The defrost in my '87 F250 6.9L has been weak and not warm at all so I replaced the old leaky heater core. I filled the cooling system and got it up to temp with the cap off but now the truck is blowing colder air than it was before. Could someone inform me as to what i'm doing wrong in this process. Thanks in advance for any help.

 

Are the heater hoses getting warm? Both? Or only one?

 

Both heater hoses are getting warm.

 

There are 2 diverter doors inside the HVAC ducting, one diverts air from the floor to the windshield and the other switches between inside and outside air. There are 2 things that can go wrong with these, the vacuum line can crack rendering the door inoperative or debris that falls down the dash vent openings will jamb up the door and prevent it from fully closing. On these older trucks I believe the dash/floor door is cable operated while the outside/inside door is vacuum driven. Also note that when changing the heater core you want to make sure it is sealed well around the edges, an air leak there will dilute the hot air and the colder it is outside the less heat you get.