So I'm having an issue with my R 56 Cooper S. It started out as a high idle at around 1500 RPMs and now it revs out of control all the way to the rev limiter. I replaced the MAF sensor and checkes if i had air leaking in after the sensor. Everything checked out but it still acts all crazy.

 

Check your throttle. Does it open all the way up on its own? If so, it could be the stepper motor that controls the valve, or the sensor at the pedal, or the electronic stuff in between the two.