whenever you have performance issues do the simple stuff first. A Compression test will eliminate areas to look at and tell you the mechincal health of the motor. The diesel compression tester wont work. ignition parts to look at include plugs, wires, distributor. Make sure those are good. I would then make sure fuel pressure is good.

Once the basics are established; once you determine where your engine compression, fuel pressure and no vacuum leaks and at and its in spec then you can start looking at other areas that can cause performance issues then you can look at engine timing, fuel injection, evap system.

another thing to consider even Though i havent road tested the vehicle is it could be the torque converter. If it takes forever to acceralate and you have determined nothing is wrong is with the engine it could be a bad torque converter. as mentioned below check to make sure the vacuum modular on the tranny is working fine. to determine if its something with the transmission disconnect the Vacuum line to the transmission. this will result in harder shifts but tell you if the tranny is shifting fine and the torque converter is working

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