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