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Thread: 89 gmc k1500 timing breaking up when connected

  1. #1
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    89 gmc k1500 timing breaking up when connected

    heres my trouble guys- just installed a new motor in my 89 gmc k1500 tbi 5.7 auto.
    am able to set base timing at "0" fine. reconnect wire (tan & black) and will idle and rev up great sitting in park, take it out on the road and states breaking up and backing firing. took it out for a road test with wire disconeccted, ran good. seemes like it's getting to much advance when under a load or even a small load, and wire is connected. no trouble codes! Any ideas before i pull out the rest of my hair? Has known good module in it. the one thing i did happen is that when i was reinstalling motor and was reattaching grounds, the smaller ribbon ground on the pass. side at fire wall broke the stud off when i was reattaching(what a great idea-lets put a ground right under the hoses for the heater core, that way it can rust and corrode) anway i ran another ground from the firewall up around the fuel pump fuse to the water outlet with the other grounds.

  2. #2
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    crabtruck's Avatar
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    Welcome to the forum

    Note on the grounds - make sure that after rework
    the original points that had grounds still do.
    They provide not just grounds but also protection
    from electrical interference to sensors.

    Truck running well with timing wire unplugged
    but not with it connected suggest IC mod or pick-up coil problem.
    Specifically, with timing wire connected, ECM is looking for pulses
    from ICmod/coil to control things, without it connected,
    it just runs off base programming.

    Ohm the p/o coil and be sure it's not shorted to ground.
    Careful with the tach lead, even a momentary short
    to ground can damage IC mod.

    Hope that helps, let us know what happens.

  3. #3
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    not really sure what the IC mod is, what should the p/o coil ohm at?

  4. #4
    Senior Member zombie1969's Avatar
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    Some things to check have to start someplace.
    Vehicle Application:1989 GMC K1500 Sierra XC 5.7L, GAS, Vin K, Eng Cfg V8Customer Concern:Has an intermittent stall. Backfires/stalls at times. Stalls at idle. Ignition timing is erratic. Runs good with the EST bypass wire disconnected.
    Tests/Procedures:Test pickup coil output when cranking.
    700 mv AC when cranking.
    If less, replace the distributor shaft, part number#10467353.
    If OK, test PCM grounds, A12, D1 at the PCM.
    Should have less than 50 mv drop.
    These grounds are on the intake manifold, on or near the thermostat.
    Potential Causesistributor Shaft
    Powertrain Control Module (PCM) Ground Supply

    Diagnostic Codes:None



    Vehicle Application:1987 Chevrolet Monte Carlo SS 5.0L1989 GMC C1500 Sierra XC 5.7L, GAS, Vin K, Eng Cfg V8Customer Concern:Lack of power. Sometimes runs good with the bypass wire grounded. Misses and backfires if the bypass wire is connected.
    Tests/Procedures:1. Check for a loose reluctor on the distributor shaft. If loose, replace the distributor shaft.

    2. Remove the distributor rotor and cap. Rotate the engine by hand to align the points on the reluctor with the points going around the outside of the pickup coil.

    3. Install the distributor rotor. Point a finger at the blade end of the rotor. Next, slide the cap into position. That finger should be pointing directly at one of the 8 spark plug wire terminals. If pointed between 2 terminals, replace the distributor shaft.
    Potential Causesistributor Shaft

    Diagnostic Codes:None
    Sent using Boost mobile 4g htc evo.

  5. #5
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    tomm. i will check all these things, thanks for all the help guys

  6. #6
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    Quote Originally Posted by pumppy10 View Post
    tomm. i will check all these things, thanks for all the help guys
    So I know its been a few months but what did you come up with? I have a very very similar problem. Just when I connect the wire it barely idles and dies..

    I did replace the connector for the ground on the t-stat housing and still no luck...

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