The Intake Manifold Runner Control is supposed to move the tumble flaps in one position at under 3k rpm, and the other position at 3k rpm and up.
My IMRC was sitting in the "relaxed" > 3k rpm position even at idle.
On top of the intake manifold are two solenoids - the driver's side solenoid turns vacuum on & off to the IMRC.
When the solenoid is open, the IMRC energizes, and puts the tumble flaps in the low RPM position. When engine RPM hits 3000, the PCM closes that solenoid, the IMRC loses vacuum, and it relaxes.
My IMRC wasn't energizing at low RPM for some reason.
I disconnected and checked vacuum lines in the system, reseated them; then checked and cleaned electrical connectors at the IMRC solenoid, the IMRC itself, and the Intake Air Control valve.
I reconnected everything, cleared codes in Key-ON Engine-OFF, & started it up.
The IMRC energized, the idle smoothed right out - I took her for a drive about 10 miles, all is well.
The ODBLink was the key - with that + the ODBLink Android app on my smartphone I had the data I needed to properly troubleshoot.
I'm an IT professional - gimme some data about what's happening and I'll fix anything.
I wanted to change the Topic Title to "[SOLVED] Help me diagnose this engine problem?" - but I don't see the control for that... am I missing something?
I managed to change the Title of my first post, good enough I guess! Thanks all!