Why does my heater turn cold after a little while????????

[89_3SFE]

I've got an 89 GT with stock everything and I'm having a problem with my heater. I'll set the heater to warm and the first time I drive the car for a little while it'll work really well and get really hot for about 20-25 minutes. Then I can hear the valve close like I turned the temperature knob to cold and cold air starts coming out. I've tried moving the temperature knob back and forth from cold to hot and nothing happens. It won't move unless I shut the car off and let it sit for about an hour. Last winter I just stuffed a piece of wood behind the radio to keep it from shifting to cold while I'm driving. I live in Idaho and the winters are freezing so I really need my heat. Hopefully someone has had this problem before and know what it is. If not, maybe you still know how to fix it. I don't, and I need your help. Please help me guys!!

[es250nut]

If you have the electronic controls this thread will probably help you:

1988 Camry Heater Cycles Between Heat & No Heat