VWs are known for idle issues due to dirty throttle plates. Air
can't get by and idle is low & rough. So I would start with cleaning
the throttle plate and doing an adaptation (capable scan tool required).
Do a visual inspection as best you can for anything that looks
out of place. Oil leaks around gaskets can be a source of vacuum leaks (as mentioned already).
A dipstick that is not sitting tight & all the way down is also
a source for a vacuum leak.
I would have to see "fuel trim" values (with the use of a scan tool)
to determine if the issue is due to a vacuum leak.
A faulty EVAP purge valve can affect fuel trim & idle, but would
usually set a code.
A PCV system that is not venting correctly. Or too much, can
cause idle issues.
As mentioned before, if the ECM is not able to detect that something
is wrong, it might be difficult to track down the actual problem
without the use of a good scan tool. To monitor ECM inputs & outputs.
Wednesday, March 14th, 2012 AT 8:22 PM