I have a 2009 R56 cooper s that is strictly a track car. we have been to 2 lemons races and everything worked great. Just before our 3rd race we overheated the car but it still ran okay. At the 3rd race I noticed a stumble at part throttle and a longer hesitation at the gear shifts before boost came up. Long story short we blew a head gasket and (fixed at the track but it blew again while testing before our upcoming race). Rebuilt head and car idles fine and runs fine at full throttle but we still have the part throttle stumble ( it is very noticeable if you hold the throttle steady between 2000 and 3000) Any ideas? i have replaced the in tank pump and the fuel rail pressures seem to stay pretty steady. I am out of ideas. The car does not throw any codes.

 

Do you have scan tool that can graph rail pressures at steady part throttle when the stumble occurs?

Other items to consider: VANOS solenoids & vacuum leak

 

I have a scanner that can monitor real time rail pressure and I have not seen any drops in the rail pressure (I have another mini that the rail pressure would momentarily drop to zero and then recover (bad in-tank pump)). This car does not do that. I did change the Vanos last night ( as it is a track car I have lots of spare stuff) with no change. Idle is very steady so I am not suspecting a vacuum leak. I have a spare HPFP that I will try swapping out tonight.