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3.8L 3 series ghost misfire

B

B_lron

New member
I'm brand new to this forum and frankly don't know if this is the right place for this, but I digress.

My 08 grand prix base has been troubled with a random misfire for years now, usually highway in 4th at 2-3k rpm. That being said I can also force a check engine light by selecting 2nd gear and cruising around for a bit. Mostly concentrated in cylinders 4 and 6, least in 2 and 5. I've replaced a cracked pcv elbow, plugs (gapped at 0.55), coils, icm, leads and MAF. What stands out the most to me are my fuel trims, short term floats around 0 but my long term is consistently negative. (about -7%) Replacing spark leads took counts down from thousands to hundreds though i'm struggling to get it lower than that. These misfire events are very occasional and will happen maybe once or twice on a 15-30 minute highway drive. I should also include that aside from a rough cold idle there are no physical symptoms you'd usually expect from hundreds of misfires. No knocking, jerking, loss of power or hesitation.

Any help would be greatly appreciated as i've been trapped by this for weeks.

Thanks
 
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What codes is it throwing? That would help us figure it out.

Beyond that, what brand of coils did you install? If it's anything but OEM then I'd get rid of them. The stock coils are best on these cars. What type/brand of spark plugs did you install? Copper, Irridium...? Have you ever replaced the 2 oxygen sensors? Is the catalytic converter factory? How many miles on the engine?
 
FordMan77 said:
What codes is it throwing? That would help us figure it out.

Beyond that, what brand of coils did you install? If it's anything but OEM then I'd get rid of them. The stock coils are best on these cars. What type/brand of spark plugs did you install? Copper, Irridium...? Have you ever replaced the 2 oxygen sensors? Is the catalytic converter factory? How many miles on the engine?
Click to expand...
p0300, 2 coils were dead, a-premium coils, ac delco iridium plugs, haven't touched the oxygen sensors, factory cat and it's got about 244,000km on it so about 151,000 miles
 
Yep, I'd clean up the MAF and TB for sure. Read up on how to clean both of those as I'm not sure how to clean the electronic TB's on these cars (mine has a cable still). That rough cold idle shouldn't exist. I can go out in freezing temps and fire my GP up and it idles just as smooth as when it's 90* out. You very well could have a vac. leak somewhere.
 


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