I'd been for a long drive earlier in the day. I started the truck and it seemed to not want to idle...strange, it always idles nearly perfect. I go for a short drive around the block to see how it acts...I let off the throttle to make a right turn and the engine shut off. It has NEVER done this. I throw it in N and restart, and it's okay. Bring it back, check it over, bump the idle up a couple hundred rpm, all seems well.

Go for another testdrive, I notice my gas gauge is acting a fool all the sudden. It's "nervous" for lack of a better word. I just filled it all the way up last night, and it was just below the F mark. At idle or when driving (even with the key on but engine off) it "quivers" rapidly over about 1/16" range...looking at it, it's moving so fast it's blurry. Also, mostly while sitting still, it randomly jumps back and forth WHILE still quivering, perhaps 1/8 tank one way or another. For instance, sitting at just under full, it'd "jump" to full and then back, and sometimes down near 3/4. All teh while still quivering at supersonic speeds. IF you shut it off, wait a few seconds, and turn the key to ON but not start, the gauge pegs way past full for just a second then returns to it's normal behavior...it hasn't been pegging wide open for that split second when the key is turned until now.

I got out the wiring diagram and found that the fuel pump sending unit wire is purple, and I found it in the harness along the framerail. To see if the problem lies in the gauge or the sending unit, I cut the wire temporarily. The gauge then immediately pegged full, but still quivered. After about 30 seconds, the gauge pegs below E, and stops quivering. Of course I hooked the wire back up and insulated it as normal, and the gauge returns to the same screwed up behavior. I've went for a few short drives and it does not change.

I've had this denali for about 4 months now...I bought it off ebay from out of state. It has 125k miles but I did a ton of maintenance when I first got the truck. Fuel filter, air filter, oil change, coolant flush and change, hoses, a/c recharge, brakes, suspension, oilpan gasket and timing chain cover, etc. The truck has run flawless since I bought it, other than a VERY slight surge at idle when in gear at times (I dismissed it as trivial).

Any ideas?

My gas guage is "nervous" too. I bought my truck a week ago, didn't notice it on the test drive. I've put on 300 miles without a problem, but I'd like to know what is causing this too.

I figure it must be in the cluster itself, or the sending unit with pump. I'm trying to get a spare pump/sending unit since the pumps are bad to go out on these trucks anyway, and I may try to get a spare instrument cluster too. I bet it's a PIA to swap out.

As a followup, I bought a spare fuelpump and sending unit after reading all the stories of bad ones on here. Also got a instrument cluster, although not identical cosmetically, electrically the same, from a 98 yukon (non denali). The instrument cluster plugged right in and fixed the problem. I wasnt happy with the lighting changes, so I put my original back in...now that I know it is just a quirk in the gauge, and not the wiring or sending unit, I am a little less worried about it. IT still seems to be accurate, just flickers around about 1/16" the entire time.