Inaccurate Gas gauge

[Country705]

I have a 1997 Silverado Z71 that has a very inaccurate gauge. I am trying to diagnosis what is causing it. It doesn't matter how much gas I have in it the gas gauge will jump to way past full and then come back down to 3/4 of a tank. This makes it almost impossible to know when I am out of gas and as a result I have ran out of gas several times because I would think I had more than I really had.
I was thinking maybe the sending unit is bad? Could it be I need to replace the instrument cluster? any help would be appreciated.

 

[aaronb]

I'm going to say its the sending unit. my gauge in my 98 did it for a couple years. I went to the junkyard and pulled another cluster and swapped it in, same problem.
then my fuel pump quit. the arm on the sending unit didn't take any effort to move and was pretty nasty looking. swapped in a new unit and the gauge was fixed.

 

[Country705]

I'm going to say its the sending unit. my gauge in my 98 did it for a couple years. I went to the junkyard and pulled another cluster and swapped it in, same problem.
then my fuel pump quit. the arm on the sending unit didn't take any effort to move and was pretty nasty looking. swapped in a new unit and the gauge was fixed.

Here is a picture from last night when I started the truck up. as you can see the fuel gauge shows a little under full.

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In this shot from earlier today the gas gauge is way past full

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[twomanymontes]

When the circuit is open the gauge goes way past full. I'd check the ground wire to the frame and make sure it's still good. If it's loose or even if it has a few strands broken, the resistance value may change enough to create this problem. If the ground is good, then next step would be to inspect the sender.

 

[98_k1500]

+1 sending unit in the tank. I actually have a good one riding around in a box in my toolbox. I robbed it off a truck i was doing a fuel pump job in that the gauge still worked, the pump just quit. Saving it in case i need it, cuz i have not found anywhere you can buy just the sender.

 

[jmc1997ck]

What if the gas gauge needle has a wicked shimmy to it while the vehicle is running? That's what I'm seeing. It reads accurate, but looking at it while it's having a seizure at a rave is bugging me...

 

[df2x4]

What if the gas gauge needle has a wicked shimmy to it while the vehicle is running? That's what I'm seeing. It reads accurate, but looking at it while it's having a seizure at a rave is bugging me...

You might try this out...

http://www.gmt400.com/forum/showthread.php?15139-HOW-TO-Fix-Bouncy-Hummingbird-Fuel-Gauge-Needle