2007 Cobalt 2.2
Engine was removed, machined, rebuilt, replaced. 4 new pistons, rings, crankshaft ground, refurbished w valve job, both timing sets replaced, all gaskets, plugs, etc. A full rebuild. When fired up first time on fresh build had a misfire on #2. I ignored it long enough to seat the rings and allow full temp warm up. shut it down changed oil and restarted and began searching for cause of misfire. After running down all the usual suspects, a leak down test revealed #2 exhaust side had a massive leak. This surprised me because the compression test showed 190 PSI like the other 3. I had assumed it would have shown somewhat differently than its siblings.
I inspected the valve lash adjusters and followers and found nothing suspect. In an attempt to cure without having to remove the head, I replaced the 2 adjusters and followers. Fail, no difference.
Removed head and found carbon behind all 8 exhaust valves. #2 was the worst by far and the culprit holding open was obvious. I determined the catalytic converter had failed and carbon and other crap was being sucked in to exhaust runners.
Cleaned all valves and seat areas. Vacuum test showed all ports (exhaust and intake) to be perfect. Replaced the catalytic converter. Re-installed cylinder head. Fired engine up. Runs far smoother but still a very faint misfire can be heard, no CIL though.
Connect the Tech 2 to learn #4 has a constant misfire.
Shut down and inspect plugs, injector, coil, and spray brake clean around intake and injector ports. No suspects. Swapped injector, plug, and coil from #1. Misfire still present on #2. Compression 190 PSI across all 4. Leak down test good.
Read this forum extensively and determined the next step should be a crankshaft (CASE) relearn. completed the procedure, no change.
Using the Tech 2, I observed the only misfire to on #4. The only hint of a change I noticed was that if I increased idle over 1800 RPM, the misfire log and graph stopped increasing. So thinking it meant the misfire was not present above idle, I decided to test drive. Fail. Definite miss present, down on power and flashing CIL for misfire.
Contacted my machine shop and they are scratching their heads too. They say sounds like a vacuum leak. That doesn't seem likely to me with these symptoms. I would expect a higher idle if that was the issue. Not many places a vacuum leak could hide on the 2.2. I disconnected the brake booster vacuum hose and blocked off the port on manifold. No change.
So, what am I missing? At this point no advice is bad! Headlight fluid, muffler bearings,