2004 QC with the 4.7 and 115k miles. I'm the second owner and the first owner is a good friend. The maintenance has been meticulous. Mobil 1 for the majority of its life at 3K mile intervals.

It overheated one time a year ago when the thermostat stuck closed. I turned it off and changed the thermostat as soon as the check guages light came on. No issues since.

An intermittent misfire started about six months ago and is especially bad in hot weather when the engine is warmed up. It idles perfectly at first start and progressively gets worse ending in a lopy idle at normal temps. I've checked everything that I know to check and can't figure this one out.

Things i've done:
1. Swapped coils from left bank to right.
2. New spark plugs SPRC12MCC4, gapped at 0.040.
3. Swapped spark plugs from left bank to right. Spark plugs have 3k miles on them and look really good with a slight white/reddish tint.
4. Fuel pressure tested at 58 PSI across entire rpm range.
5. Warm engine compression test. All cylinders were at 180 +- 5 PSI and all jumped to 90 PSI on first stroke.
5. I pulled the entire fuel rail last night and swapped injectors from left to right bank with all new O-rings.
6. Oil changed last night. No water in the oil.

At this point the only thing I can think of is a bad valve seat on cylinder 1 or a sticky exhaust valve. The injector port on cyl 1 had a bit more carbon fouling than the rest but nothing crazy.

 

I have a 98 3.9 with about 230K on it and get a #1 and #3 intermittent misfire as well. I beginning to think your last sentence about a valve seat or sticking valves is probably the culprit. I have used an oil additive to try and clean things up a bit and it seems to help out as I don't get it as often anymore but it didn't go away. Sometimes I can run for weeks or even months at a time with no misfire so again some improvement as it was pretty bad. I'm between oil changes right now and going to revert back to just oil with no additive just to see what happens. My truck is way to old for a heads redo so I'm not being too particular. Yours is just broken in so it might be an option for you. Intermittents are a real PITA. I, like you went through everything else on the engine and it's golden otherwise. Never does it under acceleration or heavy load? The overheating can cause a defect in the head gasket, it's a possibility. Just a comparison for you as I wish I had a better answer.

 

is it always cylinder 1 ?? also, have you checked for possible vacuum leaks(intake gasket) any ticking noise going on?? possible lifter, worn cam lobe, or loose rocker arm will all still show good cylinder pressure...

 

Agree with 44 above. It could be any of those.

 

Yes, its always cylinder 1. Vacuum looked good yesterday. I don't remember the exact readings but it was right with the shop manual specs.

I ran an entire can of Seafoam through the brake booster this morning, shut it down for 10 minutes and watched it smoke for 30 minutes when I cranked it up. It seems that most of the issue has gone away. No more hesitation or missing at high rpm but my idle is still wonky. It seems I have an issue that could only be in the cylinder head and/or valve seats.