Hi guys! I own a 2014 Volvo S60 T5 R-design.80K KM on the cluster, always treated with the best oils, and never been in an accident.

when the engine is warm at an optimal working temp and I come to a stop the engine suddenly lowers the revs and rattling and black smoke is coming from the exhaust.
I've tried K44 and liqui moly fuel additives and nothing... tried to clean the MAF sensor and the problem is still there. (the air filter is clean too).
no check engine light... it came on only one time when the revs went bellow a certain point and when the revs came back up it turned off.
what can it be?
video that shows the problem

 

any oil consumption going on? black smoke suggests the car is running rich or it may be sucking in some oil (say from a failed PCV oil separator) or the spark is weak. Usually this will set off a misfire code. How do your plugs look? Also a sensor could be at fault. You seem to suggest it happens only when the engine is warm which would suggest some temp sensor is at fault (ECT, engine intake air, pre-heater etc)

 

nothing unusual. but i noticed that it happens only when i come to a stop and leave it in D or shift it to N.
is i come to a stop at an S (sport) mode nothing of this happening