I have an 81 F150, built in early 80. It runs great! Dual tanks but the rear tank hasn’t been used for years. I just got it last week and filled the front tank. It registered at 3/4 tank. I saw the switch on the dash by the heater controls and switched it to the back tank to see what would happen, the gage went to empty. When turned back to front tank the gage stayed empty.
I bought a fuel tank selector valve that was supposed to fit but it is not what was on it. The valve that was there was just for the rear tank, and the fuel lines weren’t hooked up and had only one wire. Not sure what to do. Any suggestions?

 

No easy answer, fuel gauges work on electrical resistance relative to tank fill levels. For the irregular readings described, suggest a dirty switch or bad electrical connection or wiring integrity issue.

Perhaps shoot some electrical contact cleaner in the switch and exercise it several times as a relatively easy first step.

Inspect the wiring to the fuel gauges, looking for any damages such as being pinched or rodent gnawings.

Then look to inspect the terminations to the gauge on the tank, which gets to be a chore, why left to the last step. Could also be a bad spot in the sending unit - another degree of difficulty.

Essentially, a wiring diagram would be most helpful, then go down the circuit, checking the things easiest to check first and going down the list - if you don't know what the problem is, start eliminating the things it's not.

Good luck!

 

Thanks for the advice, looks like I have some work to do. This doesn’t have a EFI so it has a 3 port selector valve that’s been bypassed. The last owner only used the front tank. I’m planning to pull the panels on the dash to replace the bulbs, radio and speaker so I’ll check out the switch then.