George,

Mark listed one of the probable causes.  The switch is easy to test
with an ohmmeter per the Bentley manual (write back if you don't have
one and I'll quote it for you).

Another possible cause I can think of is the aux air regulator.  With
the engine running but cold, pinch the hose, the rpms should drop.
After five minutes of running, pinch the hose again, they shouldn't
drop.

Good luck
Bill Kenneke <kenneke@clis.com>
'85 Wolfsburg GL "The Cool Camper"