Not a professional mechanic, but I have had the front-tank-filling problem before. The research shows that there is a backflow valve incorporated into the in-tank pumps that prevents this phenomenon, as the switching valve does not prevent back flow into the tank that is not selected. The gas goes from the back to front only because the front tank is "downstream" in the gas flow. I thought about installing a small one-way valve in the outlet line of the front tank, but it would have meant taking off the bed anyway, so I just replaced the front pump instead. Works fine for three years now.

 

On what year truck is this? I think there was a year that they put the backflow valves on. And the previous years used the selectors. If I understand things right anyway

 

I had a similar problem with my 88 F-150. I talked with an old experienced Ford mechanic and he said there had been a recall. I believe he said it was a check valve had to be replaced. In my case the rear take was cross feeding to the front. I never pursued it because I can run on the front tank faster than it cross feeds.

 

I think the recall applies to '90 and newer trucks that use a different fuel system with the high pressure pumps mounted in the tanks and a check valve attached to the sender. Years ago I installed many of those recall kits. The kit contained a check valve for each tank and a new fuel pressure regulator, but is not designed to work with the older system using the low pressure fuel tank pump/high pressure frame mounted pump used on '89 and older trucks.

Edit:
Didn't realize this was an old thread. Hope he has it fixed by now.

 

they installed the return hoses incorrectly. it was recalled...check your front fuel tank for the plastic shield may need one of those.

 

mine does this too????

 

You have a stuck "O" ring on a shaft inside the selector valve. You need to replace the selector valve.
The gauge is problem is unrelated.

The stuck "O" ring.

/

 

i have an 89 f150 xlt dual tanks. my fuel selector powers the in tank pump and the accumlator is passive on newer years it was switched. mine is starting to bleed and needs to be changed soon. from what ive read on theads on this site its very common and ford has made a living selling them. i looked up the recalls for my 89 and when it came out some bozo changed the return overflows for the two tanks so if your running the front the return goes to the rear and it over flows the rear tank. the other was a sheild for the front fuel tank during a 30 mph head on it would turn into a missle spewwing fuel on impact they put a plastic shield on the front of it. the third was a snap ring on a trans shaft that would snap and cause it to leave park when running. i didnt know it was an old thread either but its good stuff. remove the baffles from your air intake get 5 hp found a kn in a salvage yard five dollars..breaths better..........

 

Kevin, if it looks like this pic, I have a brand new one I'll sell you for less than it cost me from Ford. It'll be here Tuesday and I can send it to you Wednesday.

 

its looks the same..what do you want for it

 

I paid $140 for it. I'll ship it to you for $120. Brand new still in the wrapper.