Originally Posted by
Jon S.
The 2010+ E38 operating systems had issues with the "virtual" Flex Fuel Sensor algorithm, which often caused the commanded AFR to be incorrect for the type of fuel in use - these generally command an overly rich AFR.
Problem trucks often had the rear bumper / tailgate covered in black soot, from the overly rich AFR being commanded. If memory serves, there was a GM Service Bulletin on this topic.
If you always use the same fuel type / grade, then you can setup your calibration for that fuel type solely - which eliminates the random AFR swings caused by the virtual flex fuel sensor (if that is your issue - it certain sounds like it may be the case).