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Rough Idle When Stopped in Gear


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Hi everyone,

I have an '89 that does not idle smoothly when stopped in gear. For instance, when waiting at a red light, RPM'S will quickly fluctuate from 650 to 600 back up to 650 resulting in a bit of a shudder that lasts about a second. Over time it seems to be getting progressively worse.


Very smooth idle @ 750 RPM when in Park/Neutral

RPM'S drop to 650 when in gear

Fuel system has recently been treated (problem existed before treatment)

Battery & charging system are good

Other than this little hiccup, car runs & performs excellent

Dirty IAC? Dirty MAF?

Any suggestions are appreciated!

Thanks in advance,


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Judging by the tach on my car, at idle it runs 600-650 (warm engine), and routinely fluctuates 25-50 RPM's but does not cause any apparent roughness that I can feel. I always thought this was normal - even my 95 Deville varies up to 50 RPMs at idle, usually in a range of 575-625. I've not seen an engine that can maintain dead-on idle with no variance at all. Note that my IAC has been thoroughly cleaned, and made no difference in this regard.

Bear in mind also that the ECM output scales the engine speed reading to have a minimum resolution of 25 units, meaning that it will round up or down to the nearest multiple of 25 on the display, so the actual variance may not be as much as it "looks" like on the gauge.

It may be that the slight change in RPMs is not an indication of the underlying problem causing the rough idle. The IAC is a good first place to check, although a fouled spark plug, failing injector or any number of other ignition system issues could be at play.


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Mine has always been very smooth but then I also kicked the idle up to 700 rpm to improve the a/c at stoplights. Very low idle speed is an indicator of either a ded cam or VVT.

I have always thought to myself "If only I could bump up the RPM's a little bit...". How can I do this?

In the meantime, I will clean both the IAC & MAF (probably long overdue anyway) & will report back any improvement.

Thanks to everyone for your input!

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  • 4 weeks later...
Classic signs of a dirty IAC. Bad ones will stall when coming to a stop and be hard starting also.


Finally had a chance to clean the IAC today & now she purrs like a kitten.

Thanks Padgett!

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