The simple answer is.. yes, lean the mixture out. If everything works as it should you would turn the 3mm adjuster counter clockwise till the EHA current started to rise. You would be doing this slowly and the first sign would be a few ma 2-3 say. In small steps one would then continue til the average EHA current was 8ma (except I like it richer so stop at 6ma).
The problem is it only works this way on a clean system and yours is most likely fouled. To resurrect the system I advise disconnecting the O2 sensor and hook your volt meter to the sensor end. Before adjustments you must be seeing about 1v. It takes this kind of reading to cause the EHA current to go to 0.0ma full lean correction. Now adjust the mixture (CCW -lean). Rev and hold the motor at 2000rpm. Did the O2 sensor voltage change (be sure to hold a constant throttle when reading O2 sensor voltage as the voltage always goes off scale with throttle movement - the scale is 0,0v to 1,0v)? As you hold the car at 2000rpms continue to adjust the mixture lean. You eventually will lower the voltage (or the engine will run badly). If you reach running badly before a voltage change, we have a problem.
This problem will either be a sensor problem (be sure to leave the heater coil connected when reading O2 sensor voltage - even though voltage dropping to the sensor from this circuit is the common problem of such faults) or a separate problem that won't allow the engie or individual cylinders to run that lean. Remeber the amount of fuel we are trying to achieve is the absoilute minimum for efficient running. At such settings any restriction or added air causes fouling. Cause the flame to burn out (if it were a candle).
Anyway, aftyer you get the voltage to start reading, get the number as low as the engine will run sort of good. Rev the motor a few times to 3k or more, hold it at 2k for 10-15 seconds and then reconnect the O2 sensor and see if closed loop can be maintained at around 6-8ma.
Remember closed loop is the distinct cycle of lean rich lean rich correcting. Once this is happening the current will be real sensitive to mixture. Placing ones finger lightly on the airflow meter should be seen on the mixture correction. This will show you how much the system has the ability to correct. If you place a finger on it for say ten seconds and the mixture corrects back toward zero ma what will happen when you let up on your finger???? The system will have corrected lean and will be lean and correction will run way above 8ma for a while til recovered. Watch all this happen and it starts making sense.
Bosch Master, ASE Master, L1
33 years MB technician