[shuefly]

Here are symptoms of my sick expedition:
1. Cool air no heat in front and rear
2. