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I Cured A Random Misfire Problem-3800 V6


95oldsVan
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I think this should apply to any V6 GM makes...depending on select years.

Dealing with a random misfire OBDII code of code P0300,and having bad misfires with the 3800 engine,I put new parts everywhere that did not solve the problem.

 

Even out of preventive maint. I replaced stuff(kept old stuff too) that didnt seem to be TOO bad,just Old...was the coil module,coil packs and etc.Everything was old anyways so I justified myself for replacing everything I needed to do.

 

On this particular 3800 engine,the back 3 plugs had metal plug boot heat shields with springs that slipped over the spark plug,plug boots.I always used them,because it was how the car came designed.I decided to remove them this time and the misfire stopped!

 

Going back to the problem,I had checked the plugs and noticed some burn marks around the ceramic white area of the spark plug...didnt know for sure what it was,But assumed it was a misfire spark deposit when seeing it.Removing the metal shield tubes off the 3 rear plug boots gave me a new engine again.I dont know why or for sure it misfired with the metal shields,but somehow,on install they caused a misfire under load.

 

See,before I found the problem, I had new plugs,new wires,new coils and module and the problem persisted.I used GM dielectric grease on the plug boots,and followed every step I always had for a good tune up.Had the injectors rebuilt,flow matched and tested by Rich at http://www.cruzinperformance.com

 

They needed it,and the before and after rebuild specs was just WOW!

 

I even changed the egr,new ecm,new intake man. gaskets(it needed it) and checked and tested everything with a snap on scanner.It still misfired badly under load.Still threw a code 300,random misfire,couldnt track for sure which one it was.Engine would rock back and forth when misfiring.Fuel pressure was in the normal area,didnt lose pressure or leak down,ever.

 

Finally in the end,I found the problem was with the spark plug boot shields.

I would have never guessed it,it happened by chance.Oh before I forget,and Before I threw new parts at the car,I had a GM dealer keep the car for the day and the techs had no idea what was wrong.They wanted to try the GM top engine cleaner treatment,like the seafoam treatment stuff,but I declined...I knew it was not carbon build up on the valves.I had ruled that out with the other work Ive done.

 

What DID lead me to suspect the 3 back plugs was misfiring,was I used a timing light while the engine ran and the front plugs flashed the timing light just fine,as if in proper sequence,but the back plugs would skip and flicker at times.

 

So now the engine runs like a champ and I wanted to pass this info along..sorry its long but needed to let you know I did all the obvious stuff and still had a problem figuring it out.I believe I posted about this a while back in another topic.I thought maybe the crank or cam sensor may be all thats left because the engine passed compression and valve train visual check ups for any broken parts.

 

 

Mods,this 3800 V6 engine is inside my 95 olds mini-van but since GM uses this engine in so many cars,like the GP and others,I hope you leave this here in powertrain so it may help someone have time down the road.So many W cars uses the same engine from GM powertrian division that posting this in Other Cars felt wrong.

I could have lied and said it was a 3800 GP or something but rather keep it all real.Im just glad to share my(long) story...hope it helps someone in the same boat someday.

 

 

 

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Im glad Im giving advice but damn I could have edited and made it alot smaller!

:lol:

 

I can get so long winded at times I dont realize it..just typing and the thoughts flow...but do hope It will eventually help someone in the future with similar problems to be able to narrow down whats wrong quicker.

 

:smile:

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  • 6 years later...
Dealing with a random misfire OBDII code of code P0300,and having bad misfires with the 3800 engine,I put new parts everywhere that did not solve the problem.

hope it helps someone in the same boat someday.

 

THANK YOU!!!! my story is a carbon copy of yours! '95 firebird with 3800 motor. replaced spark plugs, replaced wires, replaced coil packs, replaced ICM, did spark jump test, cleaned fuel injectors, did compression test, change plugs again (autolite xp 606), regap, change wires again, wrap wires with looming, reroute wires, check for back pressure/clogged CAT. no matter what we did the car would just stumble all over the place, have no pep, flash MIL at me with P0300 over and over and over. VERY frustrating to say the least!

as a last ditch effort, we took off the stupid F'in heat shields and like magic, the car runs like new again, NIGHT AND DAY difference immediately! WTF?? now I'm just confused as to why it ever worked with those things and I hope I don’t have melted wiresets and cracked boots to look forward to. but for now I've finally got my car back!

thanks!

