Here is a strange problem I have been fighting for months, maybe somebody has a clue, because the dealership wants to drop the tank and replace the sending unit, but admits that they have no idea what the problem is or if that will fix it.
When I fill up my wife's 99 Bonny, the gage goes to full when the key is turned on and then slowly starts to drop until it hits empty. If you hit a railroad crossing, a pothole or hit the brakes hard enough to nosedive the car, it will stop falling and slowly return to full, then it starts to drop again. This goes on until about a gallon of gas has been burned off, about 20-30 miles, then all is normal. This happens every time I gas up the car. I don't over fill it, just let the pump kick off on its own. Any Pontiac owners heard of anything like this?
this happened to my moms chevy 1500. we never got it fixed because we just looked at the miles that have traveled to decide if it needed to be filled up. the truck was a highway truck with constant mpg so we really didn't need to fix the problem.
I would say it is the sending unit or some vehicles have magnets in the tank that work the fuel levels, not for sure if gm uses the setup since they are more electronic to fix the problem. but I would replace the unit and I would bet that will fix the problem.