The other day I filled up my truck and now my fuel gauge is stuck on full. Ran it for a couple days and still the same. It was working fine before I filled it up with diesel the other day

Anyone one had this issue before that could lead me to the right direction?

Gonna remove the tank to check it out. Anything I should do in terms of upgrades while I’m there? In the past I’ve already replaced that filter tube that is known to fall off

Also how accurate is the overhead unit with “Miles to Empty”? How much are y’all getting when tank is full?

 

Also would like to note I got these codes

 

A short in the cut wire will cause it to show as full.

 

I believe an open in the wire shows full and a grounded wire shows empty. I don't know what truck he's working on but my truck has a delay in the dash as far a showing fuel level. You can pull the wire for the fuel sending unit and use a test light on it to ground and see what the dash does but it takes a while to react. If the tank has never been dropped it's a good time to do the mods.

 

Thanks Eman. That’s what I meant but it came across as backwards. It was late and I was sleepy, that’s my excuse anyhow.

Dad’s truck had a broken wire near the tank and it showed as full all the time. I was able to splice it back together through the fender well.

 

Fuel senders are a rheostat and are set up either to have high resistance when full and low when empty, or the reverse. My '99's sender has empty at 22.4 Ω and full at 145 Ω, so a dead short moves the needle to completely empty and an open circuit moves the needle to completely full. I can't say exactly for sure if it's true for you as there is no listing in my shop manual for any GEM DTCs for fuel sender failure, but yours obviously is monitoring that circuit. Also the pinpoint tests for my '99 say to test the gauge by hooking resistors to the gauge circuit and seeing what the gauge does, and using an ohmmeter on the sender to check its resistance. One would have thought if there was some way for the GEM or BCM to monitor the circuit for failures they'd have mentioned looking at the DTCs for that first, like they do for anything that any control module monitors and gives a DTC if there is a problem.

One thing that I would try to see is unplugging the fuel sender plug on the top of the tank if you can get to it, and see what the gauge does on a known to be open circuit. If the gauge is pegged full, high resistance is full, if it's pegged empty, then high resistance is empty.

Another thing to watch is the GEM has an "anti-slosh" feature where the needle only moves very slowly to the actual fuel level if the needle was in a different position at key-on. Oddly enough a worn ignition switch contact can cause the fuel gauge to peg full at key-on and then only slowly settle down to actual fuel level, which can take 20-60 minutes.

My truck does not have a lie-o-meter so I can't say how well it would work. If I had one and it said 393 miles to empty, I'd either laugh or see if my tank suddenly grew as I am usually around 9-10 miles per gallon.