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 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.