UPDATE - Multiple Cylinder Miss/Running Lean/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 04-22-2005, 09:53 AM   #1
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Default UPDATE - Multiple Cylinder Miss/Running Lean/Rough Idle

Well

After replacing plugs and wires that didn't really need to be done but I did just in case (no Bosch crap), I found an oil soaked PCV Valve with a torn part of the MAP sensor grommet stuck inside. The MAP itself was soaked in oil. I had noticed previously that removing the oil cap while the car is running would create a HUGE suction, and practically kill the car, so I figured that was a part of the problem somewheres.

I replaced the PCV and MAP with genuine AC Delco goodies, that fit better then the junk on there before. Now, no vacuum in the manifold.

This morning I ran an AutoTap in my way into work. No codes this morning. Yesterday I was getting a P0300 Multiple Misfiring Detected. It'* running A LITTLE better today. On the highway, it was PERFECT. RPMS stayed just about constant, wasn't running especially lean at all, or rich, NO misfiring. As soon as I am anywhere below 2500 RPM, I get a mess of misfiring all over the board.

A friend said it might be the fuel pressure regulator, or pump- any thoughts?
The guy I was talking to about all this where I bought the parts said he'* seeing an increase in bad injectors here, because of our reformulated gas- any thoughts?

I'm wondering still if it'* ignition, like the ICM? There is almost a pattern to the misfiring.

I need this fixed! Any gurus with any words of wisdom? Anybody?? = )
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Old 04-23-2005, 11:07 AM   #2
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It'* a vacuum leak. Look for one.

Almost all idle and low speed running rough problems can be traced to vacuum leaks. Write that on your palm and get busy.
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Old 04-25-2005, 09:25 AM   #3
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I believe I've checked out all the lines, and they pass... I think...

In front of the manfiold, down to the canister, EGR valve, where else would I look?

Could it be a vacuum leak from a gap between the TB and the UIM? (Aside from the obvious reasons why this might not be a good thing to have)
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Old 04-25-2005, 10:19 AM   #4
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Look up on the firewall between the maxifuse relay center and the windshield wiper assembly. There'* a black box that looks like a continuation of the maxifuse cover. That'* a vacuum accumulator, and the fittings in it are often leaking or broken. Many people overlook this.

Also make sure your O-rings are properly installed on your PCV valve.
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Old 05-05-2005, 12:40 PM   #5
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O-rings? I thought there was only one.

I think my '92 SSE */c has a similar problem. It'* had a rough idle (with steady RPMs at 700 or so) since I bougt it used, but it has no error codes, and I've checked/replaced all vacuum lines (including the ones near the firewall and to the fuel evaporator canister), performed tune-ups, cleaned the throttle body, replaced the PVC valve, IAC motor, TPS and MAP sensors, tranny mount, catalytic converter, and the fuel filter. I checked the coil resistance and it checked out ok. Other than the rough idle problem, it runs great and has never stalled, although sometimes it feels like it'* coming close to stalling. Fuel economy is good.

I wonder if a rough idle could be caused by the fuel pressure regulator or injectors, the fuel pump (or maybe the sock in the fuel tank is floating around as other '92 owners have reported) or maybe there'* an EGR or exhaust leak. The engine was completely rebuilt shortly before I bought it.

Any ideas?
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Old 05-05-2005, 01:07 PM   #6
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billha sounds very similiar to my problem

Mine too often feels as if it'* going to stall. Somebody I've talked to keeps suggesting something fuel related, like the injectors, or the regulator. My scan tool doesn't appear to be showing a problem, but I'm not sure.

EGR or exhaust I'm still considering too. The only other thing is just something internal engine related, mechanical problem.

If I were you I'd check grounds yet, plugs, wires, if you haven't. Anything else weird you might have to report? My plugs looked just rather white, dry, no oil. No coolant in the oil or anything like that. Oh, consider it might be ICM or PCM, I'd try swapping one of either in if I had it.

There are two rings for the PCV valve on the Series II, I dunno about the Series I SC. On mine there is a small washer around the "middle" of the PCV valve itself and a large O ring around the plastic retainer that screws on top of the spring and PCV assembly.
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Old 05-05-2005, 01:14 PM   #7
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How white is white on your plugs? That shows lean condition. Tan and all is good.
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Old 05-05-2005, 01:24 PM   #8
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My scan tool is already indicating an excessively lean condition, no code, but 14.7 long term fuel trim... and the plugs were white white
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