The idle stabilizer control unit behind the rear tail light controls this
along with the idle air valve on the top of the engine.  I would start by
checking your throttle position switch to be sure that it is clicking and
functioning at idle.  If that is working then it is either a problem with
your idle control unit or the IC valve.  Usually the unit is the bad one,
and we have rebuilt and tested units available.  If the idle control unit
is good (only way to test is to install it in another vanagon and see if it
works)., then you could get a new ISCV.  The best way to test things if
your switch is fine is to get a friend with an 87-91 Vanagon and swap your
stuff with his and see if the problem follows the parts.  There really
isn't a good way to test beside this in my opinion.  There is a test that
the idle valve is supposed to vibrate when the key is on, but I have found
that this test doesn't really tell you anything.  I have had vans that
idled fine but didn't pass the idle valve vibration test.  I hate to just
throw parts at a problem, but if I had to guess the problem I would guess
the ISCU as they go bad a lot.

Ken

On Fri, Jul 9, 2021 at 7:02 PM Ryan Cresawn <jrcresawn@gmail.com> wrote:

> On July 6, 2020 I hired a shop to fix my factory AC in my '91 Vanagon
> camper with 2.1l engine. The old compressor was leaking and the shop
> replaced it. At 11.5 months into a 12 month warranty the AC wasn't cooling
> well. I returned to the shop last week. They identified a refrigerant leak
> in the compressor and replaced it under warranty. I picked up the Vanagon
> yesterday and was told that I need to hire a shop to fix the low idle when
> the compressor is on.
>
> I drove the Vanagon home and confirmed that when the compressor was running
> the idle speed of the engine would drop from around 900-950 to about
> 600-650. The engine did not stall but it felt like it was close to
> stalling. How should I begin to troubleshoot this issue?
>
> Thanks in advance,
> Ryan
>

--
Thanks,
Ken Wilford
John 3:16
www.vanagain.com