I'm trying to diagnose the root cause of a heater problem. My symptoms don't match any of the faq symptoms, so I'm at a loss.

The heater quit working correctly and now blows luke-warm air from the dash vents, even with the temp selector set to hot. If I select the floor vent, I get only cold air. I originally suspected the heater valve was bad, so I replaced it. Still have the same symptoms.

By the way, the engine has been slowly losing coolant (have to top off the antifreeze after three or four weeks). I suspected the heater core leaking, but see no evidence of water or antifreeze in the floorboard. I think the coolant loss was from the heater valve, based on water stains around it.

Is there a test I can run to see if the heater valve is working? I've tried different temp settings, but the heater valve seems to stay in the same position. Wouldn't that point to a vacuum problem?

Any advice appreciated.