On my 86 F150, I drove last week with the fuel tank selector switch on the rear tank (about 120 miles). I did not use the front tank switch at all since I filled the front tank last week. When I went to fill up today, I was able to put 9 gallons in the front, and none in the rear. The gauge always reads Empty when the switch is on the rear tank, and it seems to be reasonably accurate when the switch is on the front tank. Ever since I bought the truck 2 months ago, I have never been able to add gas to the rear tank (I've checked for crimps, everything looks ok). Any ideas what's going on with this truck??

Thanks,
TM

 

There's a valve mounted midway on the frame rail that may have gone bad. It switches the fuel front to rear and has a bunch of lines going to it.

 

Thanks, Dave. So if this valve is bad, it will never physically switch over to pull fuel from the rear tank. Would the fuel gauge's reading be affected by this (since the fuel gauge changes when I switch between tanks), or is it completely independent of the valve?

TM

 

I don't know. There where several different versions of this system. I don't know if it was in this forum, or the electrical forum, but several weeks ago there was a little discussion about it, and there was a poster that knew a little more about it than me. They where even talking about a filter being integral in one type of valve.

 

Read the Haynes manual - it explains the whole system, and the wiring diagrams in the back show how the gauge works.

 

I've had some experience with the fuel tank selector valve on my 84.

I rode around for a week on the front tank, finally realizing it still read full. I switched to the rear, and it was almost dead empty. Barely made it to the gas station. It turns out, the "primary" tank is the rear. If the selector valve gets stuck, it will always pump from the rear. Of cours, if someone has reversed the fuel lines on your valve, it would pump from the front.

I'm not sure this is your problem, because my fuel guage would always read correctly for whichever tank I switched to, even with a busted selector valve.

The easiest way I found to test the valve and the switch is to look for the terminal on the valve, and use a test light.

Switch to the front tank, you should have 12V. Switch to the rear you should get nothing.

If it reads the same in both states, it's probably a bad switch or a break in the wire. If it switches appropriately, it is probably a bad valve, but I couldn't explain the fuel guage then.

The Haynes manual has some good info on this. That's how I figured out what was wrong with mine.

You probably already know this, but I learned the hard way. Be sure to clamp off the fuel lines before removing the valve. I dumped 30 gallons of gas down my arm, my back and all over my driveway the first time I changed it.

The second time, not a drop.