rough idle/vacuum readings

Discussion in 'GM Powertrain' started by dsfloyd, Aug 30, 2010.

  1. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner Gold Member 1000 Posts

    Ok I will start out by saying that this is not on my chevy (even though it is running a little rough too) but on my wifes car. I have a question for those who understand vacuum readings better than me. So here goes to make a short story long.

    The car has 140k on it and runs well just idles poorly. the engines are known to have intake manifold leaks etc so I figured I would vacuum test it after not finding anything with any sprays (propane, carb cleaner etc). So the problem is that in park idles pretty fine. turn on the ac and it gets a little rougher acc belts moves a little (I have replaced tensioner, ac compressor, condensor etc already). the real problem is in D or R. the idle drops to around 500 so a little lower than I would like and gets a bit rougher. kick on the ac and the belt moves a lot more and idle gets rougher (doesnt stall and rpms stay fairly consistent but drop a little). So I vacuum tested the engine and in park no ac the needle is at 20 in/hg no fluxuating so thats good. turn on the ac and it drops to 18 and still no fluxuating (tiny vibration of needle but thats all no pressure change) rev it and let go it all checks out normal. That is how all the checks say to do it. my question now is I then put it into D and the guage drops to about 15 and with Ac about 14, but still no fluxuations just running rough (below 17 is considered low). Is this drop normal in D as that is not how the test procedures explain how to do it, and I couldnt find anything on vacuum readings in gear (probably cuz its dangerous to stand in front of the car with it in gear:lol:). I was kind of hoping for fluxuations to more easily diagnose the problem but that didnt happen.

    I also rechecked plug gaps, it has been seafoamed, and only uses Chevron. I need to still do compression and fuel pressure readings. Just wondering if anyone had info on whether the pressure drop in gear was normal.

    oh yeah no codes anything like that.
    Last edited: Aug 30, 2010
  2. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    Have you checked the IAC??? It sounds like it isnt opening/closing properly to maintain the idle. There could be a chunk of carbon or else the solenoid on the IAC might be bad.
  3. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner Gold Member 1000 Posts

    Thanks, It looks like on this unit I can't replace just the IAC, I have to do the whole throttle body (which comes with IAC, TPS etc on it). the throttle body itself has been pulled apart and cleaned. Is there an easy way outside of replacing it to tell if its the IAC. a new throttle body isn't too expensive ($130) but not sure if there are further testing I could do. I still need to compression test and fuel pressure test, but that is one thing I will definately look at.
    Last edited: Aug 30, 2010
  4. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    WEll if you cant remove the IAC it is pretty hard to test since you cant look at it. IT should move when you turn on the ignition. Also if there is carbon in there is a bit harder to get out.. Because due to low idle etc i would lean twards a bad IAC before fuel pressure etc.. But i guess id check those out and then go from there if they are good.
  5. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner Gold Member 1000 Posts

    ok thanks.

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