Hi there good people!

Recently I'm working on m103 engine from Mercedes 300CE 1987. This car belongs to friend of mine. I've been checked most of the parts of Bosch CSI E system

and all within specs and the problem is the car is running rich and dies sometimes. The system has no vacuum leaks. Spark plugs, wires, coil, rotor, cap,

fuses, OVP, EHA, temperature sensor, fuel distributor, o2 sensor, throttle microswitch, throttle body are all brand new OEM parts. Although when I measure

from pin X11
(multimeter cable black to #2 and red to #3) the duty cycle on static with the coolant at 20 Celsius, reading shows 66,8% so I preasume thats good.
When I start the motor reading immediately change to 75.6% and no fluctuate, first I thought it's bad altitude sensor but the cars hasn't got one.
Also I can't adjust the airflow poteniometer - min. value which I can obtain is 0.9V and max. is 1.4V - very strange.
I've checked readings from O2 sensor ( inline from male connector under the carpet) and the following reading are: when warm 0.49V and then fluctuate from

0.2V to 0.8V so in my opinion its good.
I have no idea what causes that crap condition, never met similar problem since I've been working on most of old Benz's engines. Maybe ECU is gone? Is there

any procedure to check ECU? Any suggestions will be appreciated. Thanks in advance!