I have experienced fuel transfer from rear tank to front tank; but, not from front tank to rear tank. I think that when you switch from one tank to the other on this model truck, the tank that you are switching from shuts off the pump in that tank and closes the check valve which then will not let returning excess fuel to flow back into that tank. The tank that you are switching to then starts pumping and opens it's own check valve to allow returning fuel to enter that tank. Therefore, when a check valve goes bad (remains open after shut off) this check valve would allow returning fuel to enter this non-pumping tank. This type operation would explain the reason for my problem of returning fuel to return to the non-pumping tank -- even if only part of the returning fuel is going into the non-pumping tank. THIS IS ONLY MY IDEA OF WHAT THIS DUAL FUEL TANK OPERATION MIGHT BE. Could someone confirm this operation or describe the correct operation of this dual fuel tank operation. Many thanks in advance. Iraa

 

Yeah you got it.

 

Thanks for the response. Now, I am putting Marvel Mystery Oil in the front tank hopefully to free-up the check valve. Maybe It will free up the check valve so that it may close when switching to the rear tank. Again, thanks

 

I apologize for hijacking your thread, but I'm having a similar problem on my 88 250 If possible, I'd like to just eliminate using rear tank. Any suggestions? Thanks in advance.

 

I don't know anything about discontinuing use of one of the tanks. There was a fuel tank switching valve with six hose connections attached to the frame on the driver's side on some models earlier than my 1994. I ordered the valve before looking under my truck -- mistake, cost me $58 restock fee when returning it. If the pump in both tanks are pumping and sending fuel to the injectors under pressure and pressure is returning excess fuel to the tanks, I believe I would try and keep both fuel pumps and both tanks operating and look under the drivers side frame for a switching valve (six hoses connected -- some supply and some return hoses) and try to replace that valve. I believe that valve was the culprit prior to my 1994. Also, think about putting 10 gallons in the front tank and 5 or 6 gallons in the rear tank, and add some sea foam injector cleaner and marvel mystrery oil and see if that will get check valves in the pumps to free up and start to working again. I would't want to plug off one tank for it to dry up and pump go completely bad. Good luck.

 

If trying the marvel mystery oil doesn't work, then the pump will have to be replaced. The tank that is being overfilled is the one with a bad check valve.

 

Thanks for the insight. Unfortunately the back tank is starting to leak. Yeah, imagine that. It rusted enough on the seam (folded upward) that it's starting to seep.

 

Sounds like you will be dropping both fuel tanks. Try to get the tanks as empty as possible before dropping them. It's still not a fun thing to do though.

 

Chiming in. Amen to "the tank that is being overfilled is the one with the bad check valve." Such a hard job to drop the tanks; I believe from what I have seen here on the internet that it's worth thinking about to consider removing the bed of the truck to replace the fuel pumps or maybe change tanks if one is rusted through. I have just driven my truck about 50-60 miles since adding sea foam to the gas. With over 174,000 miles, it is really more responsive and smoother. Also, I don't notice any significant amount of fuel transfer to the front tank since adding the Marvel Mystery Oil to the gas in the front tank. Keeping my fingers crossed. Iraa

 

Yeah, on the earlier model trucks the dash switch controlled what tank's pump was running, that pump's outgoing fuel pressure caused the "dual fuel reservoir" to switch so that the return fuel from the engine went to the tank that was pumping. If the reservoir's return valve was stuck, the return fuel went to the wrong tank. Changing the reservoir or re-building it was the cure.
On your model truck, you described the problem correctly and, yes, replacing the FDM (fuel-delivery-module) was the fix, thus dropping the tank or taking off the bed.
Changing the reservoir(mounted on the frame rail) was easier and cheaper. Got mine from Amazon for $85 ?.

 

Any chance you can post a pic of the reservoir on the frame rail? I'd like to know what I'm looking for. Thank you.

 

Everything you wanted to know about the EFI fuel system used on the 1984-89 F-series/Broncos: 1984-1989 EFI Fuel System (Gas Engine) F-series & Bronco (courtesy of Steve83)