Simply, would a transmission module vacuum leak effect the idle? or can I rule this out?

 

Basically any vacuum leak can affect the idle, it depends whether the idle positioner valve can compensate for the extra air inlet at idle from the modulator valve leakage by shutting in the throttle buttervalve, if it not already at the fully closed position.

In addition any vacuum leakage to the modulating valve can affect the harshness of the shift changes.

Tails 1990 928S4 Auto

 

Correction, the above first paragraph should say the 'Throttle Butterfly Valve'.
Tails 1990 928S4 Auto

 

I prefer "buttervalve" !! Classic... :-) Reminds when my wife went into a hardware store for a craft knife, she asked for a "Herbert Knife" instead of a Stanley Knife. So now forever referred to as a Herbert in our household ever since.

 

John,
Thanks for the comment and story.

Maybe the the throttle 'buttervalve' could go along with the 'flappy valve'.
Tails 1990 928S4 Auto