I've got an 07 Cooper S that's misfiring and/or running lean in just one cylinder (#3) under anything above moderate acceleration. My mechanic swapped around the spark plugs and coil wires, and the problem remained with cylinder number 3. He also checked the compression and it was good. There are no codes or check engine lights. Has anyone had experience with this? Any feedback would be greatly appreciated.

 

Fuel Injector.

 

Do a leak down test as well as compression test