Removal of Fuel Guage Sensor Ring

My 2003 Jetta TDI ran badly on the way home after filling up at a station. Then an hour or so later I tried to start and it wouldn't start. I removed the fuel filter and drained it - no water. I replaced the filter any way and managed to get it running again but it continued to run badly (no power, jerkiness). I left it idling while I looked up some stuff on this problem. When I went back out, it had stopped running and wouldn't start.

So now I'd like to look in the tank, but can't get the darn plastic ring nut off to remove the fuel sensor.

I'd appreciate any suggestions.

If you don't have a large enough Channelock, just tap the outside of the ring on one of the nubs with a screwdriver or dowel and hammer. Mark the location of the ring so you can tighten it the same amount later.