My 1994 5.0 F150 XLT extended cab 4x4 is pumping gas from the rear tank to the front tank. I think the check valve in the front pump has failed. I do not have the switching valve on the frame.
I would like to confirm this is the problem before I pull the bed.

Thank for any help.

 

Yes, the failure is in the pump that RECEIVES the fuel.



Read the other captions in that album for more info.

 

Yeppers, you've got it nailed. This dogged the 2-tank-2-pump F150s even when they were relatively new. These use a pump in each tank feeding a common fuel supply line. Ford's dealer fix was pretty poor--they just had the dealers insert another check valve external to each tank (has the quick-connect fittings, so it was just release a hose and insert the valve between the hose and the tank). Unfortunately, these fail also. The one with the falied pump/valve is the one that receives the excess gas.

Your best option if you don't have a forklift handy is to drop the tank (carefully) and replace the fuel pump. It's also a good opportunity to check the tank for leaks and to clean any sludge out of it. Since I'm alone with only two hands, I used a couple of ratcheting tie-downs to hold the tank while I unbolted the straps and disconnected the electrical and fuel connections. The tank should easily slide out (put it on a creeper if you want) once that's done. Some shops remove the bed and do their work from the top, but I've got a Gem-top steel canopy on mine in addition to a bed liner. Be sure to have a quick-connect tool handy (or make one yourself).

A word of warning--don't shop for the cheapest pump--Airtex and other Chinese vendors sell some pretty cheap. I was warned that I'd be redoing the job in a few months if I succumbed to the urge to scrimp. I used a Delphi pump and it's held up very well for a year or so. Bosch is apparently pretty good also, but I've never used them. The OEM parts seem to be made by Walbro.

You should be able to get the work done in a day. Siphoning as much gas out of the tank as possible will make handling the thing much easier.

----
I've wondered if simply installing two solenoid valves, one on each tank and connecting them to each pump's electrical feed might, in the long run, be the best solution...

 

It's not a solution unless you gut/delete the FDM. If you leave the ruptured diaphragm in the system, it'll just dump all the fuel pressure back to the tank, and eventually the truck will set adaptive lean limit codes before it loses power and finally stops running above idle.

 

That wasn't the case for me. If I switched to the tank with the failed check valve, I could drive all day--the pump delivered. If I switched to the other tank, the one with the failed valve filled up--and I had the dealer "fix" of the extra check valve. I still have the internal pump from the bad unit (it's a Walbro) and I should probably pull it apart and see what gives.

My suggestion was to prevent any flow in or out of the fuel tank by installing a solenoid valve after each tank on the fuel line hooked in parallel with the fuel pump for that tank. So if a pump wasn't being powered, it would essentially be closed off from the fuel line.

 

Your FDM's rupture hadn't progressed to the point that the pump couldn't keep up yet. But it WILL, eventually.That's what the recall check valve does, and it doesn't require power or wiring. And since it's machined from stainless steel, it won't break, melt, crack, rust, leak, or fail from the high pressure gasoline going through it. Where are you gonna find solenoid valves that reliable for less than the truck is worth? And how are you gonna install them? You can't cut the fuel lines because you can't swage them back together like the factory did. So you'll have to find solenoid valves with fittings that the fuel lines will connect to.

It's getting more expensive, complex, unreliable, and difficult-to-source.

 

Well, if the recall check valves were any good, they would be a solution, but mine (and others) weren't--and failed. I'll agree that my proposed solution is probably over-engineed.

Probably the best solution would have been to return to the 3-pump solution--a low-pressure pump in each tank and a selector followed by a third high-pressure pump.

All in all, I think that the two-tank setups were a nightmare for Ford.

 

It sounds like you've never owned a truck with 2 tanks & 3 fuel pumps. I have. I had to maintain it, too. It's a horrible system. The FDM system is a vast improvement. Its only real issue is that weak orange diaphragm. But newer FDMs will probably last longer than OEs, so just replace them when they fail. Don't downgrade to an older, more-problematic system. Or to a more-expensive homebrew system with more failure points & modes.

 

Oh, don't worry Steve--after replacing the front tank pump, I have no intention of doing either.

I wonder why the failing pump inevitably seems to be the one in the front tank.

My (and others) original advice stands, I think--don't buy a cheap pump.

 

I don't think anyone will argue with that!

 

I am currently dealing with this problem. My '96 4.9L F150 is also giving P0171 and P0174 lean codes although I'm not sure this is related. How can I be sure which fuel pump system I have? I have talked to 3 different mechanics at 3 different Ford dealerships and they all tell me something different. I have followed the fuel lines everywhere they go and I can't find a "reservoir fuel tank valve assembly" so I think I have the duel fuel pumps with check valves on each but I'm not positive.

My fuel is going from the Front tank to the Rear tank though. I changed fuel filter and the front fuel pump 7 months ago.

 

You have the later, one high pressure pump in each tank system.
Ford changed over to that in 90 or 91.
Sounds like you need a rear pump.
That could give you a lean code when running on the front tank because your pressure to the fuel rail will be low - pressure will be bleeding off to the rear tank.

 

Well I'll probably try and change it tomorrow then. I'll reply back, thanks for the advice.

 

I changed the rear fuel pump and have driven about 150 miles since and the check engine light has not come back on! Not to mention it drives so much better! I am thinking all my immediate problems have been solved. It wasn't the most fun wedging my body up in between the pump and the bed and dealing with 20 years worth of dirt. The inside of the tank was clean but there was light rust on the bottom side where the straps are. I sprayed the area with a light coat of gloss rustoleum and called it a day. I wanted to reply to all this with my results because I ran into several threads addressing the "fuel migration" and the lean codes but not very often were they part of the same conversation. The threads addressing both were never resolved. Hope this helps somebody else out there. Keep those I6s running strong y'all!