Carbon build up in the combustion chamber and intake valves might be responsible. Most BMW engines starting in 2008 use direct fuel injection, meaning that the fuel is injected directly into the combustion chamber. The fuel injectors are placed directly in the cylinder, no longer before the intake valve. While this helps enhance performance power, as well as reducing fuel consumption, it has its drawbacks.
Exposing the intake valve to fuel and oil, after a few years of driving, results in carbon build up in the intake track, and intake valves, from the fuel and oil. As a result of this carbon build up, you might start experiencing sluggishness on acceleration, rough idle, increased oil consumption, amongst a few other symptoms.
While some fuel additives, will somewhat reduce the amount of carbon build up, after a few years of driving, the buildup is almost inevitable. So what are the solutions?
The decarbonizing process might be an expensive proposition, as quoted by some BMW dealers. Some will even recommend removal of the cylinder heads, which is quite an expensive procedure.