High idle, pretty stumped

Discussion in 'Engine & Performance' started by Yeret, Sep 24, 2017.

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  1. Yeret

    Yeret Senior Member

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    Ok all, I made mention to this in the GD topic but I think it's about time to make a thread because this issue is starting to irritate me.

    Starting about three weeks back, every once in a great while when I shift the tranny into Park, the engine idles up to 1,000 RPM and holds. It drops and rises during this time like the ECU is trying to pull it down but is not succeeding. So, I decided to replace the IAC, which I suspect was faulty anyway due to unresolved cold-start stalling issues last winter. The engine started and idled no problemo, so I decided to rev it. Lo and behold, the engine did not return to it's normal idle of 650 RPM, but instead stuck at friggin' 1,000 RPM again. I revved the engine a little and nothing changed so I revved the engine to 3,000 RPM and what do you know, it returned to proper idle.

    So, I spent the next little while revving the engine to see if the issue would happen again. And it did but there was no predicting when it would. It was totally random when the engine would return to a high idle rather than a normal idle.

    Earlier this year, I fiddled with the idle screw on my modified throttle body and had no problems with the setting until now. So, I reversed what I did and the issue was improved but the engine still wanted to high idle on occasion. Went for a drive earlier today and when I stopped at the station, the idle was fine. When I got home, however, the engine high-idled again!

    This issue has me totally stumped. New IAC, fiddled with the throttle body and the problem as of now remains unresolved. Starting to bug me. What do y'all think? Vacuum leak? Electrical bug? I've got my old throttle body in a box somewhere. If I can find it, I might try putting it back on and see if that does anything if I can't figure this out.

    If it could possibly make a difference, this problem started about one week after I got my shorties installed.
     
  2. dudeman2009

    dudeman2009 Senior Member

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    The engine cant rev if its not getting enough air, something is causing it to open the IAC to hold that RPM, which it will do on cold start for a few seconds until things warm up. A vacuum leak unless its huge (like pulling the brake booster) will not affect the idle much at all. If it only happened after shorty headers then I would suspect something to do with what the O2 sensors are reporting and how your tune is deciding to act. Do you have a scantool that will read O2 info and fuel system information? I'd be checking to see if its going open loop when this is happening, and what the O2 sensors are outputting. I'd also be curious to see what the engine temp is reporting via the scantool.

    I doubt its anything with the intake, especially since its intermittent its gotta be the computer reacting to a sensor reading somewhere. You might also try the APPS reset procedure and see what the TPS readings are.
     
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  3. SYKRAMMAN

    SYKRAMMAN Senior Member

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    ^+1, Yes I would definitely look at the o2's, might need to get a couple non foulers.
     
  4. goodtodoo

    goodtodoo Senior Member

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    You guys have helped me lots getting my 3 1996 trucks running, I,m thinking. The iac you got isn,t good. Hard to say.
     
  5. Shadow_Death

    Shadow_Death Senior Member

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    I literally cleaned my TB because of my IAC getting stuck open. Turns out there was 122k miles worth of grime built up in there and you could see the lines where the IAC would rest open and closed.
     
  6. Yeret

    Yeret Senior Member

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    Thanks for the input, guys. I've got an Innova scanner with live data function, so I'll check things out next time I get a chance.
     
  7. dapepper9

    dapepper9 Senior Member

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    These may help

    Screenshot_20170909-232748.jpg
    Screenshot_20170909-232815.jpg
    Screenshot_20170909-232822.jpg
    Screenshot_20170909-232830.jpg
     
  8. Yeret

    Yeret Senior Member

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    Just tried that adjustment procedure. Ended up turning the screw back in at least a few whole turns. Couldn't get as precise as 200 above command (which would have been 850). It was either settle on ~750 or the engine would rev up and hang on to 1,200. Seems to be okay now but I'll drive the truck for a few days before I come to any conclusions.
     
  9. goodtodoo

    goodtodoo Senior Member

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    I am not understanding this, what did you adjust?
     
  10. dapepper9

    dapepper9 Senior Member

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    Where'd you get the throttle body from?
     

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