Many thanks to eveybody who shares his knowledge!
Yesterday I checked some stuff on the car:

(1) Duty cycle

• Ignition on, engine off:
  30% constant reading on Fluke DMM (duty cycle 70%)
  ok, non californian car

• Operating temp (idling and around 2500RPM):
  84-90% fluctuating (duty cycle 10-15%)
  base mixture too rich, oxygen sensor working

-> adjust base mixture to 50%. Surprisingly, I had to turn the adjustment screw on the fuel distributor (after I removed that anit-tapering tower) clockwise to get it around 50%. I thought this means richer? Confused here.
By the way, I replaced the oxygen sensor a week earlier, so I assume it is ok.

(2) EGR valve

Not sure that is a valid test for it: Applied pressure to the valve via the vacuum connection. The valve lost pressure quickly, suggesting it may not feel the vacuum that is required to operate it. Do I need a new valve?

(3) Ignition should be ok.

Coil, wires, distributor, rotor, spark plugs: all have been replaced in the last year.

(4) Vacuum.

The cubber hoses on the idle control valves are new. Also I replaced brittle vacuum lines and elbows where I saw them. What shall I check here, does anyone know a systematic way of doing it?

(5) Leak at the exhaust manifold/downpipe?

This is just an idea. When I installed the catalytic converter, I had a hard time fitting it. I had to grind etc. Finally I could not feel or smell any exhaust gases and the car ran quiet. But if there is air (oxygen) entering the downpipe it may fool the whole system. So I may apply some sealant paste to the manifold/downpipe junction.

Cam69ss, your numbers indeed look similar to mine. Is it correct that I have to go for a second test within 48 hours in MA? Or otherwise bring some paperwork with me from a shop with stamps and stuff to prove what repairs have been done to solve the problem? So I have to go to a shop if I do not apply for a retest within 48 hours?

Have a nice day! Bruno

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1992 300TE 160 kmiles