Causes of rough idle?

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nedfunnell

Senior member
Nov 14, 2009
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I drive a 1990 Toyota Camry with a V6. 160k miles. On my way to the airport 2 days ago (bad timing), it started idling rough. When under way, it sounds and feels normal, but at idle the engine runs rough and the speed blips up and down quickly. It feels like maybe it is not running on all cylinders, but that's a wild guess. I had two cylinders go out on a different V6 car once due to a coil pack dying, and it sounded way worse, so I'm guessing it is only one cylinder out if that's the case. Also, oddly, I hear -no- difference from normal at speed. Sounds absolutely identical to before the problem.

It started at least once normally after the problem emerged.

The car is waiting for me in an airport parking lot an hour from home tonight. What should I check first?

Thoughts: arc tracing on distributor cap, borderline spark plug, bad/clogged injector
 

exdeath

Lifer
Jan 29, 2004
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Vacuum leak, idle valve, open EGR, etc, are all things that will affect idle but be insignificant (problem goes away and engine smooths out) when the engine is revved or operating at normal driving speeds and during acceleration. Idle hunting is usually a clear sign of a vacuum leak or slow responding idle control valve.

Bad ignition components will also manifest as cruise and acceleration hiccups as well but it's never a bad idea to replace plugs/wires/cap/rotor if they are old and you observe excess erosion or arcing. Arcing in the distributor cap is a sign of too high resistance in the plugs/wires.

Just recently, for example, the cause on a brother-in-law's Saturn was a $20 intake manifold gasket on a 1.9L (vacuum leak on cyl 1 causing a lean misfire at idle). Problem found by spraying carb cleaner around intake sealing surfaces and listening for the rough idle to smooth out.
 
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natto fire

Diamond Member
Jan 4, 2000
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Most commonly is the O2 sensor.
Not sure about the newest cars, but all the ones I have worked on ignore the O2 sensor at idle.

I would check the idle air control valve, and for vacuum leaks as well. Do you have A/C? If so try turning that on and listening to the idle. Personally I would pull the IAC valve regardless and clean it with carb cleaner, there will definitely be some carbon buildup.
 

nedfunnell

Senior member
Nov 14, 2009
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Thanks for the info, all. It started, idled, and drove normally tonight. Not sure what the story is on this. I've heard that some ignition issues only happen during higher humidity, so maybe that's a factor. Anyway, some more info:

Reasonably new plugs and wires (judging by that they're clean and bright, installed by/for the PO). Cap looks newish as well.

EGR valve bypassed- vacuum line plugged. It's been running fine this way for half a year, situation normal. I'll check to see that the plug hasn't fallen out, though. Is EGR usually active at idle?

Haven't checked the IAC at all, but I will.

I looked over the vacuum system a few weeks ago just out of curiousity, and I didn't see anything abnormal.
 

jlee

Lifer
Sep 12, 2001
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It could've been moisture in the distributor cap - would explain why it cleared itself up in time.
 

fanex

Junior Member
Aug 14, 2015
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there is a good chance the vibration isnt coming from the engine itself. have the engine running and examine the exhaust system right from the beginning to the end. i found out the cause of my rough idle was my exhaust pipe touching the and vibrating against the chassis in the mid section and this happened when the car scrapped the road. good luck. toyota camry


fanex... Might be a good time to check out the newer threads and leave the old ones alone...

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