my brother took his truck to the stealership & they replaced the old 1. didnt fix it.

 

the vacuum gauge doesnt bounce, fluctuate, drop past 15hg, or rise above 15hg. it stays steady which is a little bewildering. the misfires should make the needle bounce.

 

The coil thing makes loads of sense. I have seen numerous GM 3800 powered cars that had this problem, turned out that the ignition module and or coil had a tiny crack in the resin filled cavity that make it work fine cold, but after heating up the crack expanded & caused ignition poo poo.

 

Sorry, found the post after I replied to PM.


trace O2 heater control circuit. If it happens only at the switch from open to closed loop, and more throttle corrects it, I'm wondering I'd the O2 sensor isn't heating up properly and the extra exhaust burst finally gets it up to temp.


and I have not yet figured out my issue yet, save that I know now that it is officially a misfire that will get significantly worse after about 20 minutes of driving, regardless off the outside temperature. I think I'm going to have to tear down the engine again and rebuild to find it.