Re:  First point is the idle control system can't lower the idle, only raise
it. - this isn't a Vanagon-only weird system, they used the same system on
(at least) Jettas for years.  The valve is the same, not sure whether the
control module is.

The name "idle control system" is basically euphemistic.

Any gasoline engine (save maybe for those direct fuel injected)  requires a
rich mixture to start cold.  This because the gasoline portion of the
fuel/air mixture tends to stick to cold manifold surfaces rather than make
it past the intake valve to be combusted.  So, more fuel.

Along with the rich mixture, a little extra throttle is required.  Perhaps
you remember the old days of carburetors when you would floor the
accelerator to "set the fast idle cam" before starting.  The idle control
valve does essentially the same thing for fuel injected engines by bypassing
some extra air around the closed throttle, and increasing idle speed a bit.
It's really a cold start fast idle system and does little once the motor is
at normal operating temperature and air is monitored by the normal hot wire
or flap system.  Most Bosch FI systems have an idle control valve, although
I suspect but don't know for sure that direct-injected motors don't really
need it, as the fuel for sure gets there.

Kent Christensen
Albuquerque
'81 Vanagon with rubber floors and vinyl seats