I've got an '86 F150 (5.0 EFI) that won't switch fuel tanks. Fuel flows from the front tank when the dashboard switch is selected to "FRONT" but the fuel will continue to flow from the front tank when the dashboard switch is selected to "REAR".

The dashboard fuel gage will read the correct fuel levels in the respective fuel tanks yet there is only fuel flowing from the front tank regardless of which tank has been selected.

The fuel tank switching unit appears to be a diaphragm actuated valve assembly that works on the fuel pressure from the low pressure fuel boost pumps located in the respective fuel tanks. I'm guessing that this is how that works. Can someone verify this for me?

I independently checked the fuel flow from all three individual fuel pumps last year and everything seemed fine*. I am now wondering were the problem could be...

*(due to a different problem)

 

You are correct. The valve works purely off pressure from the pump tanks to switch. If the o-rings are deteriorated or trash is in the assembly, the valve can really cause you a lot of pain.

Every single thing has to be correct in the system for it to work....proper pressure from each LP pump, correct tank vent operation, fuel lines in good condition, and good condition switching valve assembly. If anyone has ever dropped either tank it is easy to kink the lines and since they are either nylon or teflon they get crushed.

You said you already checked the tank pumps and they're easy enough to check...remove the feed lines from the tank valve and jump the fuel pump relay to verify each pumps. If they do the problem is likely the valve. Some have disassembled them and found the o-rings about gone.

I successfully installed an '87 and up switching valve F1UZ9B263B in an '86. It is identical to the '86 and down except it does not have a filter in the lower bowl. Since today's gas pumps filter at a finer rate than EFI filters do (5 micron vs 10 micron EFI filter) I don't think it's an issue as long as the tanks are clean and sending unit strainers are in good condition. Heck, look at a lot of cars now that don't even use a serviceable fuel filter.

You can find the valve mentioned above on ebay for about $160. The one I took off the truck I did was already that later model valve but I never tested it. The truck had a rear tank full of dirt when I got it so I put on a new tank/pump and new valve just to be sure it was good. Also the tank had been dropped and lines kinked so I cut the bad ends off and replaced (replacements are available at most parts stores, Dorman). Once that was done, everything seemed to function correctly until I filled both tanks for the first time.....It would pump from the front tank and return to the rear tank. Found the vent on the front tank was pinched between the sending unit tube and frame so the HP pump was pulling a vacuum on the valve causing it to partially switch. Now everything works perfectly. The truck was a gift for my dad and I told him to switch tanks regularly to keep the valve exercised. With careful shopping you can replace both tanks, both LP pumps and sending units, and the valve for under $500.

BTW I think I may still have the valve I took off the truck if you want it. Not sure though....I finally got on a garage cleaning kick some time ago.

P.S. To give you an idea what a pain these systems could be I believe Ford extended the warranty on the dual tank fuel systems out to 12 years.

 

1985/86: The fuel tank selector valves have been notorious CRAP since day one!

There are two types, one has 3 ports, t'other has 6 ports. Both are available from Ford

 

Thanks guys for the replies! There's lots of good info there.

I do know that I should be switching tanks a lot more often. I would run on the rear tank for a couple months before for switching over to the front tank.

The 'no fuel flow from rear tank issue' only showed up after I had been running only on the front tank for a few months...

 

Since it did work at some point, you could try switching to the rear tank and rapping on the valve with a screwdriver handle or something to see if it'll unstick.

I believe if I were to try this, I would disconnect the outlet side of the valve and unplug the hp pump, then just turn the key off an on or jump the relay. That way you could stick a hose over the outlet to run in a bucket and see if it would push from both tanks. If that works then maybe a heavy dose of Marvel Mystery Oil in both tanks to help lube the valve o-rings.