I thought it was somthing like that.  Never really noticed this behavior
before..maybe I've never run the motor without the filler cap in place..  I
was thinking my Idle Control Valve may have failed again ($).
  Anyone else encounter this "Other" Vanagon Syndrome , illustrated (sort
of) by this incident?  Just when everything about your van has been perfect
for a while, something goes cuckoo?  I was expecting that to be the
case...But instead, it was 'operator error'...
  Thanks for the explainations.
  Don Hanson

On Fri, Jul 24, 2009 at 9:09 AM, Dave Arthur <dave.arthur@nscc.ca> wrote:

> On A WBX, and certainly your engine also... There is a hose, positioned
> between the AFM and the throttle body, which connects to the oil breather
> tower. This hose purges the crankcase. By leaving the oil filler cap off an
> abnormal amount of air can enter the throttle body without being metered by
> the AFM, to my mind this should cause a lean condition, but I'm not sure.
> Certainly it will cause a problem.
>
> Best Regards
>
> D
>
>
> On 24/07/09 12:50 PM, "Don Hanson" <dhanson928@GMAIL.COM> wrote:
>
> >   My van has an inline four VW motor..Digifant II injection, lots of the
> > same parts as a WBX.  So yesterday I was changing the oil (to 20-50 dino)
> > without changing the filter...I do that every second change..3000 mi
> (+/-)
> > intervals.  I was distracted during my refilling and somehow I spaced-out
> > replacing the cap on the filler.  Started it up and had a really poor
> > idle...very similar to what was going on just before I replaced my Idle
> > control valve (about 4000mi ago)  I was puzzeled, thinking "Jeeze, feels
> > like some connector has come loose"..but I wasn't even inside the engine
> > compartment and it happened suddenly without any hints before the oil
> > change.  Then, standing behind the van scratching my head, I noticed my
> > licence plate a bit askew and found my oil filler cap wedged in there
> rather
> > than on the filler pipe.  Put it back on and re-started...in about 10
> > seconds, the idele returned to normal...
> >
> >   So, there must be some kind of vacuum supply that's dependent on a
> sealed
> > sump, right?  Anyone want to try to explain why leaving the oil filler
> cap
> > off might cause the engine idle to go all wierd...kinda felt full rich..
> > though it was so windy in my Gorge driveway that I couldn't see any dark
> > exhaust or smell anything..
> >
> >   Don Hanson
>
>