The heater in my 1994 F-150 is blowing cold (luke warm at best). The thermostat is OK, the core is circulating and unblocked, and the switch is functioning correctly. What am I missing? Can anyone help before it turns cold this week? Thanks in advance.

 

your problem is more than likely in the air box its self. the actuator that controls the "door" from fresh air/ ac to heat may be malfuntioning.. hope i am not too late with this info

 

Take a look at the firewall on the left side where the box comes through. There is a vaccum hose there attached to an actuator bar, make sure it's not hung up on anything, and the hose is attached. No cracks, etc... You can move the actuator but don't horse it too much, you could screw it up. Other than that, make sure the coolant is mixed right, and the door opens underneath in the cab. Be careful, it's always on when the key is on, and it can be sharp!