Misses at low RPM in overdrive - 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 05-08-2003, 07:05 PM   #1
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Default Misses at low RPM in overdrive

Hi All,

I have a NON supercharged SSE that does this. It usually only happens at low RPM in overdrive.

Maintaining speed is usually ok, but going up a hill or under acceleration, the car stutters or lurches every half second or so, the harder I put my foot on the gas, the more it does it.

It doesnt matter if the AC is on or off.

My mechanic has replaced the fuel filters, fuel pump, plugs, plug wires, swapped out the ignition "computer" thats controls the plugs firing.

My mechanic seems to be out of ideas. He says there are no codes, no check engine light or anything else showing up.

It runs normally and has OK power still under hard acceleration or in low gear at high RPM
.
It does idle a bit rougher than it used to.

Oh one more thing, my gas mileage used to be 30Mpg on the highway, now I get about 22Mpg.

Any ideas ?

Thanks,

Andrew
92 SSE
97 SSEi
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Old 05-08-2003, 08:07 PM   #2
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Default One other thing....

Mine also gets worse when the car gets hot or in warmer weather.

When cold, it behaves perfect. On a cold day, it wont start doing this until I have been driving for about 15 minutes. On a hot day, it starts after a few minutes.

The other night, it was warm here (Indiana) but very humid with fog and it didnt stutter at all, even after driving around town for 30 mins.

It seems like moisture or humidity could affect this. Could it be an O2 sensor, EGR valve or PCV valve ?

I've done the usual plugs, wires, fuel pump/filters ect.
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Old 05-09-2003, 12:48 PM   #3
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Thanks for the great questions

I dont know the brand the Mech put in, but I think it was either NGK or Champion ? I bought Bosch Platinum plugs and new premium wires about 6 months ago, and when I took it to my mech for him to fix this problem, he changed my Bosch plugs for another brand, saying that Bosch and Pontiacs dont mix well.

****Your mechanic replaced the module on your car??? Why??

Yes, there is a module under 3 other modules where the plug wires go in. Are those 3 modules the coils ? He changed the module and did not change the 3 other modules (coils i guess). No change.

He told me his scanner did not throw any codes. I told him to check the O2 sensor, and he said his computer scanner said the O2 sensor was behaving normally and that the model of O2 sensor I have should last forever and never need changing ? Sounds suspect ??

I have never had the injectors cleaned. A previous mechanic said that an injector clean can do more harm than good, and generally a good injector cleaner in your gas tank works better. Is that true ? Should I shell out the $80 or so and get my injectors cleaned ?

I dont know anything about the MAF sensor. Would that show up on a scan if it was bad ?

The car does idle a bit rough, but not real bad. It used to be so smooth. It does have less power than it used to, and does only get 22Mpg on the highway now. It used to get 30 !

I'm not after a performance beast, just a nice car that runs good on the highway.

Thanks for welcoming me, you guys rock ! Now I have 2 Bonnie'*, I plan on staying here and hopefully contributing and helping others as well.

Cheers,

Andrew
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97 SSEi
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Old 05-09-2003, 01:40 PM   #4
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Default

You may have already considered this, but you have the EGRless engine which is prone to burning a valve. Have a compression check and leak down done on your car.

This would affect fuel economy, idle, and smooth engine operation at low RPMs. It is most noticeable in OD--when the TCC is engaged--thus letting you "feel" every miss of the engine more than when the tranny is slipping (not in OD).

The other possibility for fuel economy is the O2 censor. Whether it is completely gone or just on the way out, it will affect your economy. It may or may not send a code if it is still functioning but is on the way out.

Good luck.
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Old 05-09-2003, 02:06 PM   #5
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Great answer - thanks.

I will get that checked out. I will change the O2 sensor, and check the compression. Is there anything else I should look for to see if I have a burnt valve, or is a compression check enough ?

Thanks


Andrew
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Old 05-09-2003, 02:12 PM   #6
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A compression check will tell the story on the valve, no problem.

You may also want to clean the throttle body thoroughly. Sorry to say it, but on a NA 92, the valves are the most likely culprit. Deathrat is a victim of this too.
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Old 05-09-2003, 02:30 PM   #7
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Thanks, I will do the compression check and see the results.

If it is a burnt valve, is that going to be expensive ? And can I do the throttle body clean myself or is it a job best left for the 'experts'?

Thanks for the great forum, I am learning tons about these cars.

As an ex Aussie, I used to do a lot of my own work on my 1972 Chrysler Charger Coupe, but those cars were so easy to work on. Now I am just totally lost when it comes to fixing complex American cars

Cheers,

Andrew

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Old 05-10-2003, 05:28 PM   #8
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Along-

I had the same problem with my '92 SSE. I went straight to the plug wires and plugs as neither had been replaced in 150,000 miles (Actually my Bosch Platinum plugs still looked new). Well I made the mistake of using new AC plugs. Still had that misfire type problem in OD. On a hunch, I pulled the new plugs and looked at each one very carefully, especially the one closest to the header to exhaust pipe, rear of engine (that one would probably run the hottest). Well, that one nearest the crossover was a little brown, so I replaced it. Guess what? Problem cleared up. One of those brand new AC'* was defective. First time I've experienced that.

Have your mechanic pull those plugs again. A bad new plug is very rare but I proved it can happen.
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Old 05-12-2003, 02:52 PM   #9
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Thanks Ted.

I am going to change the O2 sensor, and do a compression check.

I really dont think its the plugs. I had this problem for a while, changed the plugs and wires, no change, my mechanic changed the plugs again, no change.

I am starting to think its a burnt valve and/or oxygen sensor. (This car has been overheated before a long time ago, maybe that burnt a valve)

Cheers,

Andrew

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