Please help me solve this riddle

[mxer147]

Well, winter is around the corner and I am gonna start driving my beater car, 99 Bonneville 3800, 275k miles. Thus, I’m going to work on my new issue that I didn’t solve when I stopped driving end of April. I definitely have oil in my coolant, enough to leave residue on my finger when I put it in the fill hole. The coolant level is good and stays that way. What’s weird, the engine will go into a misfire condition when it gets just above normal operating temp for a few miles, which is about 10-15 miles in warmer ambient temps. During the winter, colder temperatures, I really don’t experience the issue since coolant temperatures stays lower. The misfire is so severe, the car can barely accelerate but it hasn’t triggered an error code. I can let the car cool some and it will resume normal running conditions after the misfire condition. I’ve perused 3800 forums and similar posts suggest UIM/LIM gaskets. Since I just replaced the plenum, it’s fair to rule out UIM gasket? I replaced it to resolve a very minor coolant and oil leak. This leaves LIM gasket and/or head gaskets. Possibly warped or cracked head/manifold since the engine did get hot when the water pump seized at one point. I’ve put about 2500 miles on it, experiencing this condition. When it’s cold, it runs fine. I also replaced the thermostat and coolant temp sensor since it’s easy and inexpensive. When this initially started running poorly and wouldn’t idle, Ricco suggested a vacuum leak which I did find a suspect vacuum line and replaced it but still wouldn’t idle. I then replaced the MAF sensor (Delphi) which resolved my initial idle issue. Maybe the MAF gets wonky at higher temperatures?

Thoughts and/or recommendations please? I’m gonna start looking at this probably Octoberish after I finish the paint on my K20. I’m considering pulling plugs and doing compression checks, not really sure how to troubleshoot this one without ripping into it?

 

[SquareRoot]

Remove and replace. Let r rip.

 

[Ricko1966]

When you are having the problem, remove the dipstick put a vacuum gauge on the dipstick tube. Pull the pcv valve and cover it,plug the pcv hole in the valve cover,make sure your oil cap seals good,or pull it and plug that hole too. See if you are pulling a vacuum in the crancase,which would indicate a vacuum leak in the lower intake. Also pull your plugs,see if some combustion chambers and plugs are cleaner than others,indicating,coolant getting into cylinders which could be intake gaskets,head gaskets or cracked heads.

 

[mxer147]

Thank you for the advice, will try this sooner than later since it’s relatively easy. I’ll have to get some help to keep it running when I test this.

 

[PrairieDrifter]

3800's are great but no one seems to talk about the head gasket issues. I've had a 98 bonneville and a 94 lumina lose headgaskets.

The boneville I have no clue how long it was bad. It only started misfiring after I checked the coolant during an oil change, and topped it off. Probably a gallon, wasn't empty. But it never got hot or misfired with the low coolant, once I topped it off, it barley ran at all and was choochin white smoke real bad.

The lumina was obvious was a head gasket and never ran good after.