John Pirtle, September 12, 2002
The heater valve is in an open state by default, which allows hot coolant to flow through the heater core and heat the car's interior. When heat is not desired, vacuum actuates a swing arm and the coolant flow is shut off.
Be aware that any time the engine is shutdown, vacuum is released to the heater valve and the hot coolant is able to make its way to the heater core. For this reason the a/c takes a while to chill after restarting a hot engine.
The valve is prone to two types of leak problems: a vacuum leak preventing valve function, and warping of the internal parts preventing an adequate seal (the entire part is made of plastic). When either occurs, a portion of the air in the center vent may feel chilled while the rest is warm.
For troubleshooting vacuum see the HVAC Vacuum actuators procedure.
Hot coolant flows out of the engine toward the heater core and heater valves are unidirectional. The black side of the Porsche valve should be oriented towards the engine. In fact on the black side is the word Wassereintritt which means "water entrance".
After my valve failed due to internal warping (two years and 20,000 miles old) I decided to look for an alternative. After all, my minivan has its factory heater valve after eight years and 175,000 miles! The one I bought from NAPA has a metal arm and plunger, and is made of a very hard plastic. Note that 928 heater hoses require a valve with 3/4" fittings.
Generic valve on left, Porsche on right.
Manually-operated valves are also available.
928 S4 Home