rough idle - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 09-26-2006, 10:06 PM   #1
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Default rough idle

Seems to be mostly after start-up (I only notice it then)...I'll let it run for 3 minutes or so. Idle fluctuates, the pitch of the idle goes up, goes down (as do rpms). Not steady at all. It'* done this for a while, but today I really sat quiet and listened as it was running. It'* so far from 'smooth.' I know it can be better.

I'm guessing it could be a few things, one being a cruddy TB (which i will be cleaning this weekend).

My EGR tube WAS cracked but I replaced it with a brand new one. I thought that would help my idle but no change yet, and no vac leaks at all from what I can tell.

Ideas?
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Old 09-26-2006, 10:08 PM   #2
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If it'* not another vac leak (you know how to find 'em), it'* likely your dirty MAF sensor, or even more likely, the IAC plunger tip or seat in the TB.

The IAC meters the air needed to idle at a particular RPM, and when it gets crudded up, it can't fully seat without leaks. If you already have that planned, that'll probably do it.
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Old 09-26-2006, 10:25 PM   #3
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Quote:
Originally Posted by willwren
If it'* not another vac leak (you know how to find 'em), it'* likely your dirty MAF sensor, or even more likely, the IAC plunger tip or seat in the TB.

The IAC meters the air needed to idle at a particular RPM, and when it gets crudded up, it can't fully seat without leaks. If you already have that planned, that'll probably do it.
IAC was the other thing I was thinking actually. Since I didn't clean that last weekend (as you explained how over the phone) when I did the TB while it was on the car. I could not take the IAC off the TB due to the angle and not the right tools to reach the screws. I cleaned the MAF sensor a few months ago so that should be ok.

Once the car is warmed up completely the idle is pretty good. I'll see how things go after the TB cleaning.
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Old 09-26-2006, 10:35 PM   #4
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My 99 was acting the same way a few months back. Turned out to be a combo of the IAC and the MAF, both of wich needed to be replaced in my case, but maybe you can get by with a cleaning. Is you Check Engine light on? Mine was throwing a code for the MAF sensor at the time.
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Old 09-26-2006, 10:54 PM   #5
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Mine hasn't thrown any codes, no SES light.

How did you know you had to replace both and not just clean one or the other? Do you remember prices for them?

Thanks guys, I'm pretty sure it'* the IAC/dirty TB now. Can't wait to get this thing clean.
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Old 09-26-2006, 11:16 PM   #6
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Tried cleaning MAF, no change, Cleaned IAC, slight change.

Replaced IAC, big change but still not right. (it was at this point the SES light started coming on scanned it, showed codeP0102 Mass Air Flow Sensor Circuit Low Frequency) Replaced MAF, purs like a kitten

Just for *&Gs I put the old IAC back in with the NEW MAF and it still ran rough, long story short she ran like crap unless BOTH new sensors were in.

As far as prices, I don't remember, I'd have to look.

If yours is not throwing a code I'd lean towards IAC, but don't qoute me on that......
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Old 09-26-2006, 11:42 PM   #7
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Quote:
Originally Posted by Jack
Tried cleaning MAF, no change, Cleaned IAC, slight change.

Replaced IAC, big change but still not right. (it was at this point the SES light started coming on scanned it, showed codeP0102 Mass Air Flow Sensor Circuit Low Frequency) Replaced MAF, purs like a kitten

Just for *&Gs I put the old IAC back in with the NEW MAF and it still ran rough, long story short she ran like crap unless BOTH new sensors were in.

As far as prices, I don't remember, I'd have to look.

If yours is not throwing a code I'd lean towards IAC, but don't qoute me on that......
I'm betting my IAC is just cruddy. I've only had history codes but still nothing involving the MAF.
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Old 09-27-2006, 03:10 AM   #8
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I put the Chinese water torture and Seafoam treatement to my 92 with 165k miles. Idle smoothed right up and gas milage went up considerably. For those who are not aware this is a GM approved technique for removing carbon from the upper engine. It can solve the rough idle which remains even after you have done all of the other high milage maintenance. I can post a link elaborating on the technique if interested.
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Old 09-27-2006, 08:50 AM   #9
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Jack.. that'* waht I call a carboned up sensor.. wow.. Did you clean the IAC passage as well?

Pete... Try the cleaning before you buy anything.

Drew... We actually have the procedure documented.. er somewhere.... yeah..prolly techinfo. Do you have a nice write up with pics etc, if so..please forward to me.
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Old 09-27-2006, 02:53 PM   #10
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I'm definitely cleaning it first thing Saturday, before or after I do the brakes.

I have been wondering if I should do the seafoam too. But I'll do the TB cleaning first. My gas mileage is really good now, so it may not be that dirty inside. I don't know what the best stuff is to buy though either. My CAT is probably clogged as it is, so I may as well do the seafoam sometime and replace the CAT afterwards.
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