I too have experienced the leak in the gas tank only when I filled up about 2 years ago. The gas tank was rusted at the base of the filler neck. The reason is if you take the passenger rear tire off, you will see the wheel well doesn't go down far enough in the back so that road grim shoots onto the gas tank and lower filler neck. I'll bet a big pile of crud is there (drenched in gas no doubt). Replacement of the gas tank was the only good cure. I then made a small discreet rubber mudflap that I attached to the back of the wheel well so that this won't happen again. It is just big enough that from the outside, you can barely see it too. I really don't like the look of mudflaps.

Good luck,
JimC
1983 533i

The rear of my car feels like a boat... I hate that.
I already changed the shock absorbers. Could it be the dogbones?

I have a leak on the top of the gas tank when I fill up the tank too much, is this a known problem.

Thanks

'89 535i
'86 535i