2000 Tahoe, 5.3. When I fill this up it never seems to get full, only about 7/8. It will take about 12 gallons from 1/4 to this point so with a 27 Gallon tank I don't think it's the gage or sender. I purchased it earlier this year and it's always been that way since then. Any ideas?

 

Actually the gauge and/or sender are the prime suspects - both are known for failures.

The in-tank sending unit is wired to the PCM. The PCM then sends fuel level data to the instrument cluster via the data circuit.

The factory level sender was known to develop corrosion and affect the reading. The clusters are known for stepper motors to fail and solder joint to go cold and affect the movement of the gauge (though typically the gauge movement becomes “choppy” and/or pegs beyond the range of the gauge).

The easiest way to determine if it’s the sender or the cluster is to read live data from the PCM while the tank is known to be full. Anything less than a full reading in the PCM data (above 95%) indicates the sending unit is to blame. If it’s above 95% the cluster is suspect.

That said, if the sender and cluster check out, one other possibility is the EVAP system charcoal canister and vent valve being restricted and not allowing proper venting during fill up. Typically this presents with a pump that constantly clicks off while attempting to fill, but depending on the location of the restriction, it’s possible it doesn’t affect pump operation until it’s nearly full.