Hello, I have a 2005 r53 with about 120k miles and 17% SC pulley reduction. Recently purchased it and it gave me a lean code that was resolved with a new fuel filter, immediately after starting it for the first time with new filter it threw the p0304 code. I pulled the spark plugs and noticed the cylinder 4 plug had slightly larger gap so I swapped them all with the same brand NGK iridium spark plugs. After putting new plugs in I unplugged battery to clear ECU but code is still there. I believe it is stock coil pack but has aftermarket leads and 1 step colder plugs.
Also worth mentioning is the P0108 code that I am waiting to have system smoke tested. Replaced TMAP sensor with another OEM but noticed fuel smell on original sensor.

 

I’ve got an 05 also with 17% under pulley and 110k miles and prob will need a fuel filter soon so I’m subscribing.

 

P0108 is commonly associated with crank pulley problems. I'm assuming your crank pulley is relatively new?

 

I believe it is stock crank pulley