Gas gauge problem?

[Koby]

In my 1998 Yukon SLT the gas gauge is broken. This is a different problem than other threads I have seen on this site, so I'm making this one. Basically, when I start the truck, the gas gauge flies to full and vibrates wildly there for a few seconds, and then plummets to empty and stays there. This problems occurs even while the vehicle is not in motion. Any help is appreciated!

 

[johnd8695]

Probably just the sending unit in the tank

 

[SunlitComet]

you should use ohm testing on the sender and gauge to eliminate the possibility that the pcm is the issue. in 98 it ran thru the pcm on the way to the dash.

 

[YukonandtheHOE]

My 96 would sweep in both directions then peg the needle to full between 3/4 and 1/2 tank, swaped the pump assembly and gauge works great now.

 

[Steve Y]

Had similar issue with my 96 Tahoe. Replaced the stepper motor in the cluster and the problem was resolved.

 

[Kenny D]

OP,
curious if you have ever used Shell gasoline? Back in the day, Shell got in some hot water because it was found that their gas messed up people's sending units. My 99 suburban didn't have that issue until I started running Shell gas ( station was close to my job). By the time I discovered/stumbled upon the info, the class action suit had been settled.
It is most likely your sending unit.

 

[Koby]

I dropped the tank and checked the sending unit. The little arm and the two prongs that run against the resistor are not broken, however, it is pretty loose and the arm can move at different angles such that the prongs don't make contact either the resistor plate. I'm not sure if it is just old and needs replacing or what. But I am quite sure I have used Shell in the past, but there aren't any near me anymore, probably for the reason you stated. I'm leaning towards the sending unit being at fault here. Thanks for the replies this far!