Recently picked up a 2010 MCS and I'm going crazy trying to figure this out. I'm a technician by trade, and know my way around diagnostics but I can't for the life of me land on a solid answer for what is going on and I'm hoping someone more familiar with Minis can shed some light.

So basically the idle is pretty off. Hunts 2-300rpm, with occasional harder dips and while cruising you can feel the car surging a bit. No DTCs (but some data down below).

The car has:
S42 Turbo
K&N Intake
Forge FMIC
Turbosmart BPV
Catless DP
Borla S Catback
M7 catch can
Tuned by VR

What I've done so far and data I have collected:
First was spark plugs. New to me cars always get basic maintenance items after purchase. It should be noted it had the wrong plugs installed (which was causing a very obvious misfire separate from the current issue). Installed NGK ILZKBR7A-8G.
Next was smoke testing which was done through the oil filler cap with intake valves open. After 3 cycles, no leaks detected.

Edit: Forgot to mention I ran the Vanos self check procedure and it did pass.
Edit #2: Scanner was showing closed loop fault.

So with a borrowed Snap On Verus Edge (I work for a Japanese brand and don't have a need for such a tool as we have our own diagnostic software) I was able to get in and see exactly what the car was doing.
First thing I checked was fuel pressure, actual and commanded. Showing in the 700-750 psi range at idle and jumping to 1200ish psi with a quick stab of the throttle. Actual and commanded stay pretty close so that seems pretty normal based on what I've researched
Long term fueling was approx 7 before I reset adaptations, which returned it to 0. However short term is min/maxing. -25 to 25 back and forth before and after the reset
MAF Reading 7ish g/s at idle (didn't get to monitor under load but I will do that tomorrow.)
O2 Voltage .46ish at idle
A/F Sensor is fluctuating between -1.40 something to +2.2ish. This seems really odd to me as I am not used to seeing and A/F Sensor with a negative voltage value.

I did not have the scanner for this last bit so I'm not sure what the data shows but if I unplug the A/F sensor the idle returns to normal. Plug it back in, starts hunting. I would normally condemn the A/F sensor at this point but with no input from the sensor the DME will revert to a known good or "safe" value for fueling, which could possibly hide our root cause and not directly fail the A/F.
I'm open for suggestions haha. If I missed something please let me know. Also, if anyone could point me to a resource with known good data for these cars I would greatly appreciate it. Alldata doesn't have much and that's my only resource currently. I did pick up a Creator C410 so I could continue diagnostic without having to borrow a scanner.

 

Check that Vanos solenoid that could possibly be the source of your problem they usually get clogged up and cause
​​​​​​​problems like that.......they have a newer version now with the screens around them.....holes it's on the back of the head....

 

Thank you, that's on the list for today. I forgot to mention in my post that I did run the Vanos self check through the scanner and it passed but I want to put hands on it anyway to see if there is obvious signs of wear/debris.

 

I had an issue with the stumbling and it was driving me nuts......I had another one from a low mileage motor and gave it a shot.....
it worked and I bought a brand new one from the dealer.......