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'99 Pontiac erratic gas gage

  1. 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?
  2. 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.
  3. Electrical/mechanical - After a fill up it goes erratic, it could be the sending unit or the ground is bad. Check the wires.

    The locking ring that holds the sender unit in the tank could be corroded badly and the unit isn't firmly attached to the tank any more.

    My friend was replacing the rear fuel tank in his old Ford pickup truck. As he was removing the sender unit he had the locking ring pop off the sender unit with a cracking noise, it was paper thin and nearly dust. Needless to say that ground path was questionable but it had a hardwired ground as well.
  4. My '93 Bonneville would read dead empty after a fill-up often, even tripping the low fuel warning. It would eventually go to full, but sometimes fall back to empty. Never fixed it, just tripped the odometer after filling up to keep accurate track.
  5. Thanks, yeah the trip odometer is a real help. I guess I need to get it fixed one of these days though, my daughter will be using it soon.
  6. I have the same problem on my 01 Tahoe, The DTC said the voltage to the sending unit is to high.
  7. Replacing the sending unit in the tank will most likely fix the car, it's been an ongoing battle with all GM vehicles. Every once in a while it will be the cluster or wiring, but more often than not it's the sender. There is a float in the tank attached to an arm that sweeps across a resistor block, what makes contact with the resistor block is three little tiny metal fingers. It could be that the fingers are worn down and not making contact with the block in the full position, or the resistor in the full position has gone bad. The dealership should be able to diagnose this 100% in about an hour. Unplug the connector, which plugs into the body just in front of the tank, and put a 250 ohm resistor inline back to the gauge, it should go to full and stay there. If not, wiring or cluster. Then put an ohmeter on the sender side, with the tank full and the gauge acting up, and see if the ohmeter stays at or near 250 ohms or starts to drop out.

    With the gauge coming back to full when hitting bumps or the brakes hard, I'd put money on it that the sender is the cause. The float is getting bounced hard enough for the fingers to start making contact with resistors near the full range, driving the gauge back up. When the fuel stops sloshing, the sender goes right back to the inoperable portion of the resistor block and the gauge starts dropping.
  8. knightkrawler00

    Thanks! I knew that someone on GT would have seen this.