misfire under load - 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 05-06-2003, 12:17 AM   #1
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Default misfire under load

I got one for the techies out there. I have a '93 bonneville supercharged. It misfires under load. This starts to occut at 2500 rpm, or when the boost gauge hits zero (on its way up). It occurs when driving or if I just break torque. It'* not very severe but it is definately there. In my search to fix the problem I have replaced the following things which I found to be bad - cannister purge valve (wouldn't hold vacuum), spark plugs (worn out), plug wires (I broke one when changing the plugs, so I changed the whole set), ignition coils (secondary resistance was good on all three, but all had cracks in the housing). I even tried a new ignition module, but that had no effect. Fuel pressure is correct and the resistance on all the injectors is in spec.

Anyone have any suggestions?
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Old 05-06-2003, 12:27 AM   #2
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I've asked this question here once before... Does the fuel filter have an effect on fuel pressure, or does it only restrict flow under a load? (That Q is for everyone). Could it be that?

Is it missing at a certain throttle angle point (but not above it?) or does it happen above a certain point?

Also, a Tech I would help you to find the weak cylinder number, so you could isolate the problem.
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Old 05-06-2003, 12:52 AM   #3
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Default I have the same problem

I have the same problems .
I have a 92 ssei.
I have spent hours looking at posts.
It looks like a lot of 92 and 93 have this same problem.
Ive had a scan tool, Replaced so many things. Still the same problem.
Will go into Much greater detail soon and yes this happens in 3 gear to.
(Replaced the fuel filter to.)
Ive been living with this for over a year.
Its got to be the same problem for most of us.
Brett
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Old 05-06-2003, 01:44 AM   #4
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Car starts fine, Idle is fine,Runs good until load and boost and low rpms
but can get the problem in 3 going up a hill but a lot harder to do.
Injectors rebuilt and tested,
new coils,icm(used),maf,map,fuel filter,nose cone,sparks plugs(2 sets),wires,
orderedO2(comming soon, Delco. I dont think this will help).
No back pressure problem.
?
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Old 05-06-2003, 11:15 AM   #5
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Thanks for the suggestions guys. I gotta change the fuel filter, although I don't think that'* the problem as I do change it on a regular basis. What really intrigues me is jr's3800 comments. I have long suspected the injectors of being bad as I have been living with the same idle and starting problems he talks about for years. However at 80 bucks a pop (about $500 for the set ), I was willing to live with it. I'm not brave enough to go the junkyard route for the injectors, I'd hate to spend money on bad/worn out parts. I think I'll start by pulling the injectors and see what they look like, see if they're black like jr's3800 were. I'll keep you guys updated on what happens, although it may be a couple days before I get to it.

UserBrett - I wouldn't recommend rebuilding injectors. I believe all they can do is clean them and check the flow pattern. They can't take them apart as the housing is injection molded around the components. If there is an internal problem it'* likely the rebuilder won't catch it. I suggest trying new injectors.

This train of thought has reminded me of something I heard about GM injectors. GM used gasoline to cool the wiring in the injector. However the gasoline attacked the wiring and the injector would degrade over time. I think this only applies to cars from the early 90'*. Maybe someone out there has heard something about this?
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Old 05-06-2003, 08:10 PM   #6
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Well I pulled the injectors out. They all have a black plastic "cap" on the nozzle end! Although if I look closely I can see carbon deposits on the caps. The steel nozzle plate with the four holes are clean on all of them though. Were you saying the cap was black or the nozzle plate was black on your car?
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Old 05-07-2003, 01:33 AM   #7
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Hmmm... I rechecked the spark plugs I just replaced. Three of them seemed to have pretty heavy carbon buildup on them, for only having about an hour of run time on them. Maybe those injectors are bad...

You probably should fix your buddy'* car. Sounds like he'* got the same type of problem I do - something that will take a lot of time and effort to figure out what is wrong. A dealer won't want to do that - won't make a profit if his mechanic spends too much time on it.
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Old 05-07-2003, 02:15 AM   #8
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Well I use AC Delco, I forget the specific callout but they should be the stock ones. They are gapped to 0.060". Plugs # 2 and 5 were fouled and I thought Ahah! those run off the same coil. But then I found #6 to be fouled too...
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Old 05-07-2003, 02:20 AM   #9
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I've been noticing a loss of power and have wondered if I have had misfiring. I'm far from an expert on this so can someone explain exactly what I would experience if this was happening. I will be changing my plugs to ACDelco Rapidfire (I have Bosch now and I'm sure they're shot). I am changing my wires to Taylor 8mm (I know my wires are very old and were not changed anytime in the recent past before I got the car), and have already changed my fuel filter. Last time my car drove like this I changed the plugs and it drove fine. I'm hoping it will be better after I change the plugs and wires this time.

I also thought my injectors may be bad, but will check later if the wires and plugs don't do the trick. I know injectors are very expensive so hope they're fine, but I doubt it. My tranny might be slipping a bit too.

In the meantime, can someone tell me exactly what a misfire would be like so I'd know if this is what I have. Excuse me for not knowing as much. Still learning. Thanks a lot.
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Old 05-07-2003, 04:04 PM   #10
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Boy that'* hard to describe without being able to mimic the sound with my mouth. It'* different from a knock (pre-ignition). It'* kind of like the engine starts stuttering. What'* happening is one of the plugs is not firing when it is supposed to. You can feel/hear it each time it misses.
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