2012 MCS, 125K



I’m hoping that someone could help me with direction to solve this:



Symptoms:



- Erratic or surging idle in closed loop only (normal start-up, no bouncing)

- Idle fluctuates low enough that the lights rhythmically dim in coordination with the idle, also the rear defrost relay clicks, sometimes the brake/ASR lights flicker too.

- Defrost works, brake fluid level is good and clean, alternator is 1 year old

- No detonation

- No issues if I hold the idle 1200 and above

- No codes, no pending codes

- Vacuum of the brake booster line pegs out the gauge at +30”

- Vacuum lowers with brake application, five pushes bring it to 0”.

- Vacuum lowers with the depression of the brake pedal at first, then it lowers again with the release.

- Very rarely (three times in three weeks) the motor has cut out for a second or two, then back to life as normal.

- Low (-15%) ST fuel trim, but varies wildly to +15% too.

- Low -2-3% LT fuel trim



Recent:



HPFP replaced with a MINI dealer pump three months ago. FRP seems right, 800-2000 PSI in concert with RPM.



Fuel filter, coils, plugs all replaced within the last three months. Plugs look good this weekend.



Air filter is new.



I’ve tried chasing down vacuum leaks (suspect the valve cover, but can’t figure out how to test it)



I also suspect injectors.



Any other PIDs I should look for?



I’m sure this is obvious to someone, or at least obvious where to look!



Thank you.