If it's one bank it would suggest that it either gets more than fair share of air which means restricted airflow on the other bank so 2 main issues would be valve timing or blocked exhaust/catalytic converter. Or it does not get enough fuel so could be blocked or sticky injector but ECU should be able to detect that and throw a fault code for single cylinder. Couple of things to check are misfire count per cylinder and knock sensor voltage to see if 1 or couple cylinders differ from the rest, then maybe swap injectors between banks. Fuel pressure/flow could affect as well so testing fuel pressure under load would be beneficial.
Also do a compression test on all cylinders both cold and warm and see if it is the same for all cylinders / both banks
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Currently 8less
2011 Q7 S Line 3.0TDI, 2016 Tesla Model S 90D
8 history:
2006 A8 Sport 4.2TDI quattro SOLD,
1997 S8, reached end of life with gearbox failure
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