Thread: Fuel gauge not working after car sat for a couple years

I have a 99 Grand Prix 3.8 2 door, been in the family since 2001. Back when the 3rd recall on these came out for the oil leak and fire the car was parked for a couple years. This spring I got it out and have been driving it since with a few minor bugs here and there but nothing I haven't figured out. About a month of being back on the road, the fuel gauge would randomly drop to E then up to full then read accurately. This went on for several weeks with no rhyme or reason as to why it would stop and start reading correctly. Then it finally stopped altogether. Every time I start the car the needle is at full and slowly drops to E unless I'm in P or N, then for whatever reason it's a pretty quick drop. Sending unit and pump were replaced right before the car was parked and all tested fine last week when I was having wiring issues at the plug on top of the pump. Now there is a resistance going through the plug that changes as the float goes up and down like it should. What is the next step to diagnose this? I've heard speculation from others as to the cluster and various things but I hate wasting time and money throwing parts at something. Any help would be greatly appreciated.

What happened was the new fuel pump had a slightly different plug so I modified it so that the new pump worked with the OE wiring. In doing so, I didn't realize it had the ability to wiggle loose. I found that out about the same time the gauge completely quit because I went to speed up to pass a car and the car completely died and wouldn't restart. I did a few quick fixes which didn't permanently fix the fuel pump issue so when I finally did a proper fix that has now resolved the fuel pump problem, I kinda expected the gauge to work but it still doesn't. Wiring at the tank is good, probed the wires after they were connected, and when the sending unit was out I checked the resistance and it changed when I moved the float and the resistance stayed constant when I held the float still. So next step is the cluster? Is there a way to test it first? Am I correct with the resistance test on the sending unit?

I think it was about half full and it sat for about 5 or 5.5 years now that I think about it. It was ran for inspection each year but that's about it. I'll check into the cluster. I don't know if I'm not quite understanding you but other than vibrations, I don't see why the canister would still be an issue. I fixed the prior modification that I had with a proper, permanent fix and checked for resistance in the body harness past where it plugs into the top of the pump with everything reassembled. Is there a good place to get wiring diagrams so I can check the connection further up? Possibly up to the cluster?

I did get the issue solved finally. I forgot about a wire I had traced back when I was diagnosing the fuel pump issue that I had after getting the car back on the road. The harness that, IMHO stupidly, is located between the bottom seat cushion and metal structure of the car that the cushion rests on had a snapped wire while I was testing. I thought it was just to the rear speakers because the one will intermittently work if someone is sitting in the back seat and the other never does so I just skipped over it at that point. I reconnected that wire and a couple others which had broken apart and that resolved the issue with the fuel gauge and the FTP sensor code that I had. I admit, I was a very hyper little kid and did a lot of bouncing around back there but I feel like GM could've placed the harness beside the seat at least.

Anyway, figured I'd post this so maybe it'll help someone out in the future.