I think I found it!
Possible DME software issue, BUT, I think the DME is recording misfires on the WRONG CYLINDER.
So the customer said he replaced (or had them replaced) the spark plugs. He was very smart in labeling each one and saving them once removed. #1 was sooted up around one half of the side of the plug, the rest were just worn and normal looking.
Lo and behold the NEW #1 plug looked VERY SOOTY around 1/2 of the side of the plug. Interesting, eh? Borescope in the cylinders shows nothing conclusive, BUT you can see the tip of the injector protruding into the combustion chamber. #1 injector tip was very sooty as well, while the others were relatively clean.
So I swapped #1 injector, coil, and spark plug into different cylinders each and test drove again. The problem is not gone but much better. The sooty #1 injector tip is still sooty, and the plug that goes with that injector was also starting to get very sooty. The plug that was originally next to injector #1 was nearly burned clean again.
So I think the DME is falsely reporting the misfire on #5. I think it is a bad injector, and it's in cylinder #1. I will update with pics in a bit...
E92 335i/6MT Dinan Stage 3: intercooler, oil cooler, intake, flash, exhaust, LW dual-mass flywheel.
Vishnu PWM methanol custom trunk mount, Diffsonline 3-clutch Salisbury custom diff, Ohlins Road and Track coilovers, Vorshlag camber plates, much more...
E28 M5 w/ buncha stuff, R171 SLK350