Wonder if anyone has has a similar problem or knows where to start looking, I have a 91 F250 with two fuel tanks that seems to pump fuel from front to rear and vice versa. The selected tank pumps into the opposite tank. I can't seem to find a decent schematic either so I have know idea how the pumps and plumbing are set up. I'm getting a little fed up dumping fuel on the ground at Ca prices!

 

According to the shop manual, for 1990 and newer trucks, each fuel tank has an Fuel Delivery Module (in-tank fuel pump) that has a shuttle selector valve. The switch in the cab controls two things, 1) which FDM will have power and pump gas to the engine and 2) select which fuel sender will send signal to the fuel gauge. There isn't any mechanical fuel valve that routes the return fuel to either tank so both tanks are always connected to the engine and therefore and unfortunatly to each other. That shuttle selector valve in each FDM prevents the FDM with power from sending return fuel to the other non-powered FDM and fuel tank, that way the non-powered tank should not overflow... Unless the shuttle selector valve becomes defective. So I guess you have defective shuttle selector valves in each FDM. And guess what, the Shuttle valves cannot be replaced, you have to get 2 new FDM's. &^#@!!(*# !!!

 

I'm trying to reply so I don't sound completely dumb! I'm assuming the pump with pressure forces the selecter valve over to make sure the the tank and pump with press is the only one suppling gas, is there any other way to check out the FDM,.i.e can you hear them or do you have run a press chk or some other test.

 

There's no way to check the FDM, they are sealed self-contained pumps. These truck don't have a returnless fuel system so you have gasoline flowing to the fuel pressure regulator(s) and whatever the FPR doesn't use to feed the injectors has to go somewhere and that's back to the fuel tanks. So you have two fuel lines per tank (3 if you count the vent line going to the evap canister and the intake manifold), one line going from the tank through the actual fuel pump and check valve assembly and then onto the FPR and another line returning unused fuel from the FPR back to the tank via FDM's shuttle valve. Both feed lines and both return lines come into a couple of Y fitting right before the fuel filter on the driver's side frame rail under the cab. Even if both pumps were running at the same time, the fuel pump check valve would keep the pump with enough pressure to overcome the other pump from receiving a reverse flow of fuel and the shuttle selector valve would keep the extra fuel not used by the FPR from going to that tank. Whenever there's no power to a particular FDM, the shuttle selector valve for the returned fuel automatically closes. The check valves on the fuel pumps themselves are one-way valves that also close automatically when there is no positive fuel flow. One of two things is happening here, either the fuel pump check valve in each FDM is bad or the shuttle selector valves are bad. Either way, there's nothing you can do to fix that but to change out the FDM's. Did that make any sense?

 

yes, it makes a lot of sense, thank the lord some one knows what they're talking about out there, it seems basically I'm out of pocket. Time for prickerly armpits I think! You know, when the gas runs down your arm....

 

It's kinda weird that BOTH FDM's decided to go bad. It's almost like you had the old 6-way selector valve system on your truck. In the older style fuel systems for carburtated engines (1986 and below, 1987 for 460), there is a 6-way valve on the driver's side frame rail that has 4 inlets on the back side and 2 inlets on the front. The 4 in the back are tank 1 fuel supply, tank 1 fuel return, tank 2 fuel supply, and tank 2 fuel return. The two on the front of the selector valve are fuel supply going to engine and fuel return going from engine back to the tank. Now there's a third system that has an in-tank low pressure fuel pump and an frame rail mounted high pressure fuel pump. (used on 1986-1989) The switching component on these trucks is called a dual-function reservoir. The output from both high pressure fuel pumps and the return lines for both tanks are routed in the back of this DFR (it looks like a round pot with a square top and the 6 lines come in to the square top) and the other end has the fuel line going to the engine and the fuel return line from the engine. The actual switching here is like the carburated trucks, it's mechanical. In either case, your problem would make a lot more sense and would be a lot more likely to happen with the two older systems. (Before you do anything, make sure that for some reason, your truck doesn't have one of those older systems...)

 

ford has replacement check valves that go outside the tank to fix that problem <ford had a recall for that problem> the check valves cost about $28.00 each you need 2 per tank only replace on bad tank. i found out this one week ago have a90 f150 same problem

 

What if...I install a 4-7 lbs fuel regulator to feed a carburator ?
Do I need to use the return line for excess fuel ?
Thanks.

 

I had this problem on my 90 f150 and the dealer installed a check valve at no charge.

 

Have a 1989 F150 5.0L EFI with dual tanks. It has a pump in each tank, and a fuel line junction with 4 lines connected to supply/return on each tank on the rear, and a supply and return line to the engine on the front. There is also a main fuel pump in front of this junction.

The rear tank is filling the front tank to the point of it running into the street, and at $2.10 a gallon that is no small matter. Should this vehicle have the newer system with the shuttle valves or what ? Could I add simple check valves to the lines on the front tank to prevent this, or what is the preferred fix on the 1989 ?

You can't imagine how grateful I was to find this string on the net.

Good to meet you,

Dann White

 

If you have the 3 pump system, the switch valve is pressure operated by the in-tank pumps. Perhaps one of the in-tank pumps is not working properly, or the valve itself could be stuck. If there is a physical valve I would check it first, but if you happen to have the lines just y'd together, it's likely a problem with one of the in-tank pump check valves.

 

I follow that, is there an illustration of these fuel systems that can be downloaded anywhere ? It wasn't clear to me whether there was a two-pump ( post 1990 )and a three pump system ( pre 1990 ) and if so it would seem that if this junction is just a Y and has no electrical or mechanical valve, I could remedy this with a one-way (check valve) valve.