Try using the x11 diagnostic plug. Use a multimeter and set it to VDC and use pins 2 and 3 while engine is running. The voltage will oscillate between a narrow range of values ex., 2.3 - 2.9. Spray carb cleaner around the air filter housing boot near the fuel distributor and see if the values change. If so then there is a leak somewhere there. Also, try tapping on the throttle body air potentiometer sometimes it will get stuck. It happened to me yesterday and my idle shot up to 2k. I had a long extension and slid it down to the throttle body and gave it a few taps and it was back down to normal idle.

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1990 190E 3.0L