So for the last month Ive been putzin around with my 99 3.1L monte carlo. The car has about 210,000 miles on it so its a little over the hill. Ive been doing all the maintenance myself to avoid high costs at a garage. I had a numerous problems that I will try to list all of now. Car overheated, bubbles in the coolant reservoir(blown head gasket), sweet smelling exhaust. Lower intake seal was ruined, coolant mixing in oil (Not going into cylinders, was an external leak), Car ran rough,(cylinder 1 misfiring) and No heat in the car. So this is what i did, I bought about 300 dollars worth of troubleshooting tools(Ohmns meter, Diagnostic DTC code reader, various other manuals and trouble shooting tools). Replaced all the seals (excluding the head gasket) because i went to a dealership at the very beggining and had them do a compression check to see if the head was blown. They said the head was FINE, but it would be likely that it would go,(the car was overheating and had all the symptoms of a blown head before i did maintenance) After i had replaced the lower intake manifold seal and sealed the walls with RTV(the right stuff) I then went about trying to figure out the Misfire. I changed all new spark plugs, calibrated every one at .060, replaced all the wiring, changed the oil and filter, checked the Coil packs, resistance on them as well..Still got a misfire, then changed the Fuel injector,the problem was GONE. After doing a short drive and flushing my Coolant(to put the BLUE DEVIL IN) now the car says i have a misfire again, the Code was CLEARED off the computer, but now its back...And im clueless as to why. I attempted to fix the head gasket but cleaning the coolant out and putting in new 50/50 coolant and ran it from a cold start up just like it said with the Blue devil(the bottle says its guarenteed or your money back, permanent fix, not temp.) So i did that and it did fix the heating problem, i didnt get above 120 degrees and i get HEAT FINALLY! Might be just because for this problem i had to take out the T stat. btw, I have also changed out the T stat twice since the problem, Changed out the water pump, changed out the rubber lines. Exhaust still spitting out alot of smoke and my misfire can be heard through the "Puh puh puh" in the pipe. My only guess would be that because the engine overheated that the Gasket around cylinder one has melted/warped, therefore causing less compression/bad compression and forcing my cylinder not to work. I was hoping that blue devil stuff would work, i did everything it asked but still didnt work, the gap might be too big. IF ANYONE KNOWS WHAT ELSE I COULD POSSIBLY CHECK BEFORE I RIP IT DOWN TO THE HEADS PLEASE LET ME KNOW. I know this is a little lengthy but i dont want to have to keep replying for weeks in order to get all the details out. Ill post the Diagnostic report i did on cylinder one today as well. Any suggestions are welcome. Thank you.

Duncan` 3.1L 99 monte carlo LS [align=right][/align]

 

Ur car has 3 coils right?

 

My guess on the mis-fire would be a bad coil pck. I replaced everything on my "01, and the coils last. That was the problem.
You got lot's of miles on this 3.1. It's possible that you have other major problems like bad valves. Might be time for a valve job. Did you get a full compression test done? What does your oil look like?