1. Welcome to FordForumsOnline! Our community is here to help you with any questions you have related to your Ford. If you are looking for a friendly community to hang out with other Ford owners you can look no further than our forums here! Registering is easy and free!

1988 F150 dual fuel tank issue

Discussion in 'Engine, Transmission/Axle & Driveline/Axle' started by 70XL, Jan 10, 2017.

  1. 70XL

    70XL Active Member

    Joined:
    Jun 28, 2011
    Messages:
    249
    Likes Received:
    34
    Location:
    SK, Canada
    Vehicle: 1988 F150 XLT Lariat 4X4, S/C, LWB, 5.0l, 5 spd.

    Problem: Fuel disappearing and leaking out on the ground. This is a low use vehicle for me, winter beater, garbage hauler, etc. I haven't used the front tank for many years because it had a leak, so I only run off the rear tank. On Dec. 11 I fill up and a few days later, the next time I drove it, the gauge (which also has never worked properly as long as I've owned the truck) is acting like it is only half full. Did someone syphon gas out, or did the sender break even more?

    I kept driving it until the gauge showed empty and only a fill would tell me if the tank was empty, or still had fuel in it. Sure enough, it took 68 litres to fill and I only drove it about 80 km. I shrugged and thought I had been a victim of theft. A couple days later I notice the ground under the truck is wet and I can see gas dripping, and dripping rapidly at times. We were / are in a good cold snap, -30C, and I couldn't get syphon, or transfer pump hose into the rear tank. The odd thing was the dripping was from the front tank area, not at the rear of the truck.

    The gas was dripping off the shield on the front of the front tank. I started the truck and switched the selector to the front tank and the gauge showed full. It would seem that somehow the selector valve got stuck between the two and the rear tank is filling the front tank, and the front tank obviously still has a leak. Is this a common situation, or has anyone else had a similar experience? Any advice on testing, or repairing? If that selector switch is open to both tanks are both low pressure pumps energized and running? There are low pressure pups in each tank and a high pressure pump on the frame rail under the cab.

    Sorry for the novel.
  2. Dominick 1

    Dominick 1 Well-Known Member Respected Member

    Joined:
    Dec 26, 2014
    Messages:
    2,329
    Likes Received:
    231
    Location:
    Parkersburg Wv
    See if this helps
    http://redirect.viglink.com/?format.../motorsports/articles/FuelTankValve/index.htm
  3. 70XL

    70XL Active Member

    Joined:
    Jun 28, 2011
    Messages:
    249
    Likes Received:
    34
    Location:
    SK, Canada
    Interesting info in that link. Thanks Dominick. Maybe a damaged diaphragm is letting fuel bypass from one to the other. The engine runs fine, so enough fuel is getting to the high pressure pump. Right now I'm running off the full, but leaky front tank, and I'm hoping the rear in tank pump is not staying running, which could burn it out when it runs dry. Does the selector valve do the electrical switching, I don;t see a wiring harness in the pictures?
  4. Action

    Action Moderator Staff Member Respected Member

    Joined:
    Mar 22, 2014
    Messages:
    4,869
    Likes Received:
    448
    Location:
    Phoenix, AZ 85008
    The system is design a little different than what would normally be the thought process. The switch on the dash only activates the selected tank pump and gauge. The part in the link only prevents fuel from going to the opposite tank. It is more of an automatic switching valve that switches when the pressure is greater in one system (front or back) than the other. If the dash switch is front then the front pump should be on generating pressure while the rear tank pump is off and pressure is going to or is at zero.

    It would make sense that this part is bad if you already had a leak in the front tank.


    >>>>>>>>>>>>Action
  5. 70XL

    70XL Active Member

    Joined:
    Jun 28, 2011
    Messages:
    249
    Likes Received:
    34
    Location:
    SK, Canada
    I think I may have found a good explanation and reasonable solution to the situation. On another forum I found a thread on the very issue I'm experiencing, it seems that if the selector valve is not cycled regularly by switching tanks, then an o-ring on the valve stem can stick and the return flow from one tank will end up in the other tank. If I read it correctly it will only cross circuit with the rear tank return flowing into the front tank. There were some pictures and illustrations of the assembly, so it can be fixed, should be repairable, without replacing the unit as long as the diaphragm is not torn, or it is not damaged during disassembly. I guess I'll need a fuel line disconnect tool to remove the 6 lines. It was -35C this morning, but is supposed to be around freezing (0C) on Sunday, so I can probably wear shorts while working in the drive way.;):)

    Attached Files:

Share This Page