Ok, so per Rev Workshop manual:
ENGINE TUNING Service repair no - 19.22.13 The position of the Idle Air Control (IAC) valve can be checked using TestBook and adjusted if necessary through the by-pass screw in the plenum chamber. The bypass screw is covered by a tamper proof plug which can be extracted using a self tapping screw. All vehicles: 1. Ensure air conditioning and all electical loads are off. Vehicle must be in neutral or park with air suspension in kneel and disabled. 2. Carry out tuning or base idle setting procedure as applicable using TestBook.

Anyone has a test book or could you please share how to carry out tuning?

 

Setting base idle is an easy procedure and does not require TestBook. It involves blocking the idle air control valve flow path, blocking the vacuum advance ('94 & '95 only), then running the engine (warm idle) and adjusting the bypass screw to reach desired RPM (around 500 - 525 as I recall). Blocking the IACV flow path on '96 - '97 models does require closing the IACV, so maybe that's where TestBook comes in, but there is a way to force it to close (turn key on, engine off, then unplug the IACV, not completely certain?).

 

GEMS = Reset Adaptions for IAC.

 

Ok, so I actually discovered that LAUNCH scanner has under the "maintenance" section for GEMS, Idle Tuning - I think they copied procedure from the REV, because they literally use the same language: "The bypass screw is covered by a tamper proof plug which can be extracted using a self tapping screw and etc.". Then LAUNCH will show you RPM and at how many "steps" you are in. Based on that, Launch is advising you to either go up or down with the screw. And you repeat the whole process until target is achieved. Target window is between 15-30 steps at 700rpms. I just finished doing it. I'm at 17 steps on idle now. Before i was pushing 70 steps...whole "tuning" was done when engine is at the "operating temp".