A couple of weeks ago my wife and I went to dinner. when we came out of resturant the truck would crank but it took forever to start. We got home it started up alright the next morning. We went into town and the truck started fine. stopped and had lunch the truck took forever to start again. The next day I went to an appointment. This time the truck wouldn't start I switched to the rear tank and it started. I wen to the gas station fueled up the rear tank it starts great. Now the rear tank says it's empty. So I go and fuel it up again it takes 7 gals. I have not put any fuel in the front tank for three weeks today now both tanks indicate that ther is 1/2 of a tank of fuel in both tanks. the rear tank is the original tank the fron tank is an after market tank.Holds 45 gals. I had this problem once before with the tanks. Fuel pump went out in front tank also it was pumping the rear tank full. now it seems that the rear tank is pumping into the front tank. Any suggestions I guess my mechanic is going to get the truck on monday. One question is do I replace both fuel pumps or just the front one.

 

The receiving tank has the bad valves in its Fuel Delivery Module (FDM) when one pumps to the other.

 

By receiving tank, Do you mean that the check valve in the front tank is bad?

When this happened the first time I had to replace the fuel pump.

 

If it is the tank that is filling then yes.
The pump is also inside the FDM.

The Fuel Delivery Module (Image Below) 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. To service the assembly, replace it.

Reservoirs and Filters
Fuel reservoirs are used to prevent fuel flow interruptions during extreme vehicle maneuvers with low tank fill levels. The reservoir is located in-tank and is either moulded or welded into the tank or is part of the Fuel Delivery Assembly.

Filters and Screens
The system contains four filtering or screening devices:
1. The fuel intake sock is a screen located on the intake side of the fuel pump and is part of the fuel pump assembly.
2. The filter screen on the fuel rail port of the injector is part of the fuel injector assembly and cannot be serviced separately.
3. The filter screen on the fuel inlet side of the pressure regulator is part of the fuel pressure regulator assembly and cannot be serviced separately.
4. The fuel filter assembly is located between the fuel pump and the Schrader valve or injectors. This filter should be serviced periodically by replacing it.

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.

Venturi Jet Pump
A portion of the high pressure flow from the pump is diverted to operate the Venturi Jet Pump. The Venturi Pump draws fuel from the tank into the assembly. This process ensures an adequate fuel supply to the pump during extreme vehicle maneuvers and steep vehicle attitudes with low-tank fuel fill levels.

The inlet of the Venturi Pump has a nylon filter on it to prevent dirt and other particulate matter from entering the system. Water in the fuel tank can pass through the filter in small quantities.

 

Thank you for the information. I guess I really need to let my mechanic fix this. I should probably replace both fuel pumps. Again thank you.

 

... so, if I have a '95 F250 ext-cab with dual tanks where I have just installed new fuel pumps in both tanks - and now when running on the Front Tank, the Rear Tank overflows - I need to replace the Rear Tank FPM - correct?

 

Yes you need to replace the rear FDM.

 

Hello,im having the same problem and curious if the FPM was the problem? Thnx in advance..

 

I think you mean FDM but yes the rear one has a bad valve in it.