Morning mate I had the same thing 3 weeks ago, I have a post in the M power lounge, the long and short of my problem was the car sounded like it was pinking around 2300rpm and it through up the management light, did a diagnostic and it showed up a constant misfire on cylinder 6, swapped around coil packs and no change, checked the plug on said cylinder and it was new, did a compression test on all 6 cylinders and cylinder 6 had none, got the head off and the head gasket was gone between 5 and 6, the job took 14 hours, the gasket set and bolts were £168 inc VAT from BMW but the problem was the tooling for the timing a good specialist may have it, if not then you can buy it at around £600 plus VAT, hope am wrong and your problem is a simple one but after a lot of research its a common problem, they are getting old. Regards Stephen