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GMC C1500 stumbling/hesitation when cold

This is a discussion on GMC C1500 stumbling/hesitation when cold within the Technical / Maintenance forums, part of the General Discussion category; I have a 1989 GMC sierra 2WD C1500 with a 305TBI. For the past 2 years I've had trouble with ...

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    Angry GMC C1500 stumbling/hesitation when cold


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    I have a 1989 GMC sierra 2WD C1500 with a 305TBI. For the past 2 years I've had trouble with it in colder weather.

    When its absolutely ice cold, the truck runs great for about 3 minutes. After that period of time, it begins to bog down considerably under acceleration. If you let off the gas pedal and press again, it sometimes gets better. This continues until the truck warms up to full operating temperature. Even at full operating temperature, things still dont seem quite right.

    Yesterday, I disconnected the temperature sensor thus running the truck in open loop continuously. Truck runs great with power even when cold, but knocked a little probably cause computer wasnt controlling stuff.

    So here is what I have done so far:
    1. Fuel pump was replaced about 2-3 years ago
    2. Ignition module under distributor has been replaced twice.
    3. pickup coil has the recommended resistance
    4. IAC valve has been replaced
    5. distributor cap, rotor, wires, and plugs about 2-3 years ago. White deposits indicating lean burn on the tips
    6. Map sensor replaced
    7. Throttle position voltage changes as indicated from 0 to about 4.5 V with throttle
    8. Temp sensor is giving recommended resistances at the proper temperatures
    9. Ignition coil has recommended resistances according to the service manual
    10. Fuel pressure runs at approximately 10 psi. Stays there while driving with very little fluctuation. It does go to zero after the fuel pump shuts off with key on.
    11. EGR valve has just been replaced - it was bad
    12. K&N air filter
    13. Injectors dont drip and the spray seems to be an even cone shape.
    14. Oxygen sensor was replaced this past summer.

    Thats all I can think of. Any ideas and help would be much appreciated!! I'm quite sick of dealing with this.

  2. #2
    GMC RULZZ NEUMANNZZ's Avatar
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    Re: GMC C1500 stumbling/hesitation when cold

    egr valve an exact replacement?
    fuel filter?
    base timing?
    O2 GMC RCSB 4.8L V8 AUTO 3.73's
    rebuilt/beefed up 4L60E(at 43,000miles)
    Matching Ninja 500r


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    Re: GMC C1500 stumbling/hesitation when cold

    EGR valve was an exact replacement from NAPA. It looked identical to the old one and now doesn't throw any codes...it did before it was replaced.
    Fuel filter was replaced with fuel pump. Fuel pressure stays at 10 psi even while driving and hard acceleration, so I dont think its a fuel pump/filter problem.
    I checked the timing with a timing light 2 summers ago. It checked out fine, and I did disconnect the EST wire while checking it.

    Thanks

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    Re: GMC C1500 stumbling/hesitation when cold

    Anyone have any more ideas on this? I've been stumped for quite some time. It seems related to ECM/TBI because it runs better with the coolant temp sensor disabled, but I could be wrong.

    Thanks

  5. #5
    GMC RULZZ NEUMANNZZ's Avatar
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    Re: GMC C1500 stumbling/hesitation when cold

    coolant temp sensor test:
    here is the test for coolant temp sensor using multimeter set to ohms
    and ir temp gun.


    212f 177 ohms
    194f 241
    176f 332
    158f 467
    140f 667
    122f 973
    113f 1188 ohms
    104f 1459
    95f 1802
    86f 2238
    77f 2796
    68f 3520
    59f 4450
    50f 5670
    41f 7280
    32f 9420
    23f 12300 ohms
    14f 16180
    5f 21450
    -4f 28680

    check grounds on thermostat housing stud.
    O2 GMC RCSB 4.8L V8 AUTO 3.73's
    rebuilt/beefed up 4L60E(at 43,000miles)
    Matching Ninja 500r


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