Rough idle - IAC question after scantool - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 05-02-2007, 12:00 AM   #1
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Default Rough idle - IAC question after scantool

Does anyone know what the IAC position should be at idle? Is there a general range that it should be at? I've had rough idle issues for quite some time, and the engine sometimes pulses (sounds like small revs, one after another, if I let it sit an idle for a while). But generally, it idles at 1000 rpms, but a bit rough for my taste.

My IAC position at one point while at idle said 40 according to the scantool. I'm curious if I could diagnose a bad IAC from the tool, and figure this thing out once and for all.
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Old 05-02-2007, 04:30 AM   #2
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low 40'* is optimal. If you get to mid 50'* start looking into your fuel system.
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Old 05-02-2007, 10:22 AM   #3
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Cool, thanks. Can you also explain your answer. What does the number "40" mean anyway, or "50."
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Old 05-02-2007, 02:49 PM   #4
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Originally Posted by bonnie94ssei
Cool, thanks. Can you also explain your answer. What does the number "40" mean anyway, or "50."
Time to buy an FSM Pete.
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Old 05-02-2007, 09:33 PM   #5
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The IAC is a bipolar stepper motor, The count is the number of steps the PCM has applied. Think of it like a water spigot with an electric motor strapped on the end. Bigger numbers are farther open.

When initially starting the PCM runs it closed, Then counts it open to 75 or so when it has been reset. The PCM will learn what your engine likes and bring it to that number on the next start. Once it gets the engine running the PCM will step the motor inwards to bring the RPM to where it wants it.

http://en.wikipedia.org/wiki/Stepper_motor
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Old 05-02-2007, 09:50 PM   #6
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Thanks!

When I cleaned my throttle body last October, and cleaned the IAC, I had hoped to fix my rough idle. I have a perfectly clean TB but my idle is rough. I realize it can be because of several things, but when I cleaned the IAC, I had pushed in on the shaft. When I pushed it in I thought I damaged it.

Since low 40s you say is ideal at idle, I'm guessing the IAC should be fine. So I should look into other things.
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Old 05-02-2007, 10:34 PM   #7
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IACs don't really fail like that. It either works or it doesn't.

It'* internally threaded, so if you actually moved it in wore the threads a bit, But the PCM will still run it in first.


Rough idle? vacuum leak? low fuel pressure?
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Old 05-02-2007, 11:14 PM   #8
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My idle is fairly rough and it seems like on startup it'* the worst. It will idle fine for several minutes, the rpms will slowly start drop once it gets warmer, but then they'll kick back up. If I let it idle for a while you'll hear pulses, like tiny revs one after another. Boosty heard it a few times. The roughness is just a feeling that you can sense through the pedals, like the engine is shaking softly.

No vac leaks AFAIK. I haven't done any spray testing, but I'm sure all connections are good (won't hurt to check again). EGR pipe is brand new. My boost gauge at idle doesn't indicate a vac leak, it'* pretty steady at -10.

Fuel pressure last time I checked was great at idle, a bit higher than 'normal' actually.

TB of course is crystal clean. No misfires according to the scantool. Maybe the TPS could be an issue?
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Old 05-03-2007, 02:09 AM   #9
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It'* easy enoguh to check the TPS.

If you're running high fuel pressure it can be just as bad as low fuel pressure. There'* equations out the for finding your fuel flow, see if you're retardedly outside the realm of what the PCM can adjust for.
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Old 05-03-2007, 10:11 AM   #10
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I agree, I should probably look into why my fuel pressure is higher than normal.
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