6th Cylinder Inconsistent Misfire, Help Please - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 10-11-2015, 07:43 PM   #1
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Default 6th Cylinder Inconsistent Misfire, Help Please

My 2002 Bonneville started running rough this afternoon. TheCEL started flashing. I turned around to go home to check the code and lightwent out. At intersections you could feel the misfire but no CEL light. When youaccelerate from the stop the engine runs rough and the light would start flashingagain. When you let off the gas at cruising speed (40mph +/-5) the engine wouldsmooth out again and the CEL would go out.
With the CEL on I checked the code and got p0306. The sparkplugs are new, changed the wires with the plugs, replaced the upper intake andlower gaskets a month ago, while I had them out I soaked the ends of the fuelinjectors in gas and brushed lightly with a soft toothbrush to remove debris. Carran great for the last month. About a week ago I started feeling like it wasgetting a little sluggish accelerating. I wasnít sure if it was me or the caruntil today.
Today I checked the wire connections and the length of the#6 spark plug wire and found nothing. I tried swapping the coil pack to see ifthe misfire would follow, it didnít still getting intermittent P0306 code.
I am stumped!! The misfire is not constant. It is most noticeableat idle, worse when engine is cold, noticeable when accelerating, pretty muchdisappears at cruising speed. When it is parked and idling it will run smoothfor 20-30 seconds then misfire once, run several more seconds and misfire twoor three times. The time intervals and number of misfires is not consistent.
Please help Iíve got to keep this car running. I can barelyafford the parts let alone replacing the car.
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Old 10-15-2015, 02:34 PM   #2
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my engish no so goods much looket at yous
question with nos solution
nos ones here nos how to fix the auto for yous?
them needs the auto repair classses
ones your new part is possable no goods
whats you thinks
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Old 10-15-2015, 06:29 PM   #3
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You should test the Ignition control module as well as inspect the wiring that bolts to the front of the ICM, The ICM is the "plate" that the coil packs are bolted to..Most local parts stores can test it for you Also you could switch the plug wire with that of another cylinder and see if the misfire follows.Sometimes they are not visibly damaged
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Old 10-16-2015, 09:01 AM   #4
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You should test the Ignition control module as well as inspect the wiring that bolts to the front of the ICM, The ICM is the "plate" that the coil packs are bolted to..Most local parts stores can test it for you Also you could switch the plug wire with that of another cylinder and see if the misfire follows.Sometimes they are not visibly damaged
thanks for responding. Since May a national tire place has installed 4 ICM'* on my car. luckily I only had to pay for the first one. when they went before I got a p0300 random misfire code. this time around it is specific to the 6th cylinder. I like your suggestion on the switching of the spark plug wire.


I am going to work on the car this weekend. I replaced the intake manifold gaskets last month. because I used a cheap harbor freight torque wrench it is quite possible the bolts near the 6th cylinder have come loose. I have secured the use of a snap-on torque wrench from a friend. I am going to go back and re-torque everything and see if that fixes the problem. If not I will be back to being stumped.
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Old 10-16-2015, 01:38 PM   #5
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You have addressed the obvious potential causes. I would check fuel pressure next. A failing pump would give a random misfire, but since you already did everything else ...

Check for spark on the #6 plug. That will tell you if the issue is spark or fuel. Could also be a mechanical internal issue, but we are not there yet.

In case you have a bad plug, switch #6 with another.

Seems to me the #6 wire runs near the front O2 sensor. Make sure it is not rubbing
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Old 10-16-2015, 02:39 PM   #6
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You have addressed the obvious potential causes. I would check fuel pressure next. A failing pump would give a random misfire, but since you already did everything else ...

Check for spark on the #6 plug. That will tell you if the issue is spark or fuel. Could also be a mechanical internal issue, but we are not there yet.

In case you have a bad plug, switch #6 with another.

Seems to me the #6 wire runs near the front O2 sensor. Make sure it is not rubbing
Good suggestions, Don't even want to think fuel pump. plus it is specific to cylinder 6. If it was the pump wouldn't it be random misfire on all cylinders?


I was thinking possibly a bad injector?


I am going to start with re=torqueing the upper and lower intake bolts with a real torque wrench. if that doesn't do the trick I am going to install a new plug, if I have to go any further than that I will switch #6 plug wire with #4 to see if the problem follows. I think I will also switch fuel injectors from one cylinder to another while I have the fuel rail off.


I'll update my progress sometime Sunday
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