After getting home decided to check for codes since I have about a 45 mile commute. I ended up having error code 17 (O2 sensor signal cable shorted to positive or ground. So I followed the service manual troubleshooting steps Tests #20.0 - 20.8. Typically I start with the tests that check for resistance with the engine off, that way I can easily rule out a break or shortage in the wires which may affect all the other tests. Since I ran most of the tests last night checking the CTS I figured most of the wires were in order, especially since I just swapped the engine last year and redid engine harness and connectors while the engine was out. So I started with test 20.0 which requires plugging in the mm and checking voltage at the ecu connector and sure enough, the voltage I got was .94 with no fluctuation. So the choices were either the O2 sensor, which I previously tested. The wiring, also tested and was good. The ecu, which was swapped out against another and was good, and finally, the mixture. Bingo. Since I now knew what to aim for I slowly turned the mixture screw to see which way was correct to get the voltage to start dropping...(left). Voltage dropped to .76v and began fluctuating. I still had to adjust it ever so slightly to get it to the point where going to RNor D from P, did not cause a surge or sudden dip in idling.

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1990 190E 3.0L