I'm gonna be buying my moms 1990 f150 4.9L with dual tanks. The front tank leaks so it is not used, so the rear is the primary tank now. Recently the gas gauge pretty much went haywire it read way past full with only 5 gallons. And it'll move while driving. So what can it be causing this issue? Thanks Brandon.

 

Sounds like the intake sending unit. A lot of these trucks i delt with have this problem. It could also be a ground issue.

 

I don't think I've seen an F-150 from our generation where the sender wasn't acting up in one way or another. lol