We have a '89 F250 that's been sitting for a couple of years. Got it going and it runs great, has a 460 with dual tanks. Recently we've noticed the fuel levels in the tanks acting oddly. If the rear tank is full it will feed the front tank (?) to the point the front tank overflows. I assumed it was the dual function reservoir so we replaced it with a wrecking yard unit, didn't change anything. Weak front fuel pump? Does anyone have a link to a fuel line circuit for the dual tank system? Can't find much on it.

 

Sounds like you got a bad one from the salvage yard. When the "O" ring sticks on the valve shaft this will happen.

"O" ring stuck on valve shaft, spring should push it to the stop.


Here is the electrical for it:


Here is the fuel system diagram:


Here is selector valve:


/

 

THIS is why I'm a member of this forum, you guys are great. Thanks! Just what we needed. FORD FOR EVER!

 

My 89 did this very thing. I changed a lot of stuff and never could get it to quit doing it. Finally decided to check the pump pressures. The rear pump was just barely strong enough to feed the high pressure pump on the frame, but not strong enough to move the check valves in the switch valve. Changed it and now everything works perfect.

 

my 96 f150 is doing the same thing. The front tank when filled is moving feul to the rear. I can't even understand most of what you all are saying. I'm going to have to do more research.

 

You have a bad rear FDM (fuel pump module) in your 1996. You fuel system is different then the 1989 we are talking about. You do not have a selector valve or a high pressure pump on the frame. All of that is inside your fuel tanks.



The FDM:




/

 

SubFord is 100% correct on this (and thanks SubFord for all of the diagrams and pics/drawings!!). My '96 does the same thing, but only with the opposite tank as yours. My plan is to take the bed off this spring for easy access to the fuel tanks/lines, and then I'm going to replace the whole kit and kaboodle as they say: new tanks, FDM's, and all gaskets and seals. The front tank has a small pinhole leak anyway, and while my plan will cost a couple thousand maybe, it will be taken care of the fuel system once and for all. It would really grind me to go in there and fix only the bad FDM, and have the other one go out in a few months. The components are almost 20 years old, so i figured they've lived out their functional lives.

 

ok if both tanks are empty and I fill the front. I end up with fuel in the rear so that means a bad rear fdm? seems like it would be the other way to me but just wanting to make sure?

 

calixt0 for the 1996 truck question.
You should start your own Thread as this may confuse the OP with the 1989 truck that has a different fuel system then yours.
Yes it does.

Why would it be the other way around?
The rear FDM has bad valve(s) in it allowing fuel to enter when its pump is not running.

The Fuel Delivery Assembly consists of the fuel pump inside a reservoir that is attached to the pump and sender assembly. In a two tank system there is one assembly per tank. The sender assembly handles the switching of the fuel through internal valves.

Shuttle Selector Valve:

The Shuttle Selector Valve opens the return line to the tank when the fuel pressure on the shuttle diaphragm exceeds the opposing spring force. The return flow is directed to the appropriate tank on vehicles equipped with dual tanks.

Supply Check Valve:

The Supply Check Valve, which is normally closed, opens when the outlet pressure from the energized pump exceeds the opposing check valve spring force. When the pump is de-energized (i.e., engine is shut off), the supply check valve closes to maintain pump prime and fuel supply line pressure.

 

haha when you put it like that it makes perfect sense. I mean that literally not being sarcastic.

 

Subscribing to this as it has great info. Thanks subford.

 

I have a 1989 f350 that we just got done restoring. We replaced all taNKS, pumps, and fuel reservoir assembly with a new unit. However, now the rear tank just fills the front tank to overflowing when rear tank selected...WTF?...are you suggesting the rear tank pump is putting out too low of pressure. we changed out the new fuel and reservoir assembly to a known used one I had and still does the same thing...should I ****can both of the pumps and try one more time to replace them?

 

It would be only the rear pump. The pressure from the rear pump must be great enough to lock the selector in the fuel reservoir assembly to send return fuel to the rear tank.

 

so you are saying it's the return fuel that is filling up the front tank because it does not have enough pressure even with the high pressure pump replaced on the frame rail.

 

Didn't Ford issue a bandaid repair for this by installing check valves external to the tanks?