Yes, it is a mixture problem for sure (much too rich)
Is this a model, which uses EHA (Electro Hydraulic Actuator - attached
to the side of the fuel distributor)?.
This device fine adjusts the fuel pressure and thereby mixture
according to feedback from the O2 sensor. When defective, it will
create symptoms as described.
I had exactly this problem on a 1987 190E 2,3 (different engine but
same injection system).
If your mechanic tried to adjust the mixture while EHA or O2 sensor
was defective, it should be readjusted after correcting the problem.
The Air Flow Meter (big flap in intake) controls the fuel pressure
according to the setting of mixture adjustment screw, and basically
the engine will run with this alone (as it was the case until
electronics were added, and as it is still the case until O2 sensor is
warm). The fuel pressure regulator provides a stable pressure to this
system, and a defective fuel pressure regulator may also cause wrong
The electronics consist of O2 sensor, ECU ("computer"), some other
sensors (engine temperature etc.), and the EHA, which as said fine
controls the mixture in a closed loop. Fault in any of these parts may
cause the problem.
On alldatadiy.com you can find information on how to troubleshoot the