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Subtle stumble when idling

  #1  
Old 03-14-2017, 03:39 PM
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Default Subtle stumble when idling

Got a 2002 Monte Carlo 3.8 with 83K miles.

Have started noticing a very subtle fluctuation in the tachometer while idling. Can't feel anything from inside the car, but under the hood, you can hear a change in the engine. A brief hesitation every 2-3 seconds. Car seems to run fine under power. Has never set a DTC.

Hooked up the scan tool, no old codes. RPM on the scanner shows a fluctuation from around 780 to 840 RPM. Again seems to happen irregularly every 2-3 seconds.

Removed and cleaned the throttle body, it was moderately dirty. Also removed and cleaned the IAC, also mod dirty. No improvement.

Changed the PCV. No better.

Hooked a vacuum gauge to the intake manifold. Shows 20-21 inches. The needle tends to 20, with the upticks to 21 timed with the stumbles.

If I open another hose to the manifold, the vacuum drops to 10 inches and the RPM increases as expected, but the stumble goes away.

Does this mean the stumbles are due to inadequate intake air, as in a bad IAC? Or something else?

Any help appreciated.
 
  #2  
Old 03-14-2017, 06:03 PM
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IMO it doesn't sound like a big enough deal to worry about at this point.

If you really want to keep looking around, maybe try cleaning the MAF and checking out the plugs? Also if you have a scan tool, try checking other sensor readings to see what else changes immediately before that happens.

I wouldn't think the IAC would have an issue with airflow as it has to be open under other conditions where it has to open more to flow more air - cold starts, ac on, etc. Certainly it could have a sticky spot right in this area of travel, but if you've cleaned it as part of your tb cleaning, that isn't very likely.
 
  #3  
Old 03-14-2017, 11:42 PM
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Originally Posted by bumpin96monte View Post
IMO it doesn't sound like a big enough deal to worry about at this point.

If you really want to keep looking around, maybe try cleaning the MAF and checking out the plugs? Also if you have a scan tool, try checking other sensor readings to see what else changes immediately before that happens.

I wouldn't think the IAC would have an issue with airflow as it has to be open under other conditions where it has to open more to flow more air - cold starts, ac on, etc. Certainly it could have a sticky spot right in this area of travel, but if you've cleaned it as part of your tb cleaning, that isn't very likely.
I had also cleaned the MAF when I did the throttle body--still no improvement.

The conventional wisdom about small changes in manifold vacuum, if the readings are otherwise normal, is that it typically represents an ignition problem. No misfires are ever recorded on the scan tool.

Plugs and wires are about due for a change, guess I'll go that route.
 
  #4  
Old 07-16-2017, 01:04 AM
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Did you ever figure it out?
 

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