Originally Posted by professor

You may want to use a scope to test as you tweak the air/fuel mixture.
Here is a sample view of a rich mixture:



And here is a sample view of a mixture properly adjusted:



You need to do this on a test port where you can measure Lambda. On my 190E it's on pin 3 and 2 of the test port near the driver side fender.

You can also use a voltmeter on the same pins on that test port to measure the duty cycle.
Here is a sample view of a rich mixture at idle (notice the rpm meter above) and the higher voltage than reference on the meter below:



If you can't get it with air fuel mixture tower (which seems to be your problem) you will have to adjust the EHA. First make sure it's working properly by measuring the fuel pressure differential between the upper and lower chambers, then measure the current to the EHA and it should be near ZERO Amps + or - 1 mA. If it's too positive then it's rich and you will need to adjust the EHA screw COUNTER CLOCKWISE and please , pretty please DO NOT MOVE more that a hair at a time or you can kiss that one goodbye too. Be very very gentle