What are the symptoms of a bad idle control valve?

87 volks cabriolet...hard to start but when running..runs roughWhat are the symptoms of a bad idle control valve?
Will tend to stall when cold, especially when putting the car into gear. You'll probably have to keep your foot on the gas to keep it from stalling at idle if ambient is cold.





It will also usually ';hunt'; for an appropriate idle speed while idling, even when warmed up.





As I recall, with just the key on (not engine running) you will feel this thing (with your hand) ';humming';. Further, if this is a 3 wire gizmo, you should be able to find continuity with a meter between the center terminal and each of the two outside terminals of the valve. On the control side, you should get about 10V between the center terminal (positive) and each of the two outside terminals (controlled grounds).





This replaces the old manually adjusted ';air idle valve'; on a carb.