I believe the "combi valve" is not much more than a fancy check valve for the crank case vapors.
The valve function is dependent on boost or lack thereof. During idle and light throttle cruising (intake manifold vacuum), the valve is open allowing crank case vapors to vent into intake tract. Boost causes the valve to close (avoiding boost leak).
This valve should be located on the hose leading from the crank case breather (breather is located on drivers side of engine block just behind and above where the oil dip stick goes in).
edit: on a GTI 1.8T, the breather is on the front side of block.
[Modified by Electron Man, 2:53 PM 1-9-2002]