I would check the simple stuff first such as fuel pumps, vacuum leaks. Then do the more harder stuff such as testing the electrical components such as the wiring harness and sensors, components of your fuel system. Between that you should be able to find it.

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