Idle air control

[302Z28]

I am still chasing an idle problem on my '97 Cav 2.2. After having replaced a multitude of sensors and solenoids, the problem seems to have gotten worse. It idles at 600 RPM in neutral, but seems like it is ready to stall at any moment, and sometimes does. It is pulling a steady 13" of vacuum. That seems somewhat low? I have been all over the manifold / head / throttle body area with a can of carb cleaner and cannot find any evidence of a vacuum leak. I pulled the air plenum off and noticed that the idle air control plunger is open about 1/4". This seems like an excessive amount to be open at an idle, or does it open fully when the key is turned off? I replaced this item a few months ago, but was told that it is self adjusting after several on / off cycles of the engine. If this solenoid is stuck open, it would cause a vacuum leak that would be undetectable from the outside of the motor.
Is there any way of adjusting this, or is this condition normal?
Thanks,