HELP :-( Idle Air Control Problem on a 1996 C1500 Vortec 4.3 5-speed

Discussion in 'Chevy C/K Truck Forum' started by OILBRNR, Feb 27, 2012.


    OILBRNR New Member

    Hey everyone,

    Hate that my first post about my truck is problem related, but it is. Pardon my ignorance as I have been searching for days for a solution to this problem and I have not seen anyone successfully fix it.

    I have a 1996 (Build date of 10/95) C1500. It is equipped with a Vortec 4.3 V6 mated to a 5-speed transmission (unsure of what model). This is my work truck and I depend on its dependability. When I purchased it, it had a stumble anytime the throttle was held open just enough to hold a steady speed. I knew it needed a tune up, so here is what I have replaced so far...

    New Coil
    New Spark Plugs (Autolite platinum)
    New Spark plug wires
    New Distributor Cap
    New Distributor Rotor
    New Idle Air Control Unit (on the side of throttle body)
    New throttle position sensor
    New Coolant temp sensor (in the thermostat housing)
    New Ignition Control Module
    New Plenum and Throttle body gaskets

    So here is where I am now. Overall the truck runs great. It runs quiet for what it is, no weird noises. The issue is that when I start it, it immediately revs to approx 2k rpms, and after 5 to 10 seconds it settles to a nice idle. THEN, once driving, whenever I push in the clutch the RPMs simply hang wherever they are. For instance if I shift at 2500 rpms, it will stay there for about 5-10 seconds or I can drag it back down by putting it into the next gear. Sometimes, it hangs so well that it literally throttles the truck down the road.

    Troubleshooting I have done so far has been to try and detect vacuum leaks with some carb cleaner. No matter where I spray I can not get a bog down.

    The craziest part of the whole thing is.... when I unplug the IAC (on the throttle body) the truck acts completely normal. It starts up nicely and immediately goes to a nice idle. In between shifts it drops down as it should.

    OILBRNR New Member

    No suggestions anyone?

    I have tried the IAC reset procedure over and over. I also left the battery's negative terminal unplugged for over an hour, hooked it back up, and then run the reset procedure. Still the same thing. Start it up and it holds 2000 rpm for 5-10 seconds. In between shifts the revs hang (at whatever rpm you shift at) for about 5-10 seconds. It seems that the IAC is constantly being adjusted by the ecm.

    Is there a possibility that my ecm is bad? I have also heard that there is a update to fix this issue that can be had at the Chevy dealer?
  3. BCartmell

    BCartmell New Member

    I have the same issue but with the 5.7 cant seem to track it down either, i have not unplugged the iac valve though. im interested to hear how this turns out. If you find an answer somewhere else i would really appreciate it if you let me know.

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