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FWIW, OP never mentions what kind of wires he used. I've always reused the heat shields and have never had problems with misfires since using ACDelco wires. When I first started driving, and had my old '91 Euro, I replaced the wires with some from Fleet Farm. They lasted about 3 months until they started causing problems. I later replaced them with "premium" parts store brand wires which lasted less than a year, and had one wire short out. I then replaced that lone wire with an old "good" one and all was well until I sold the car many years later. Every other vehicle I've owned, I went with Delco's and never had a problem reusing the heat shields. Just my $.02

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GM isn't in the habit of spending money on anything that isn't actually needed.

 

Remove the shields at your own risk. I see baked plug wires in your future.

 

Has GM always been known to make the most intelligent decisions?

 

Maybe that goes back to them not spending money on things that are, or are not needed...

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  • 8 years later...

What series was this 3800?

I have series III it's only misfiring under boost/load but misfires badly.  No misses at idle or rev, no code thrown. 

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Probably was a Series II 3800.  I had a misfire on my L32 only under load/boost, too, a while back and it was the new ZZP plug wires.  They sent me the revised wires at the time and problem solved. Could be the metal wire covers like his problem, too.  Most people ditch them.  My manifolds are header wrapped, so I ditched mine on the current car.  Guess with drive-by-wire, you would have to have someone give it throttle, but you could check for arcing by listening or in the dark.

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That's bizarre. There's no arc or misfire at idle or even Rev neutral. Why would it start arching under load? I'll check it out but those rear plugs are an ordeal every time. Took hours to completely a compression test. 

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On 3/31/2021 at 6:54 PM, GTPsurvivor said:

Why would it start arching under load?

Increased cyl. pressure = increased kV = more likely to jump gap elsewhere. Usual tip in miss on tcc lockup is a coil, but it could be wires or plugs related too.

Also, friendly recommendation that you start a new thread. 

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There may be no need for a new thread. I took off all the heat shields and voila, reached full boost with no misfires but only while cold. When warm there are still some random misfires but not nearly as many.  I hope the rubber boots don't melt as someone said, but not sure about the remaining random misfires. I plan on building a smoker to check for vacuum leaks. 

Thanks

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It's using Autolite pro premium high temp wires but they were from the junkyard, so perhaps suspect. I figured if it were a particular wire or two, it would throw a code like it did before, but there are no misfire codes ever on the scanner, no matter how many times I hook it up. 

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Are there black marks on the plug(s)?  I would replace them with new ACDelco wires.  If you're worried about the plug boot covers, you can get some of the cloth ones. 

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On 4/2/2021 at 7:19 PM, GTPsurvivor said:

It's using Autolite pro premium high temp wires but they were from the junkyard, so perhaps suspect. I figured if it were a particular wire or two, it would throw a code like it did before, but there are no misfire codes ever on the scanner, no matter how many times I hook it up. 

Get a scanner that will read misfire counters and narrowing down which cyl or cylinders that are misfiring would be easy. There are tools that will read this in the $50 range for what it's worth(possibly some cheaper than that).

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14 hours ago, mfewtrail said:

Get a scanner that will read misfire counters and narrowing down which cyl or cylinders that are misfiring would be easy. There are tools that will read this in the $50 range for what it's worth(possibly some cheaper than that).

I have a scanner that reads misfire and it has thrown those codes before and correctly identified a bad wire. After I replaced that, no more misfire codes, but still misfired. It didn't throw a single code for the massive heat shield misfires. Not even the 0300 that it did before. That's why it's been so confusing. 

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On 4/5/2021 at 10:46 AM, pwmin said:

Are there black marks on the plug(s)?  I would replace them with new ACDelco wires.  If you're worried about the plug boot covers, you can get some of the cloth ones. 

No black marks but I will invest in new wires this weekend. 

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19 hours ago, GTPsurvivor said:

I have a scanner that reads misfire and it has thrown those codes before and correctly identified a bad wire. After I replaced that, no more misfire codes, but still misfired. It didn't throw a single code for the massive heat shield misfires. Not even the 0300 that it did before. That's why it's been so confusing. 

I don't mean misfire codes, I mean "misfire counters." Codes do not get thrown until so many misfire counts occur. Having the ability to see the counters means you can see a cylinder misfiring before a code sets.

Edited by mfewtrail
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