I think that the idle speed "hang" in neutral @ coast down is a program that reduces exhaust emissions upon de-accleration. Back in the old days, around the early 70's, some cars had de-accel valves. I know my 72 V6 Capri had one. It did the same thing that my 90 Z does when you coast down from speed in neutral to a stop, I'm that old too!
It seems that the program will not let the motor return to idle speed instantly but rather the idle speed seems to come down in two steps & takes a few seconds to get back to 6-700 rpms as I coast down to a stop in neutral in my Z. I even notice that if I coast down to a stop in gear it seems like I loose my compression breaking for brief periods & then it's there & then it's not & then it's there again. I figured that since my ECM was not setting any codes that this must be part of the emissions program?? I don't know enough about EEC motors to say for sure but mine does this all the time and it's passed emissions testing two yrs on a dyno here in NYS so I just figured that it is "normal". I would think it prevents a rich condition during closed throtle de-accleration??? I know that's what the idea of the "de-accel" valve was in my Capri, rich emissions prevention @ de-accleration.