need help, fuel gauge jumping all around

2004 lesabra, I just put in a new fuel level sending unit directly from Mr. Good wrench and it still jumps around. the Gauge will go all the way around the face. Is just the gauge serviceable? This vehical has the small fuel gauge in the upper right hand corner of the cluster. Thanks for any help.

I really don't know if this would help but in my local newspaper there was an "Ask a Mechanic" column about annoying trouble for gas gauges and this was in relation to GM fuel systems.

The gist of it was that there is sulpher contaminants in the fuel and they will disrupt the electrical signal from the sender unit..... It goes on to mention that you can purchase something called

GM Fuel System Treatment Plus (part #88861012) - apparently this chemical will clean deposits off the fuel sender.

I also have a problem with my sending unit and I was going to give this a try before I spend any money on a new sending unit. I don't know the actual cost of this additive but I would imagine somewhere in the neigbourhood of 15 -20 bucks.

If GM has a part number for a fuel additive I'm guessing they must know of a common problem with their cars.

I had this problem on a 97 Lesabre, but a salvaged sending unit fixed the problem. I tested the sending unit using a digital volt meter. The ohm value can be measured. For the 97, the range was 0 to 240 ohms. A half tank was right at 120 ohms. Hooked on to the wire right where the sending unit signal came out of the tank. Another way to do it is to substitute variable resistors from Radio Shack with known values and see where it reads on the gauge. This can isolate the problem and answer if the problem is in the dash gauge or the sending unit. It is possible the new unit is defective. How did the gauge behave before replacing? Did it bounce around exactly the same? On the 97, the resistance reading goes through the computer before reaching the dash. The PCM buffers the reading to prevent abrupt movement of the gauge for tank slosh, etc. I am not sure if the gauge in the dash is separately servicable. On the 97, the gauges are all in one gauge cluster unit.