Runs rough at low idle or go slow - no bad codes showing! - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 08-09-2003, 03:40 PM   #1
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Default Runs rough at low idle or go slow - no bad codes showing!

Hi'all. New guy here! I have a '91 SSE with the 3.8L . I am curious what feedback you can give me on an engine issue.

Starting the car is fine. It idles fine. The problem is that when you put it in Go, and slowly pull away - the engine either runs really rough, misses or sputters, runs poorly... until you give it some pedal. It WOT just fine, it goes under power little or big just fine. But when pulling away slowly, or say using cruise control and going to coast/small accel... I get this rough engine run.

No codes show on diagnostics, just 12...12...12...12 forever. What gives?

Help
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Old 08-09-2003, 06:28 PM   #2
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First I would clean the IAC. Is relatively simple but has a major effect of slow speed/idle operation & gummed up IAC will not set any codes. After that I'd clean the entire throttle body. Does it tend to stall when coming to a stop ? Does it act differently when cold than hot ?
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Old 08-09-2003, 06:37 PM   #3
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I'm not too sure what that is. But I agree with padgett, cleaning the IAC would be a good place to start. Does it make a ticking sound at all? I've got a 91 SSE too. Don't have any pics yet though.
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Old 08-10-2003, 05:15 PM   #4
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That sounds exactly like my problem, although i'm getting a code 34 low maf sensor voltage.. meaning either my maf sensor is messed or there is a vacuum leak..
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Old 08-12-2003, 08:18 PM   #5
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Default Re:Runs rough at low idle or go slow - no bad codes showing!

:o Thanks to everyone, I am planning to clean the upper assemblies, throtle body, combustion chamber, sensors etc...

I'll let you'all know if that fixes it!

Regards,
Muddtank Mark Christy
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Old 08-17-2003, 07:49 PM   #6
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Default Cleaned IAC, still runs rough - but different.

Cleaned IAC, still runs rough - but different

Took the MAF and the air tube off, found that the two tiny screws that hold the IAC in had RED LOCKTITE on them , which made it almost impossible to get them off, but brute strength and the right size std screwdriver won out, but it was close :o

I cleaned the carbon off the pintle of the IAC, lubed the little spring some and wiped out the cavity where it goes. Very Black in there!

At that time I also thought I'd remove that funny screen...


I do believe that at WOT, the screen gone made some noticable diff but it also caused it to almost die during idle/stopping, and then the OBD would surge the idle up to 1500 in an attempt to correct the rough rough idle. So back in went the screen!

Next, I tried driving slow with the cleaned up IAC. Here'* the result. It idles just fine in park/stop. If you do a stop-1500rpm startout, it runs fine. If you do a stop-2000rpm starout (means no speed to immeadiate rpm value with throttle but still slow accell), it'* fine. But... if you start at a dead stop, and feed it a tiny bit'o throttle, it runs rough right between 1500-2000 rpm, AAMOF, right at 1750rpm-1900rpm. Chugga-chugga... I don't get this!

So, now I go to the Haynes book...


I didn't check this dimension, so I'll have to take it apart again, measure this setting and put it back. BTW, a new IAC is about $85-95!!! So, I'll just have to get along with it doing this until I find a job again!

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Old 08-17-2003, 10:24 PM   #7
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Default Coils plugs wires and voodoo

I'll check that out. It seems odd that it would only have this problem at a specific RPM. Wouldn't a failing coil or wire or plug (just put new plugs in about 3 m ago) cause an overall bad engine feel, especially at WOT?

Brainstormin now! :(
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Old 08-17-2003, 11:00 PM   #8
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having the same prob...went to the junkyard today to get a new MAF, 12 bonnevilles there all with pulled out MAFs, so im starting to think, hey this is a common problem...
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Old 08-18-2003, 09:19 PM   #9
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Possibly the TPS has a dead spot in it if it happens at the same throttle angle every time.
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Old 08-19-2003, 01:33 AM   #10
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Default Old IAC goes SPROINGGGG, buy/install new one, now it'* WORSE

Bad day . I dropped the O-Ring down into the netherworlds, couldn't find it, so I had to go find more O-Rings. While I was showing the guy the IAC, it went SPROINGGGG

On the threaded part was some material, a little spiral of brittle stuff that didn't look good ... so I decided to buy a new one. I talked the parts store down to $70, because I could get on online for that.

Got home and read the instructions, this is where I get pissed off. For a short story, go to this link on my web site, it will explain it better than I...

http://www.muddtanks.com/bonneville_...tm#HAYNES_SAYS

So, I installed the new one after NOT adjusting the new one at all, it was NOT sticking out farther than 1.125", so in it went. Now it stumbles at ALL RPMS, unless I am acccellerating really hard. I even heard some pinging, which means too much air or a lean mix.

HEEEEELLLPPPPPPPPPPPP I just need to know is HAYNES correct, and I should make it longer, or is the part mfgr right? If anyone has the real shop manuals, please let me know at [email protected]
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