hey all

my 2001 f150 super crew, the heat only works on speeds 1-3. all 4 speeds blow air fine, i'm talking about the temperature. not the blower. could it be the heat resistor?

 

if the blower is running, and you dont have hot air, then the heater core is plugged, the water is not hot, or the damper is broken in the duct that mixes the hot and cool air to get that rigth temp.
--- is the temp gauge about 1/2 way up ? that tells you the thermostat is working.
---are the hoses to and from the heater hot ? (under the hood). heater outlet should be not as hot if your getting heat transfer to inside air.
---is the air mixing motor and damper door moving ? This is behind the dash below the radio. Might be able to take off the plastic and see more. I think you can lay on drivers floor and look up under the radiator and see the motor. Have someone stroke the **** from hot to cold and look to see that things are moving.

 

I'm thinking the blend air door isn't totally closed for heat because it's partially broken. I believe you remove the cover over the hump between the dash and floor to access the blend air motor.

 

wouldnt that make it not work at all? it works fine on the lower settings, nice and hot.

 

either the increase in air is moving the door to cooler... or your heater core is partly plugged and not transfering heat. Truck is 12 years old, that might be a good place to look.