My fuel tank gauge is messed up. I fill my truck up and it shows full,but once I get in and pull out of the gas station it shows I have 1/2 to 3/4 a tank instead of a full tank! So is there something I need to fix or just replace?

 

probably a bad sending unit.

 

It could also be a bad instrument panel voltage regulator. It's a small silver box on the back of the gauge cluster in the top right corner. It lowers the voltage from 12V to about 5V. If the IVR isn't working properly, it can cause the gauges to read abnormally.

I replaced mine with a used factory unit after I watched as all my gauges jumped up 1/4 gauge when I turned my heater on. According to them, I gained 1/4 tank of gas by driving a block. After replacing the IVR, the gauges have been reading much steadier.

 

I agree with Kirby. Check the instrument panel regulator first and if that tests out ok then its your sender unit. It sounds also like possibly your float may have a small hole in it and it will float a little bit when you first fill the tank but after a little bit of time the float fills up and then it sinks to the bottom of the tank giving you the poor reading.

 

If the fuel, oil pressure & water temp gauges all fluctuate & vary together, a likely cause
is the IVR and/or corroded electrical connections on the printed circuit.

If it is just the fuel level gauge having problems, a likely suspect is the sending unit.

To test:

Unplug the wiring harness from the sending unit at the tank; with the engine not running
but with the ignition switch in ACC or RUN, ground that wire and the gauge should swing
fully to one direction (prolly FULL if 80-86). Unground that wire and don't let it touch
anything, the gauge should swing fully in the other direction.

If that test passes, your gauge & wiring are fine, the problem is in the sending unit in the
tank.

If/when you remove it, remove the triangular cover at the pivot and look inside, see if you
find anything that looks not to be in order.