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Thread: 939A2 hit a bump and now wont go over 40mph

  1. #11
    Former SSG 98G's Avatar
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    Quote Originally Posted by WillWagner View Post
    OR, more simpler things first, look at the fuel shut off, if it is broken, it wont have full fuel internally, also check that the throttle lever actually goes to full throttle stop, and, look at the AFC sensing line/fitting, the like to break at the fitting at the head and the steel bung in the pump liked to waller out of the aluminum housing. Just sayin' before you start taking things apart, look at things.
    Description didn't do it justice. Here's a more accurate description -

    Sounds fine at idle. Won't go over 20mph. Falls on its face and dies. Full throttle wasn't reaching the throttle stop. It was only going to 2000rpm in neutral at WOT.

    Adjustments made. It now reaches 2300rpm in neutral at WOT. With the lever against the throttle stop. Still falls on its face and dies. Will restart after sitting a moment. The throttle limiting screw with the wire appears to NOT have been tampered with. Wire is intact and no tool marks on the screw.

    It's a wrecker. The tank selector has been bypassed. Its feeding from the driver side tank only. Made sure the supply line to the lift pump actually goes to the supply fitting on the tank and that supply/return were not transposed.

    I replaced the small fuel filter on the block. It does not have the larger filter on the firewall. No improvement.

    We blew compressed air down the fuel return line starting at the front of the IP towards the fuel tank. Unobstructed.

    We blew compressed air from the tank selector bypass line to the fuel filter fitting. Unobstructed.

    We stuck the end of the tank bypass tube into a jug of diesel. No improvement.

    We removed the fuel pressure sensor, related fittings, and inspected for cracks. None found. Reassembled and sealed. No improvement.

    We disconnected the fuel line at the inlet to the fuel filter, pulled the kill, and cranked for 30sec. 195ml of fuel was moved by the lift pump. Standard is 150ml or better.

    We tightened the fittings between the lift pump and the fuel tank. No improvement.

    Starts fine. Sounds good at idle. Good oil pressure. Oil looks good with no diesel smell. No play in the turbo shaft. Air filter looks good

    If I'm throwing parts at it I'm going to start with the lift pump.

    Suggestions?
    Last edited by 98G; 09-19-2019 at 01:04.

  2. #12
    4 Star General Jbulach's Avatar
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    If youíve already tied the fuel shut off solenoid back to eliminate it, the lift pump would be my next guess, but I would put a gauge on it to make sure itís making pressure as well as flow before throwing parts at it. If your pressure reads low, you should be able to crimp off the return line. If the pressure goes up to, or above normal, it should be a bad overflow valve. If it stays the same itís likely a bad lift pump.
    M925A2

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    98G (09-20-2019)

  4. #13
    4 Star General snowtrac nome's Avatar
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    Take a look at the fuel return banjo bolt, there is a little screen and check ball in them. If something has plugged or restricted it there, you wont fill the pump, and the engine will act as if its starving for fuel.

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    98G (09-19-2019)

  6. #14
    Former SSG 98G's Avatar
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    Today we drove 100miles each way and bought a lift pump from Cummins in El Paso.

    All issues resolved with a new lift pump. Truck is now running like it should. I'll drive it to KS using just the driver side fuel tank and then restore the selector valve and passenger side tank once I'm there.

    20190919_181452.jpg

    20190919_181527.jpg

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    silverstate55 (09-20-2019)

  8. #15
    General Ajax MD's Avatar
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    There's something about a truck with a giant vise mounted on the bumper that is just cool.
    Motorpool:

    (Isn't owning one of this insanity enough??)
    1971 M813 "Ox" (named for Dewey Oxburger)

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    98G (09-20-2019), silverstate55 (09-20-2019)

  10. #16
    Former SSG 98G's Avatar
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    Made it about 100miles and blew a tire. Good thing I brought a spare with me from KS.

    20190920_120335.jpg

    Drove it the rest of the way to Socorro NM uneventfully. Parked it there while running some errands with the redtruck.

    20190920_162244.jpg

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    silverstate55 (09-24-2019)

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