89 Sub engine swap...fired and running now. need to prime it? Low idle and cycles

Discussion in 'General Chevy & GM Tech Questions' started by TMAL1, May 9, 2012.

  1. TMAL1

    TMAL1 Member

    89 4x4 GMC 2500 sub engine swap with 91 gasoline motor.

    Fixed the TBI issues. Fired the motor. Timed it to 0 advanced with the wire unplugged. It runs and drives but must be primed to fire. No spray at start. It has a VERY low idle, 200. It randomly cycles rpm at idle, smells rich and at the lowest point of the rpm cycle it nearly dies and the check engine light briefly flashes on. The new O2 sensor is in the Y collector and is in the factory position. New exhaust directly to the cheap short headers. (It drove to and from the exhaust shop).

    Things I've done:
    Did the paper clip trick...got no codes. However...I mistakenly left the jumper in once and fired it...it ran up to high idle 1200 and slowly came down to 700 and purred. Turned it off after a bit and retried it...back to the same low idle. <----WTF?
    Tested the ECM in the dist...tested good. Replaced it with another old one...no change.
    Completely replaced bad TBI. The IAC motor broke and I replaced it with another old one I had from a donor TBI. No idea of the condition of the replacement IAC motor.
    Checked, rechecked and rechecked all grounds. Then checked them again.
    Tried to test the TPS voltage at key on. THERE IS NO VOLTAGE AT KEY ON FROM THE 3 TPS WIRES. I have voltage at the pods and nothing out of the TPS wires. Replaced TPS with another used one I had with same result. I have not tried to test the TPS voltage while running...I assume it has some as it runs and drives. Visually inspected the wires in the loom. Nothing looks wrong with any part of it visually.
    Key on...the coil is obviously hot.
    All gauges work well with the exception of oil pressure is pegged beyond max most of the time but then randomly reads correctly.
    Unplugged the battery overnight to reset the comp, with no change.

    New parts: Fuel pump, filter, cap, rotor, wires, plugs, motor, headers, 140 amp alternator, muffler.

    Replaced or tested parts: ECM, TBI, TPS, coil, IAC motor(no idea of the condition of the IAC other than cleaned and looked good)

    I am hoping with enough symptoms someone can help me determine what is wrong with this thing? Or at least give me good advice on a direction? It ran for years with no issues except for a dying motor.
    I will likely try the pick up coil next. Not sure if it can be tested or how much one is?

    Thanks again for the help on my on going nightmare.
    Last edited: May 9, 2012
  2. Chris Miller

    Chris Miller Rockstar 100 Posts

    Check all the grounds going to the ECM, as well as the main grounds.
  3. osgprometheus

    osgprometheus Rockstar 100 Posts

    What year ECM did you eplace it with? An 89, or 91?
  4. dipstick

    dipstick Member 2 Years 100 Posts

    Sounds like truck is in limp mode . Limp home till fixed. Test other sensor for 5 volt reference if none bad ecm.
    Tps should have 5 volt ref. with key on .50 closed throttle 4.50 volts at wot.
    Another reason i think it is a bad ecm no fuel pump at start you have to prime.
    When you turn key to on ecm commands fuel pump on via the fuel relay.
    Truck will run if ecm bad just will not run good at all.
  5. TMAL1

    TMAL1 Member

    Mystery solved...wire from battery to alternator doesn't go to starter.

    Note to self: don't wait 7 months between engine removal and install...

    Thanks for every ones help. I will be posting pics soon.
  6. osgprometheus

    osgprometheus Rockstar 100 Posts

    Glad it was a simple fix man.

    Another burb lives!!!!!!
  7. TMAL1

    TMAL1 Member

    Not quite sure what is wrong with me but...after being sickened by the pathetic lack of power from this new motor. Headers, no cat, cam...

    I am considering doing a TPI conversion. I may need an intervention.
  8. dipstick

    dipstick Member 2 Years 100 Posts

    Still sounds like it is in limp mode to me.

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