My grommet leaked when I bought the car, so replaced it. Had to sand down the area that slid into the grommet because it was not smooth. I put some grease on the grommet and filler tube and shoved it in. Was a really tight fit. That cured my leak. Did verify the tank vent was intact while doing this. Sounds like your grommet is damaged, not sitting straight on the tank flange, or the filler tube needs some help. The part that goes into the grommet must be smooth and dent free to ensure a good seal.
