We're seeing a concerning issue with our '07 R56 cooper S (automatic, 75k miles).

We aren't getting the noise associated with the tensioner failure (it did have this noise, and was fixed under warranty a couple of years ago).

It does this most noticeably on a cold start but will do it from a warm start to some extent.

The power delivery and idle stumbles at low rpm. It almost sounds like a misfire (no CEL). It doesn't ever seem to stall though it comes close.

When you start driving slowly from cold you can feel the surging, which seems to improve once warmed up.

It's had new oem spark plugs at about 70k, and all other maintenance is up to date. It's never had the walnut shell blasting.

The only odd thing which may or may not be worth mentioning is the camshaft visible through the oil fill hole has a visible pattern to it on the lobes, almost like an even, repeating zigzag pattern.

Any ideas would be much appreciated.

 

It is quite possible you have a carbon buildup issue.