Some possibilities:
Blocked heater core
Baffle control compromised: The heater is always on regardless of control settings (heater core is always being subjected to circulating coolant). However, there is a baffle which is controled by a bicycle type wire that connects to your hot/cold blend control knob. If the baffle isn't opening all the way, that would explain why you're only getting hot air at low fan speed settings. At high speeds, the cold air won't have enough time to heat up if the baffle is only partially open.
Thermostat to low
Leaking heater core is evident by windshield fogging up with a greasy (coolant) vapor.
|