I have a 2002 Ford 150 HD and I can't get the heater to blow hot it is just warm at low fan speed when I turn to high it gets cool. I change themo and flush the system I have no blockage far as I can see. I upper rad hose is just harm not hot after I drive the truck for miles. What could be the problems. The electric water pump is pumping the coolent is cold.

 

I'd change the thermostat.

 

Check the heater core hoses(careful they can be hot) they should be about the same temp....if not the heater core itself might be plugged.....

 

I am having the same problem. I have changed the thermostat (which was bad enough). I hope it's not the heater core or the blending door, I have heard horror stories about both.