Dan,
The cold idle circuit of most Bosch fuel injection systems incorporate
an "auxillary air regulator". When you start the engine, this valve is
open, allowing additional air (and hence gas) to be consumed by the
engine. It is electrically operated... a 12V lead attaches to the top.

As the engine gets warmer, the 12 volts acts on the internal valve of
the auxillary air regulator, causing it to close its valve. The
internals of this regulator get gunked up over time, and the mechanism
to close the valve gets tired.

A way to test if this regulator is doing its thing is to start the
engine, then pinch the hose coming from the regulator to the air
intake housing. There should be a decrease in rpms. After running the
engine for 5 minutes or so, pinching the regulator should cause no
effect on rpms.

I've tried to repair these regulators, but usually the innards are so
tired, they die again quickly afterward. So a new replacement is usually
called for.

Paul Henderson
'84 Vanagon WasserBoxer
Bedminster, NJ, USA