I have a 2008 Mini Cooper S that I rebuilt last fall after the timing chain failed. The top end was rebuilt, timing set replaced. I replaced the turbo at the time, the HPFP, coils, spark plugs, vanos solenoid, multiple sensors, valve cover (PCV issue), injectors (rebuilt injectors), MAF and probably a few other items. On cold startup (warm outside) the car runs rough and will misfire on cylinders 1,3 and 4 sometimes triggering the CEL. The only codes that show are the combustion misfire codes. Once warmed up and running, the car runs perfect including on long journeys. The car has good compression and I've checked for and repaired a couple of small vacuum leaks. The other symptom I've found while LIVE MONITORING while driving is, when I release the gas and coast it will misfire a couple of times (1-3) on one or more cylinder and then be fine. You won't notice the misfire though. It never registers a misfire while driving and giving it even the slightest bit of gas. This would seem to indicate to me that it is likely a fuel issue, but with a new HPFP and injectors, I'm at a loss and the diagnostic isn't proving much guidance. THOUGHTS? Pretty much all parts are OEM. The MAF may be aftermarket. Also, I have swapped plugs and coils around and the misfires remain on 1, 3 and 4. Also, it starts perfectly fine no hesitation and then when it settles to idle thats when it runs rough and misfires for a minute or two.

 

Cold start up issue: Test the coolant temperature sensor.

 

Post your short term (STFT) and long-term (LTFT) fuel trim numbers.

1. Idle
2. 1500 RPM
3. 3000 RPM