Hello, It's nice to be here. I'me more of a lurker and poster, and I apologize that this has to be my first post. I researched my problem all day today. I have a 2012 Cooper S N18 with 85k miles on the clock. Yesterday I installed and Borla Type S exhaust and a K&N CAI. A couple of week prior to this I had to install a new thermostat housing and coolant pipe (Myself). This morning I was driving down the road and the CEL came on and went to limp mode and running roughly. I checked the code and it was the P0300, P0301, P0302, P0303, and P0304. I came on this website and on youtube.

I decided I'd replace all the plugs with NGK Iridiums, and I ordered 4 new coils from Amazon which got here this evening. While I was waiting for the parts to get here I did a wet compression test the recommended way. 1# 200, #2 180, #3 150, #4 180. I get the plugs first and installed them then went for a drive. The CEL was still there and it ran fine but as soon as I put a load to it it started misfiring bad, back to limp mode. The coils came in and I installed them and hit the road. It ran well. I gunned it hard and it did not go into limp mode. The CEL is still there and the codes are all there with the exception of P0304. Can someone tell me it the code will go away or is there some other underlying problem. I'm hoping I don't have a head gasket, or valve issues. Thanks for any assistance.

De Oppresso Liber

 

Update: I was able to clear the codes. I did a sea foam treatment just for kicks and it did smoothing out the idle a bit. The reason why I was not able to clear the code with my scanner was the car was running. I did some more research on this website and found out how to clear the codes without starting the car. This is a great website. Thank you very much