Still dealing with random mis-fires on my 2010 Cooper S. Engine is completely rebuilt with forged rods and pistons. Head is rebuilt all new valves and seals. Had it surfaced, Arp head studs. Pretty much everything on the motor has been replaced and only has 54k on it. Doesn't mis-fire everyday. Some days it starts and runs perfect and some days its starts and misfires for about a minute and than runs perfect. Some times the misfire is after its been driven. I was thinking HPFP but the pressure reading have been fine 735psi at idle and 95 on in take pump. Plugs and coils are new. Just started set a CEL for mis fires but runs normal. Any one have any thoughts on where to look next? Thanks In Advance

 

Probably HPFP. When they initially start to go they sometimes do it intermittently. Internal bypass valve starts sticking. Were you monitoring rail pressure when the misfires were occurring? If not try that. Connect you tool of choice each time before startup and try to catch it in the act. Also try spraying some electrical contact cleaner on the canon plug pins going to the HPFP.

 

Thanks for the suggestion. I usually watch the rail pressure on start up. The lowest its dipped to was 694 psi. Ill try cleaning the connector next. Its been real cold last few morning 14* and it started and ran perfect each and every start.