Problem with the gas gauge

The indicator in the gas gauge is running wild from one side to the other. I was told that it's either something in the tank or the dashboard. I've noticed that the indicator shows the correct gas amount the first couple of seconds after the ignition (then it starts bouncing left and right) and also shows the correct amount after refueling.
Thanks for any help.

Are all of the other dash gauges working okay? I went through this on my wife's 97 Lesabre. The fuel gauge would read okay at certain tank levels and then drop and bounce back up. Turned out to be the sending unit in the tank. It functions as a variable resistor reading between 0 and 240 ohms. The high end, 240, if a full tank. Should read about 120 ohms on a half tank. I put in a salvaged unit. The first salvaged one I put in was also defective. Returned it and the 2nd one worked.

I substitued variable resistors from Radio Shack with known values to test the gauge. The resistors provided accurate gauge readings, so I knew it was the sending unit.

Might check to make sure the sending unit ground wire is good and grounded right first. If that's okay, look to the sending unit. I have read that a certain brand of gas is corrosive to the sending units. Can't recall right off hand what brand it was. I don't think that was the problem with mine though. The sender was just defective.

i have problems with my gas guage also. I have a 97 buick lesabre. Where is the sending unit ground wire by the way so i can check it thanks.

It has been about 2 and 1/2 years since I looked at it, but from what I recall, there is a black wire that comes off the tank (connected to the sending unit). There are 2 wires coming off the unit. One is black and is a ground. The other sends the signal from the sending unit to the gauge (maybe lt brown or white--not sure). May have to reach way up in there on the pass side, laying under the car in back. You can follow the wires with your hand and reach up in there to access them. I tapped into them with a straight pin and a clip wire connecter when testing. Of course, to get to that sending unit to remove it, you have to drop the tank. It is really not that bad, but must drop the exhaust. The fill tube was kind of a pain. Would be nice if it could be accessed through the trunk like on the Park Ave.

I tapped into the ground wire and clipped it to a known good ground to see if it helped/made the gauge work. It didn't help, so I tested the other wire. I ran a clip to the wire that runs to the ECU/gauge. I twisted several variable resistors into place that added up to 60, then 120, then 180, then 240. The gauge would read accurately based on the values of the resistors, so I replaced the sending unit and that fixed it.

The signal from the tank (ohms) goes through the ecu. The ecu only "buffers" the signal to 1) adjust for tank slosh and erratic readings on inclines, etc. It could bypass the ecu and go directly to the gauge, but you would see gas movement in the gauge needle (from what I understand).