Pull your plugs while the car is running and ground it on metal so you dont get shocked and see if you are getting spark. I would then see if by pulling them 1 by 1 to see if the motors behavior changes. If it doesnt then you have a problem.

You may not be getting fuel. check your fuel pumps for operation. While the car is running and when you start you should hear them working. You can remove them from the car and see if they are working by cleaning them then testing on a spare car battery

Check all fuses.

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