I have made the experience with my 4/4 that the exhaust pipe is very sooty due to the warm-up phase. This soot remains. If I clean the tailpipe of the very warm engine, it doesn't turn black again or only very little when going on further driving the hot engine. Maybe this is worth a try. Then you can see if the engine runs permanently too rich or only in the warm-up phase.

'14 4/4 graphite grey