I recently purchased a 335i and experiencing the same problem
not a good start. When I purchased the car I noticed there was a very and I mean very mild hiccup or maybe a misfire at idle. The idle didn't seem to raise or anything and there was no power loss. No lights, codes etc. A couple of weeks after I bought it, I decided to service it myself before the next official service scheduled. As I am sure most of you would agree, the service intervals recommended by the computer are just ridiculous. Anyway, I changed oil and filter, changed the air filter, sprayed some carb cleaner in the intake while engine was running warm, changed power steering fluid, and finally changed the SPARK PLUGS the day after as the engine needed to cool down. The plugs I bought were BOSCH plugs. I found the exact model number of the bosch plugs from realoem.com and then hunted for them online. I purchased them from a reputable website and not only the plugs matched the same code, but it was also listed for N54 engines 135i and 335i. The plugs looked exactly the same as the ones I took out, except they didnt have BMW written on them. When I did the oil change I spilt some oil around the oil cap and the oil found its way to the spark plugs holes on cylinders 3 and 4. Funny enough, I did not notice any negative effects the night after the oil change. I took it for a hard drive after servicing it and it ran fine that night. The following morning I changed the plugs (torqued to 28nm the lowest setting on my torque wrench) and spent a good while cleaning the split oil from coils and plug hole. Since the spark plug change the hiccups are much more profound and they happen more frequently. It is also moving the rpm from 600rpm to just over 700rpm and stays there until I restart the car. The rpm increase never happened prior to changing the plugs and the hiccup/misfire was not as profound. After reading all the posts, I am confident it is related to the plugs/coils. I purchased a BMW diagnostic cable and have managed to get all official BMW softwares running, GT1 DIS INPA PROGMAN etc... INPA seems to be the most useful. Does anyone have any suggestion on what I should look for/monitor on the live data from INPA software? any solutions so far?