Okay, that's the punch line, here's the "just the facts ma'am" story:

Drove to the dealer to diagnose a (solid) yellow check engine light. The fault codes were 2789 and 274D. After clearing the code and reinstalling the oil fill cap correctly, the technician said watch this. With the engine running smoothly, he removed the fill cap, and in seconds the engine was idling rough. I was amazed and embarrassed because I was the most likely guilty party (AKA Operator Error).

He also advised me to be sure to fully seat the O-ring sealed oil dipstick to prevent a similar engine code episode. I knew to check the gas cap, and how a loose one can cause a code, but a loose oil cap?

Hope this little story helps someone out there in the ether!

Question: when the technician opened the oil fill cap with the engine running, there was the pop (like the lid coming-off an applesauce jar) suction (air going into the engine)? Further explanation please.

 

My 11 clubman was delivered with the oil fill cap slightly off - the idle got worse over a period of a few days. Same fix as yours. These engines use a vacuum pump to generate the vacuum they need - if you get a leak you will know it....

 

Thanks for posting your experience. I did a little research on engine pressure in the valve covers/crankcase and came-up with this link:

http://www.britishv8.org/Articles/Po...lation-PCV.htm

If this does not explain the pop that I heard on idle when the oil fill cap was removed, I love to hear about it.