You can look at the color of the spark plugs to see if they
have been for sometime since the adjustment of the fuel mixture. If it is ruuning too rich, the tips of the plugs will more on the black side. Also, your MB mechanic can check with a voltmeter, the lamba value which relates the Oxygen sensor. I have been told that the value should be between 0.6 to 1 Milli amps.
Did you check your O2 sensor, if it has been replaced??
Good Luck