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Subtle stumble while idling
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dave111
New User
Mar 14, 2017, 12:34 PM
Post #1 of 5
(2839 views)
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Subtle stumble while idling
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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.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Mar 14, 2017, 1:00 PM
Post #2 of 5
(2825 views)
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Re: Subtle stumble while idling
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I suggest a higher end code reading as there has to be a pending code or more to help. You've done the common reasons now could really use more info.
Vacuum of 21Hg if just idling like that seems high to me and really shouldn't be that steady so perhaps just the spot you chose?
T
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Hammer Time
Ultimate Carjunky
/ Moderator
Mar 14, 2017, 1:30 PM
Post #4 of 5
(2806 views)
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Re: Subtle stumble while idling
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First you need to understand what a pending code is.
The programming of the computer for certain codes requires that the computer see the specific situation on 2 consecutive trip cycles. The first time it sees the problem, it sets a pending code. If it sees the same situation on the next drive cycle it will set a code and turn the light on. If it doesn't see it a second time, it erases the pending code and starts over so it's very possible for the issue to occur once and not repeat immediately to set the hard code.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
(This post was edited by Hammer Time on Mar 14, 2017, 1:31 PM)
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Mar 14, 2017, 1:46 PM
Post #5 of 5
(2795 views)
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Re: Subtle stumble while idling
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? Vacuum reading to a blower/hvac door may be tempered by a storage ball so essentially useless. That could maintain the highest it was and be steady. Actual intake vacuum live is being watched by all the controls the engine converted to electrical input for computer to make adjustments. You are bouncing idle 60 or so RPM so it should but isn't reflecting that.
Engine should once warmed up and only some deviation for A/C or steering hard should be steady and it's just where you checked it that threw that off.
Back to codes that will read what's pending to help find the circuit that causes the erratic idle/stumble,
T
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