The car is a 2006 BMW 325i (N52B25 engine). The problem started out as a performance issue (car didn't pull quite that hard), after a while it turned into more serious issues like low rpm stuttering/jerking, increased fuel consumption (x1.5), the latest issue being 3-4 stalls on cold start, then very rough running for the first 2-3 minutes.
I've managed to monitor lambda values and voltages for the O2 sensors (it has 2 banks, 1 probe before- and one after the catalytic converter for each bank). Bank2 looks rich while bank1 appears lean. There's quite a difference between long- and short term fuel trims also. Inspecting spark plugs for each bank confirmed this - bank1 plugs were burnt out nicely, while bank2 plugs were black.
This first image was taken while cruising (partial load, cruise control on). The second one is full throttle at low RPM (heavy jerking/stuttering).
There are two things I'd need some advice on:
I suspect one of the O2 sensors in bank2 are faulty - based on the values it's the post-cat probe, but it's important to note, that if the pre-cat sensor is faulty, it would be more likely to produce such dramatic results, then the secondary sensor.
Can a bad O2 sensor cause cold-start issues? As far as I knew, O2 sensor information was not taken into account until they were warmed up (20-40 seconds at least), so for a cold engine it doesn't really matter if they work properly or not.
+1 - One more thing that can cause different fuel readings between banks - the catalytic converter. Can it be so faulty, that it completely screws up fuel management like this?
Thanks for any ideas or suggestions. Cheers!