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5.7 Vortec IAC - Idle Problems

This is a discussion on 5.7 Vortec IAC - Idle Problems within the Technical / Maintenance forums, part of the General Discussion category; Hey guys, I'm having some trouble with the idle on my truck. It began idling at ~2000RPM and it had ...

  1. #1
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    5.7 Vortec IAC - Idle Problems


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    Hey guys,

    I'm having some trouble with the idle on my truck. It began idling at ~2000RPM and it had thrown a CEL. I scanned it and it came up for the IAC valve. Before dropping $70 on a new one, I did some research and decided to clean the throttle body and the IAC itself.

    What resulted was quite different. I started the engine and it shot up higher, and slowly worked it's way down to proper idle speed. I called it a day and assumed it was good to go, but once I drove it the next day, it began idling high again.

    Now, let's say I come to a stop at a red light, the engine would eventually work down to proper idle speed again (From ~1200 to around 7 or 800). But when I come to a stop, let's say a parking spot, throw it in park, and it will jump up to ~1500 or 2000RPMs, maybe higher. When I let it sit, over a couple minutes, it will work its way down slowly again.

    Think I should just buy a new IAC valve? What other things cause this issue?

    For reference, 1998 Chevy K2500, 5.7L Vortec.

    Thanks!

  2. #2
    GMC RULZZ NEUMANNZZ's Avatar
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    Re: 5.7 Vortec IAC - Idle Problems

    high idle usually the result of a vacuum leak or sticking iac valve. is it still throwing the code for iac valve? did you disconnect the battery to erase the pcm memory. allow a few drive cycles to relearn.
    O2 GMC RCSB 4.8L V8 AUTO 3.73's
    rebuilt/beefed up 4L60E(at 43,000miles)
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    Re: 5.7 Vortec IAC - Idle Problems

    Quote Originally Posted by mindset View Post
    Hey guys,

    I'm having some trouble with the idle on my truck. It began idling at ~2000RPM and it had thrown a CEL. I scanned it and it came up for the IAC valve. Before dropping $70 on a new one, I did some research and decided to clean the throttle body and the IAC itself.

    What resulted was quite different. I started the engine and it shot up higher, and slowly worked it's way down to proper idle speed. I called it a day and assumed it was good to go, but once I drove it the next day, it began idling high again.

    Now, let's say I come to a stop at a red light, the engine would eventually work down to proper idle speed again (From ~1200 to around 7 or 800). But when I come to a stop, let's say a parking spot, throw it in park, and it will jump up to ~1500 or 2000RPMs, maybe higher. When I let it sit, over a couple minutes, it will work its way down slowly again.

    Think I should just buy a new IAC valve? What other things cause this issue?

    For reference, 1998 Chevy K2500, 5.7L Vortec.

    Thanks!
    I have the exact same problem in a 2000 tahoe with a 5.7L Vortec. the first thing i replaced was the IAC. it ran great for 1 day. then went back to high idle. since then i replaced every piece of vacuum hardware, hoses, etc. replaced intake and head gaskets. completely cleaned intake and rebuilt heads. upgraded fuel injector. complete tuneup, new map sensor. After the top end rebuild the truck ran fine for almost a month. I thought I solved the problem. But it came back. I've been data logging to find a sensor thats out of spec that might be causing the problem. but everything looks fine. If I unplug the IAC the idle stays low (sometimes a bit too low) but does not get out of control. so the computer is telling the IAC to let more air in but WHY?
    Last edited by azsound; 05-31-2013 at 05:56 AM.

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    Re: 5.7 Vortec IAC - Idle Problems

    subscribe
    Mike Gallagher

    97 Regular Cab / Shortbed / Black
    87 Grand National
    63 VW Sun Roof
    04 Saturn Red Line Vue

  5. #5
    GMC RULZZ NEUMANNZZ's Avatar
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    Re: 5.7 Vortec IAC - Idle Problems

    what we dont know:
    whats vacuum actually reading?
    iac counts? if applicable
    actual rpms via scan tool?
    do all the wires and connections pass continuity checks?
    does the brake booster hold vacuum?
    fuel pressure? is it within specs
    is the high rpms the result of a/c being commanded.
    Last edited by NEUMANNZZ; 05-31-2013 at 09:37 AM.
    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: 5.7 Vortec IAC - Idle Problems

    Sorry for the late reply guys.

    Well let me take another go. I replaced the IAC, still idling high at the same RPM. I checked my TPS and the TPS wiring according to Haynes, and all checks out as good. 5V through the connectors, and .6V throttle closed, and ~4.4V open.

    I have been searching around for vac leaks, but I can't find any. I don't have tools to check for fuel pressure, nor a clue how to check brake booster vacuum. And A/C doesn't work, I believe the clutch is fried on it.

    Gonna head out and check IAC wiring, via Haynes specs.

    Any other input would be great.

  7. #7
    GMC RULZZ NEUMANNZZ's Avatar
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    Re: 5.7 Vortec IAC - Idle Problems

    ac clutch fired meaning is it putting a drag or load on the engine?
    to test vacuum on brake booster you need a hand vacuum pump tester to apply vacuum to booster. you can use the gauge on hand vacuum pump to check for vacuum also. definitely check iac wiring.
    O2 GMC RCSB 4.8L V8 AUTO 3.73's
    rebuilt/beefed up 4L60E(at 43,000miles)
    Matching Ninja 500r


 

 

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