need help i have a 99 f150 i have replaced the heater core thermostat blend door and hoses to the core and still have no heat when i reach into the heat plentum box cold air blows through the ac core and the heater core gets hot but no air blows through the heater core and the only thing that blows out of the vents is cold air and also the blend door is open any help would be greatly appreciated i am freezing in this truck

 

Under the hood are both heater core hoses hot? If so are you absolutely sure the blend door is open?

 

cold air from outside is pulled into the box and directed partly thru the heater core and partly around the heater core........ the BLEND DOOR determines the percentage of air thru the core and mixes the cold and hot to get the required temp. I will attach a drawing below.

 

skip down a couple posts and you can see a drawing of the air flow and doors.


https://www.ford-trucks.com/forums/9...-please-2.html

 

That is a good diagram. What conditions shut and open the outside air door? I would think max air would shut it, but anything else?

 

"Max A/C" (or recirculate) setting is the only one that switches from outside to inside air. All other switch positions use outside air.

 

That's what I figured...thanks..

 

I had the blend door on my truck do what they normally do so I had it replaced. I found out what broke mine was the actuator. It was worn enough that the part that fits into the blend door wobbled so bad it broke the mating socket on the door. I would suggest if anyone replaces the blend door also replace the actuator. I also suggest that you use a Dorman brand door, they have a reinforcement band on the door socket.

 

on another note, I had the control arm on the outside air door break on our 2002 EXP several years ago. The door would flop around and cold outside air would drop on the passengers feet in the winter.. I ended up removing the glove box, pushing the door all the way closed and RTV a pop sickle stick as a wedge to keep it closed FOREVER. End of story. No outside air anymore.