There are a number of possibilities:
Throttle switch. Even Digifant should be doing a fuel cut off closed
throttle @1800 rpm.
Temp 2 sensor-connector, wiring. Shared by both ECU and idle controller.
High reading or bad ground says engine colder than it really is. Disables
fuel cut off, O2 sensor feedback. Rich mixture may further increase idle
speed.
Bad valve, opens too far for signal given.
Bad idle controller.
Over advanced ignition timing.
AC system low of refrigerant, no compressor load on engine.

87 and up used a larger compressor and at some point an even larger one was
used for replacement, I think SD-509.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Karl Wolz
Sent: Wednesday, October 24, 2012 10:06 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Speaking of idle stabilizers ...

As my AC cycles off and on, my idle changes from 900 or so to around
1800rpm.  Seems to me the IS is trying a bit too hard.  Suggestions?

I have a spare, but pulling the tail light is a lot of work. ;-).

Karl Wolz
Sent from my electronic umbilicus