Check to see if your valve "hums" when the ignition is turned on. If it
does, most likely it isn't defective, although it could be "gummed up".

Another possible problem (and in my opinion, more likely) may also be the
idle control stabilizer control unit. It is an electronic "box" located at
the right rear of your van. The right tail light assembly needs to be
removed to access the idle stabilizer. If you do take it out, when you
replace it, make sure all the plug-in connections are properly seated.

If you know another late model (86-91) Vanagon owner, maybe you could try
to swap both the idle valve and/or the control unit first to locate your
problem.

If you do need to replace either, try a junkyard (or even eBay) first.

Good luck!

Michael

Personal VW Web Page:
http://www.velocity.net/~vickrail/moshevw.htm