Mysteriously Running Rich

[M3 Number 86]

if your car is truly running rich, that means you have some sort of overflow of fuel due to faulty/crapped out components such as injectors, lines, etc, basically more headaches than a small leak in the exhaust system.

have a look at fuel trims along with o2s1 voltages for both banks...if your fuel trims are positive, then it is running lean. if your trims are negative, then it's running rich. (edit: lean/rich conditions, not necessarily running but car thinks it is).

I would confirm with your brother what he saw in the fuel trims/voltage first. Ask him what he saw in the data and report back here if you can - he should remember what he saw since you said he confirmed that it's running rich.