You might try this, sounds like it could be the problem.


. Inspect Heater Control Valve. The heater control valve is used to shut off hot coolant from entering the core when the heater is in the off position. A heater control valve can be actuated by either a vacuum line or a cable from the vacuum heater control valve to the heater core. This hose should be warm; if not the heater control valve is stuck and needs to be replaced. After the repair has been made refill the coolant level and recheck system.



Check Vacuum Feed. Most heater systems are operated by engine vacuum. This vacuum is used to actuate servos to move the temperature adjustment doors inside the heater plenum. If engine vacuum is not present the heater will not operate properly. To test for this condition remove the vacuum feed line from one of the vacuum servos. With the vacuum feed line removed insert a vacuum test gauge. If no vacuum is present, open the hood and inspect all small vacuum lines from the engine to the firewall and repair as needed. If a hissing noise is present under the dash when the engine is running a vacuum leak exists and must be repaired to restore heater function.