what would cause the fuel guage in my 2000 gmc sarfari to move up and down when the van is in park, then stabilize when the van is in drive. once the tank is half full it continues to move up and down no matter what gear the van is in.
Your fuel gage is failing. It is part of the fuel pump assembly which resides inside the fuel tank. It’s a rather expensive fix considering that the tank has to be removed to replace the pump assy which is also expensive. You may want to remember to reset the trip odometer when you fill with fuel to get an idea how far it will go on refills. Then, when the gage fails completely you will know how far you can go on a tank. Once the pump fails, you will have to replace it. It’s probably only a matter of time.
Quite Possibly The Fuel Gauge “Sending Unit” Has Gone Bad.
GM Has had many of these go bad. I replaced one in my driveway. What happens is that sulfur in gasoline reacts with the metal on the tiny “fingers” on the sending unit’s “wiper” and they disintegrate.
The tank will undoubtedly have to come out or at least be dropped down a bit. The sending unit is available without the whole fuel pump assembly. The fuel pump should be fine, but will need to be withdrawn from the tank with the sending unit. As a matter of fact, GM made these sending units available at a reasonable cost ( around $50, I believe) and replaced lots under warranty. Don’t let somebody tell you that you need a fuel pump, too, unless for some other reason it has gone bad.
Be sure to have the problem properly diagnosed before replacing any parts.
Do the technicians a favor and have the fuel tank nearly empty when you go in for repairs. Knowing your approximate MPG or range, fill up and then drive to near the end of the range to determine when you are low.