Reoccurring P0304 Misfire - 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 08-19-2009, 03:39 PM   #1
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Default Reoccurring P0304 Misfire

My 99 SSEI is popping up this code when driving under load after warmed up. About a month ago the light came on, and it had 2 misfire codes and an EGR code. I found a loose plug wire (different cyl), replaced all the spark plugs and did nothing to the EGR. I read that the EGR can cause the misfire code, so I wanted to see if it would come back. It worked well for a few weeks around town, but once warmed up on the highway at speed I can get the light to come on again. It'* only the misfire code though, everytime.

Only happens when going uphill, or passing someone, feels like the car is surging, or lockup torque converter is trying to unlock. Just overall lack of power. If you step it down, you can feel the supercharger kick in, the boost comes up, but the car is fighting it the entire time. Feels like a cylinder dropping, or getting flooded out the entire time you're on it. About now is when the CEL starts flashing and sets the code. Let off, and everything is fine again. The car has no problems when cold and is a joy to drive.

I Don't think the EGR has been touched, the car has 200k on it, and I try to keep up on the routine maintenance. The wires aren't due to be replaced, but nearing the end of their life. Delco wires with NGK copper plugs, and new fuel filter earlier this year. This is my third set of NGK through the car and first misfire issue, so I don't think it'* that. Anyone have a post on cleaning/inspecting an EGR? Seems odd that the EGR could be the culprit without triggering a code though.
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Old 08-19-2009, 03:52 PM   #2
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Because it'* happening only when hot I would suspect a coil. Try witching them around and see if it follows that coil. If not that leaves the wire, the plug, or the injector. Also check the plug wire to make sure #4 is not to close to the O2 sensor wire.
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Old 08-19-2009, 06:32 PM   #3
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I'd replace the spark plug wires. I just had a similar problem with a 96 SE thought it was the torque converter. bad wires only 2 yrs. old.

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Old 08-19-2009, 10:43 PM   #4
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I swapped the coils and tried to reproduce the problem. Seemed to run fairly well until WOT. The car would accelerate well but it hesitated 3 times on the way to redline. I was expecting the Check engine light to come on, but it never did. It ran differently so I think it could be the coil. The 1-4 coil had been replaced at some time as it did not have numbers on it. Could be the #4 wire has enough extra resistance to cause the misfire. The wires have about 50K on them and 4yrs old so I'll probably replace them once I test them. My FSM'* are packed up in storage, anyone have specs for testing the coil packs?
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Old 08-19-2009, 10:44 PM   #5
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Oh yeah, and thanks for the replys guys. Much appreciated!
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Old 08-19-2009, 10:45 PM   #6
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Old 08-20-2009, 03:16 PM   #7
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I JUST WENT THROUGH THE SAME PROBLEM YESTERDAY! mY WIRES LOOKED GOOD BUT I CHANGED THEM ANYWAY AND PROBLEM SOLVED
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