Greetings, everyone!
I finally got around to probing my car with a digital multimeter in an effort to diagnose the rough running issue.
Symptom:
Rough idle, especially under load. Virtually undriveable. Engine smooths out when revved to about 4K RPM.

In addition to the EHA valve, I just replaced the O2 sensor.

Here's what I found:

Current to EHA, Key On/Engine Off: 19.6mA
Current to EHA, engine at idle: -1.5 - 3.5mA (varies)
Current to EHA, idle & O2 sensor unplugged: 0.4mA

X11 port, Pin3-Pin2 (measured in DC volts, since I don't have a duty cycle function)
Key On, Engine Off: 3.8V (about 72%, assuming 13.7V battery)
Engine idle: 6.5V-8.2V (rises steadily from 6.5 to 8.2, then drops back to 6.5 every 5 seconds or so)
Engine Revving (varying speeds): 4V - 11V; varies
Engine idle (warm): typical range 6.2-7.2V

I discovered that the tamper-resistant cover on the mixture adjustment tower had already been removed, so I tried adjusting the mixture a bit. Using a 3mm allen wrench, I gently pressed down on the screw until I could feel it engage and I could feel resisance to turning. I adjusted the the mixture screw about 1/2 turn in either direction with the engine running and while monitoring Pin3 of the X11 port. Oddly, the voltage at Pin 3 remained virtually unchanged at 6.8V, and the engine did not seem to run any differently when he screw was turned. It seemed to have no effect.

I also tried unplugging he EHA and O2 sensor with the engine running. Neither seemed to have any effect; same rough idle.

Can anyone make heads or tails of this? Your input is sincerely appreciated!

Mika