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I have a 2001 Firebird with a 3.8 motor. 344,000 miles. Replaces heater control module because of rear window defogger circuit board was wasted. Heater blows cold air but some tepid air will drift in at highway speeds. Engine does not overheat at all. I think that the valve that allows warm engine coolant to flow into heater cord is not functioning. So where is valve located, in engine or passenger compartment? Is the valve operated by vacuum? Could the controls be part of the problem? When I installed new controls I made sure that the knobs position matched the old unit. Thanks
 

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Ramblin' Wreck
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First, did you hook up the blend control correctly. It is what selects the air path, either through the evaporator for cold air, or through the heater core for warm air. Can you tell the difference with the ac on. Moving the control should select cold or ambient temperature if it is working now.

If that is working: there is no coolant valve, it flows through the heater core all the time (or maybe not in your case). Your heater core is probably clogged. Try removing the heater hoses at the engine and forcing water through them with a garden hose in each direction.
 
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