I have an '83 ZXT that idle's normally with the ACC vacuum line capped.
If I re-connect the vacuum line to the ACC valve, the car almost stalls.

Any thoughts on the problem? The ACC valve seems fine (works normally).

When the engine is cold, the idle is good even if the vacuum line is connected, only as the engine warms does the problem materialize.

Welcome to ZDriver!

Are you connecting the vacuum line to the intake manifold or to the EGR / AAC actuator pump under the AFM?

The vacuum line is factory connection. The AAC line connects to the actuator pump (V.C.M) near the AFM. Right now it is disconnected and unplugged. I don't believe there is anything wrong with the AAC since the car idles just fine not connected. If I connect the vacuum line, the idle drops to almost stall (i.e. the AAC is actuating and letting in too much air).

IIRC, the AAC default is open. If you are hooking up vacuum, it is closing it, limiting the air flow. The same problem occurs when you remove the AAC all together and use a block off plate.

Double check that all the vacuum lines are run correctly, and that the EGR / AAC lines haven't been switched. Either way, if it runs fine disconnected, you can just leave it be.

What good is it - if it's not needed? (AAC blocked off).