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Thread: Please help and thanks for the accept. GMC 3500 not staying running

  1. #21
    Join Date
    May 2021
    Location
    Indiana
    Posts
    11

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    Also just read the story Robyn on the rat..wow. Live rat in a turbo, got to be a first.

  2. #22
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

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    Stranger things have happened. If you have good ground and good Batt+ voltage to the pump, and it doesn't pump, it's a bad pump. They are simple, meaning, apply power and they just work, or they don't. No magic involved. A failing/failed pump can also overload the circuit during the process, so recheck the relay and fuses. Before dropping the change on a new pump, check it again just as before, but this time, check the voltage on the circuit WHILE it should be pumping. If the voltage drops, the circuit won't carry the load, or the pump is internally shorted. If you have any indication of either, proceed accordingly.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  3. #23
    Join Date
    May 2021
    Location
    Indiana
    Posts
    11

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    Issue resolved 🎉.

    It WAS the fuel lift pump. I put a new one in and didn’t even need the wiring work. If anyone comes across this for future references here are a few points to consider:
    1. The pump does not have to completely and immediately die, mine worked intermittently then finally failed. This put quite a bit of error in my problem solving as since it ran on one test I assumed it was good then went to the wiring as suspect. So I would test it multiple times and for an extended run.
    2. On testing the charge from the battery I skipped over and neglected a small point made by Dmax. The fuel pump will only run for 4-5 seconds then shuts off power. I tested the wires from the engine and found no charge so assumed I had a short. In reality the key was on and it had been well over 5 seconds so the system shut off charge to it. That didn’t cost me much cash (yet) but it did send me off on a wrong tangent costing a few hours of work and reading fruitlessly.

    speaking of cash here is what it cost:
    new relay $20
    new fuel filter $35
    new toggle switch and assorted wire and clips $5
    new pump $60

    A total about 120 all done, some of which was unnecessary due to my errors listed. All in all I feel pleased.

    Thanks for the help and I will probably be back eventually, hopefully not soon.

  4. #24
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

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    That's fantastic! We'll leave the light on for ya.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  5. #25
    Join Date
    Nov 2002
    Location
    Martinsville, IN
    Posts
    3,163

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    Quote Originally Posted by DmaxMaverick View Post
    Your broken OPS was probably still working, despite its appearance. It doesn't control the lift pump on OBD II trucks (1996+). The PCM will have a tantrum if it's missing, though.
    The oil pressure switch does control the lift pump on OBD2 trucks. The circuit diagrams are very similar from OBD1 to OBD2. In short, both the fuel pump relay and the oil pressure switch must be in working order for the lift pump system to function properly.

    If you need the circuit diagram, a subscription to AlldataDIY.com is only about $15 for a year. They have the same information that the GM service manuals have.

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