engine misfire only on uphill 1995 Pontiac Bonneville 3.8 series II SE - 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 02-17-2014, 11:11 AM   #1
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Default engine misfire only on uphill 1995 Pontiac Bonneville 3.8 series II SE

Hi,
last time I communicated I had trans problems, since then I found out that my eprom was for SEI. Located a used eprom for my engine and replaced my computer, which had turned to crap, with a new one. Knowing that I probably would still have trans problems I took the car for a drive only to find a strange juttering and hesitation which seemed to be coming from the front right wheel. A reputable diagnostician drove the car and said that he thought I had an ABS problem. I pulled the ABS fuse and the gas mileage went from 17mpg highway to 24 and the severe hesitation that had been occurring went away. Everything was fine with the exception of the 'Check engine' light which I thought was probably the tranny problem. Then the misfire started when engine at operating temp and under stress (going up hill, turning when going from stop to go, and or pressing down on the throttle). This condition doesn't last very long and then everything returns to normal. My first thought was that there was water in the fuel and I treated it with HEET, several times but the engine misfire remains.
Any and all ideas would be appreciated.
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Old 02-17-2014, 12:01 PM   #2
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When was the last tune up?
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Old 02-17-2014, 12:09 PM   #3
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Good question, needs to be done - can I do it myself?
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Old 02-17-2014, 12:30 PM   #4
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Very easy. Spark plug socket, a ratchet and new plugs and wires. Done.

I suggest removing 1 sparkplug at a time and replacing. Sometimes its very easy to mix the 6 wires up and get them crossed around. But if you replace them one at a time, its avoidable.
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Old 02-17-2014, 01:25 PM   #5
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Did that two years ago; how often should it be done?
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Old 02-17-2014, 01:52 PM   #6
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depends what you bought and how many miles that is, cheaper wires and plugs likely only last a couple years. another thing to check is corrosion on the coil and icm terminals, they make it feel the same as worn plugs/wires
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Old 02-17-2014, 02:32 PM   #7
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Thanks Justin, I'll do that!
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Old 02-17-2014, 03:40 PM   #8
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Good tip on the corrosion Justin, I removed my ICM, and coil packs a while back, and had a bit of corrosion there, so I used a little brass wire brush and removed it.
If you replace the plugs and wires, I'd also get some MAF sensor cleaner, remove the MAF and clean it, then give it another clean every time you replace your air filter.

Is your misfire only on one cylinder, or is it a random cylinder misfire?
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Old 02-18-2014, 07:29 AM   #9
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Thanks William, but I need a little help on the location of the ICM, and coil packs. I assume that the coil packs are where the plug wires originate, in my case in the front of the engine, but where is the ICM? I know, or think I know where the MAF is located. Is MAF cleaner like Gumout, carb cleaner?
The misfire seems to be random. There is sometimes a sneeze, like backfire thru the intake. The misfire clears after about half a city block and has good power; sometimes returns and sometimes doesn't. I am going to work on the car Wednesday as temps will be around 70.
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Old 02-18-2014, 07:33 AM   #10
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use only maf cleaner to clean it, carb cleaner leaves a residue. the icm is under the coils, you take the little bolts out and lift them up and hit the tabs with some scotch brite or 400g sand paper. should put dielectric grease back on the terminals of the icm and coils
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