Still working the issues with rough idle and hard starting with me 126-560SEL (86 year). I have noted the following:
-EHA current at start (cold) is very low. It should be 75-100 ma. When warm the current is within spec (100ma) but when the engine cranks hard i notice the current is zero.
-At idle, the current to the idle air valve is about 180ma -much below the spec of 600 ma. This seems to indicate that the ECU is calling for higher air flow (lower current to the idle air valve) but there is no actual change in the idle speed.
-throttle pos pot voltage is right where it should be and the pot is putting out good variable resistance.

I do observe that when cruise control is initiated and i go into "decel" mode (going down a gentle hill) that the engine speed searches (chugging in the technical term) which is really bizarre. I also observe that when AC is switched on the idle speed does not increase.

I know this is getting deeply into the engine management system but i would really like to get a better fault idea before starting to replace parts.

Any help appreciated.

Have you checked % duty cycle on pin 3 of x11? A fixed number might be a fault code. Have you monitored EHA current after engine warmed up to see if engine runs in closed loop? Have you tested coolant temperature sensors? Also check throttle valve switch, I think 1 & 3 should be shorted at idle position.

The cruise control is totally separate from engine. The surging indicated cruise control amplifier needs repair.