That happened to our cars during an extreme italian tune up (we were on the freeway together) I was following our 83, and all the sudden this big "POOF" of black exhaust blasted from the pipe while it was rev'd hard, then it got cleaner, and was running much nicer later on. Same for mine. Was that carbon being blasted out?
It is indeed hard to keep these car's in good shape here in MI. 
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-diesel is not just a fuel, its a way of life- 
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)
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