GM Fuel Gauge/Sender Question

[Zadroz]

I have a 1997 Pontiac Bonneville that has a really bad fuel gauge.

The gauge remains pegged at full until about 1/2 the tank is used. It then decreases rapidly. The problem is that if you allow the gauge to fall much below 1/4, you run the risk of running out of gas before you find a gas station. I can deal with this but it makes my wife so nervous, she doesn't want to drive the car if the gauge is even approaching 1/4.

This vehicle has OBD-II. The wiring schematic shows that the fuel sender output is routed to the PCM which then sends a fuel level signal to the gauge.

Using an Autotap OBD-II scanner to directly read the fuel level sensor output shows that the sender output is quite close to the actual fuel level at readings up to 56% full. That is the highest it will ever go.

Now here is the question. It seems reasonable to guess that the PCM has "learned" that 56% means the tank is full and has scaled its output to the gauge accordingly.

If my guess is correct, the gauge problem is caused by a faulty rheostat on the sender and changing it out will solve the problem. Since the part costs about $100 and I would need to drop the gas tank to swap it out, I don't want to do this without knowing that it will solve the problem.

Can anyone provide any guidance on this?

[MagicRat]

Originally Posted by Zadroz

If my guess is correct, the gauge problem is caused by a faulty rheostat on the sender and changing it out will solve the problem. Since the part costs about $100 and I would need to drop the gas tank to swap it out, I don't want to do this without knowing that it will solve the problem.

Welcome to AF.

The sender is actually a potentiometer, and is often the culprit in such matters.

You may be able to buy one and just connect it to the car's wiring harness without actually dropping the tank. Then you can slowly move the float arm through its range and have a friend watch the gauge and see if it follows the arm movements in a reasonable fashion. If the problem still exists, you may still be able to return the item (check before you buy) and save the work and expense.