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Posting just to relay my misfire story (knock on wood):
Bought the car in August from my mom who has owned it since new, 63k miles on a 2008 335i. Car had a pretty serious misfire when getting in the higher RPMs and especially in boost. She didn't really notice/care, but had all the maintenance records. The plugs and coils were only 10k miles old, a couple injectors had been replaced, but not since like 2013. I did a couple full throttle pulls and got the car to throw a code, cyl 3 misfire.
1) Replaced the plugs and the cyl 3 coil. Car wouldn't even idle properly. Threw the stock coil back in and it ran fine. So it was either bad out of the box or I didn't seat it properly. So basically all the plugs were new Bosch replacements and the car ran much better. Still misfiring ever so slightly, but much better than before.
2) Replaced all the vacuum lines, no change.
3) Finally got the intake valves walnut blasted, this completely cured the misfire. The car runs perfectly smooth at full throttle all the way to redline. The pic of the intake didn't look too bad, but lots of gunk around the valves themselves. I was surprised that could make such a difference.
Now the car is getting driven daily on my 50 mile/day commute, so it'll get 15-20k a year on it instead of 5-6k. My mom did a ton of very short trips, so I'm betting putting more sustained highway miles on it will be better for the car in the long run.
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