Sounds to me like your running/idling issues are the result of a vacuum leak. You need to have all of your vacuum lines changed if they haven't been already. They tend to "bake" under the hood, particularly after 10-15 years. Even a small vacuum leak can affect running conditions of the car.

My second thing (which you said you replaced) is your wiring harness. Any 1994 or 1995 car MUST have this done.

Lastly, you need to have a mechanic pull codes from the car. Have them reset it, drive it for a week or two, then have them pull the codes and see what comes up. Often this provides a HUGE clue (if not the offending part or system) as to what is going on. An hour or so of labor ought to be enough to get what you need.

I'm not sure about the M119 engine on the 140 cars, but at least on my 1994 E500 it is a two-piece design, with an aluminum lower part that bolts to a steel upper pan. perhaps the lower pan can be removed to access the sensor? Just a thought.

Cheers,
Gerry