I have an 08 base model Cooper that recently had the thermostat housing and crossover pipe replaced, and now on startup, the revs just drop and it immediately dies, unless I give it some throttle right away. When it's running, it also idles kinda rough intermittently. Throttle response from idle can be very laggy as well, and has caused me to stall out a few times now in stop and go traffic. I've also noticed a strange kind of airy/throaty noise from the engine between 1k and 2k rpm, most noticable during deceleration. If anyone has any info regarding this, I'd really appreciate it, this is driving me insane, as I am a delivery driver and have to deal with these issues constantly.

UPDATE
Check engine light came on, throwing code P0012 for camshaft position sensor; intake camshaft overretarded

 

That is a good lead...check your VANOS located right next to the air intake to the left.

 

So I replaced the Vanos solenoid and cleared the codes yesterday, and now the MIL is back on with P0015, exhaust cam over retarded, and P0012 pending. So, from everything I have read so far, I'm guessing it is as I feared and my timing chain is just loose and needs to be replaced....

 

the N12 engine did not have the timing chain issue that the N14 had. You have two vanos on your car, one for the intake and one for the exhaust. replace the exhaust side as well and see if that solves that issue. My guess is that both vanos need to be coordinated by the DME and if one isnt responding, then the coordination will be off.

 

Any updates on this? I am having similar issues. See here.