My 1993 T100 v6 was blowing luke warm air, thought the heater core was plugged but discovered the valve in the heater line on the back of the firewall wasn't opening moved it to open by hand and have plenty of hot air ,tryed the lever on the heater control for cold to hot but all it would do is close a flapper under the dash,none of the controls would even try to move the valve on the firewall.I figure the cold to hot lever should. Any comments?