Auxiliary air valve has an internal metal valve that dies.  There's a
test procedure in Bentley.  pinch off the aux air intake pathway when
cold and see if it affects the idle.  If not, take out the valve and
then throw it in your freezer to see if the valve opens up.  If not,
consider replacing the valve.

There's a range of other possibilities like cracked vacuum lines that
close when heated up.  The '90 has a heater on the O2 sensor, so I
don't think that your O2 sensor is taking that long to warm up.

I would check the aux air valve first.  I have a 83 and am not sure
where this valve is on a 90.  On an 83, it's bolted onto air into
runners and is fairly easy to take out of the van.

-- Craig

On Tue, 1 Feb 2005 12:59:27 -0800, David Hally <davegh@pacbell.net> wrote:
> I tried searching the archives and came up with zip,
> so its back to the list. I have a '90 vanagon GL. Just
> recently it started dying when first started when
> cold. It has always run perfectly up to this point. I
> thought at first it may be bad gas, but it didn't go
> away after several tanks. It idles fine once it is
> fulled warmed up. I didn't realize that there is not a
> "fifth" injector on these vans. What should I check
> first? Intake air sensor? Water temp sensor? Aux air
> valve? I could not find anything obvious loose or
> cracked. ANy help is appreciated.
> Dave Hally
>