Before touching any electronics check for vacuum leaks otherwise youll be there longer than necessary.

After cheking for the vacuum leaks and doing a pressure test, then check the ovp and mas, fuses and relays, sensors, harness and make sure the electrical devies are working. Also build your a self tester to check the mas for error codes.
Having a very similar problem with my 190E2.6 and I have replaced the OVP and MAS and car still doesnt run. So the point is buying this or that part will not necessarly solve your problem.

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