Thanks for the info.
Now I have found some threads in this forum (very active and helpful, by the way).
My 190e 2.3 8 valve (1992) does come with catalytic converter.
That's what I will do. Please correct me if something is wront or bad interpreted:

- Leave the car warm up to, say, 90 deg or so.
- Check the pin-3 signal of the diagnostic connector with an oscilloscope for duty-cycle.
- If it is not around 50%, I will touch the 3mm hex screw until it is: now a doubt: should I do this with or without the lambda probe connected (02 sensor)?
- The idle should stabilize. Should it change the number of rpms? What is the good number? Now it is about 700, but I think that's a bit low, what about 850-900?

Any help will be very appreciated.

Best regards,
Sergio