this may be normal and is nothing to worry about. When you first the engine it will be running in a rich state causing the idle to be rough. Your car is in open loop. As the car warms it becomes more lean and goes to a closed loop because the engine coolant has reached the proper temperature

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)