The valve closes under vacuum. The default position of the heater valve is always on. Temperature control is accomplished with an air blend flap on the top of the heater box, which switches between hot and cold air. The center vents get cold air under all circumstances. Hot air means there's a problem with vacuum control, and tracking it is tricky.

The first thing to do is to check the engine compartment vacuum system. A leak any place from the pump outwards can account for this problem. If you have a mityvac, start at the pump and move forward through the system looking for leakage. Something like the rubber connector on the EGR valve or a leaky rubber T could explain everything.

There's a multiport switching valve behind the dash on the right side that distributes vacuum in response to electrical commands from the head unit. To access this valve, you can either drop the underdash panel or remove the glove box liner. It's tie-wrapped to a steel support just above where the passenger's right knee would be. The easiest way to test the pods is to test their feed lines at the switchover valve.

If any of the five pods is leaking, the heater will blow hot all the time. Other than the heater valve, you have pods for air blend, defroster, air recirc, and foot well control. Replacing any of the latter four pods requires removing all or part of the dash. If you end up replacing the blend pod, also replace the rheostat, because you never want to do this job twice.