' trunk with a high sill that might allow a few inches for a loop, but probably not to the height of the tires. And since I actually have an access hatch for the sending unit and have to open it to fix the ground will give it a shot. If it works, then I'll do a more permanent mod, but with that expansion and contraction of the vapors, am not convinced. YET

Whatever the fume outcome - I am still enjoying driving the car