98 Taurus intermittent heat

98' Taurus 220000 km dual overhead cam 3.8L. My wife starts the car in the morning and drives the kids to school about 20km down the highway with excellent heat. She idles while dropping them off and the heat goes cold and stays cold for the entire return trip. She plays with all settings and variations on the temp control etc. with no heat. I have read something in the forum about a temp control valve? Do these symptoms coincide? Where do I find this valve? Please help!!!

Does the engine coolant temp gauge indicate that the engine is up to temp when the heat doesn't work, does it show below middle, or does it bounce around? If it bounces around or drops corresponding to the lack of heat, you might have air trapped in the cooling system. Have you checked the level of the coolant in the cooling system? Does the car have manual HVAC controls (knobs) or the Electronic Automatic Temperature Control (EATC)?

If you have the 3.8L engine, it's not DOHC. If you have DOHC, it's one of the two 3.0L engines. Understanding which may help us help you.

-Rod