Brian,
I could not trace the line under the intake manifold exactly, lost sight of it also, but I figured it must go to some control valve that controls how much vacuum gets to the HCV. This valve must open and close as a function of the setting on the heater temperature dial and the interior temperature. I am guessing it is either at the rear of the engine compartment or under the dash (probably in the engine compartment).
You got me curious, found the parts on the PET, section number 813, illustration 813-25 (in the PET for '95 cars). It shows a couple of valves indexed as number 9, part name/number 944 659 215 01 "air valve", and 944 659 215 02 "heater valve". The illustration shows both a vacuum line and electrical connection, so I am guessing those are the ones.
In terms of heater performance, there must he some other control of flaps and airflow. When I did the test drive, I found that in fact cool air came when the heater temp dial was set all the way to cool. And likewise when I set the heater temp to highest, the heat in the cabin was as hot as ever. And at about 24 degrees it was warm (not cold or hot). In one test, I had the heater all the way hot, and then moved the temp knob all the way cold, and the cold air started within 2 seconds. There is no way that the heater core would have cooled that fast by sending vacuum to the HCV to stop the flow of coolant. There must be some control of some air flow flaps as well. Also there was previous discussion about tradeoffs between the plastic and metal parts, the metal has a louvered control whereas the plastic has a slider to block the flow. I guess the louvered valve works fine, couldn't detect any problem with the performance of the heater.
Roland
