Hello!  A friend suggested I seek help here.  I have a 2010 M3.  The engine fell into a “hobbled” mode.  It appears to be running on 1/2 the cylinders.  Dealer had it for 3 weeks and said they couldn’t know for sure what was wrong, suggested taking apart the engine for $12k or replacing for $20k.  Two independent shops suggested they believe it’s the computer which makes sense as it bounces back into normal mode often.  Just need an accurate diagnosis so I can fix the true issue.  Thoughts or suggestions?  Appreciate it, love my ride and want it well again!

Hi John

First you have 2 throttle bodies if one is malfunctioning that could be a cause though a scan should show it up. I have also read of a failed O2 sensor causing a similar effect 

I take it all vacuum lines and sensors have been checked ? All the above should show on a fault scan.

What did the dealer think the issue was that was so serious that a new engine was the solution (apart from loads of profit) 

Dave