1991 F-150 4.9L - Seems like I'v always had the rear tank transfer fuel to front (someone told me it was suppost to do that) but, after reading these forums I understand it is not supposed to. I haven't worried too much about it until recently. Not too long ago the guage on the front tank quit working but, I knew there was gas there since it transfered from the rear. I would just run on the front tank until it ran out and then switch to the rear tank, fill the rear tank and run on that until it showed a 1/4 or so and repeated the cycle. A few days ago I made a trip to move my son to college. I started to fill the front tank which only took about 2 gallons. I then filled the rear tank. I ran off the front tank knowing it was full. I only drove about 70 miles when it started cutting out. Switched to the rear tank. Ocassionally I would switch back to the front tank which would run 10 -15 minutes and had to switch back to rear tank. I suspect the front fuel pump is getting weak as the whirring sound is much fainter than from the rear pump.

I can run with just the rear tank since I don't drive the truck much but don't want to have a full front tank of costly gas that can't be used. It sounds like the cross over fix would require the rear tank pump be pulled. Is there any other way to ensure all the gas from the rear tank gets returned to the rear tank so I can just use that one?

 

The rear tank is fine, do not pull it. You have a bad check valve in your front pump, pull it.

 

When you say pull it, do you mean repair or replace? Can I disable it somehow so that gas from the rear tank won't flow to the front tank and just use back tank?

 

I meant to replace the FDM. Some places just sell the fuel pump and some sell the pump inside a white plastic reservoir with the check valve and the shuttle valve. I think Ford sells the complete FDM.

To block it off I would think you would have to cut the Fuel Pressure Line off the front FDM and mash it shut and seal it somehow. In the early 90's Ford sold a check valve that would snap in here (I think this was the spot it went) on the pressure line to keep the fuel from going into the non-running tank.

Here is a diagram of the FDM:

http://i54.photobucket.com/albums/g9...yAssembly2.gif

 

I'm going to have to get my book out and study the fuel system flow (if there is one). So I understand, If I install a check valve in the high pressure side of the front tank line, that should solve the problem of cross fill?

 

Yes it would if you can find one.

Here is a diagram of the system:

http://i54.photobucket.com/albums/g9...FielSystem.jpg

 

Thank you for the information and quick response.

 

I think there is an important point here that has been missed. The shuttle valve on the front tank fuel return is the source of the problem. The shuttle valve is simply an on/off valve that is activated by the fuel pump in the FDM. You front tank's valve is sticking open all the time, allowing return fuel to trickle into both tanks when the rear tank fuel pump is working. I don't think there's any "quick fix" for this one. If your front tank pump is failing too, just replace the FDM. It's about $135 according to partsamerica.com. The greater risk here is from not fixing it, because the faulty FDM will allow fuel to fill in that tank until it overflows as you drive down the road.

 

Of all of the posts that I have read on here and other forums this is the first time I have heard of a Shuttle Valve doing this, it has always been the check valve in the FDM. I know Ford had problems with the Check Valve on the High Pressure output of the FDM in the 1993 year run and it will let fuel in the non-running tank but I never heard of the Shuttle Valve doing it.
Have you found this to be true someplace?
If so please post as it has always been the consensus on this Forum that it is the Check Valve.

The Check valve works with the Hi Pressure line and the Shuttle valve works with the Low Pressure return line.

 

Sounds like either way, I'm going to have to replace the FDM.