Our '07 Cooper S has been running rough from cold, and just threw a CEL. The dealer's had it for a while and diagnosed a leaking exhaust valve on cyl #4, requiring the head to be sent off for machine shop work.

The car's had the tensioner recall and only ever gets shell V power gas, and has 75k miles on it.

Has anyone heard of this happening before?

 

Why was it leaking? There are a few ways that they can leak, ranging from totally catastrophic (e.g., the head of the valve pops off and bounced around inside the combustion chamber) to practically harmless (e.g., a bit of carbon between the valve and seat).

Most of the failure modes have slightly different possible causes.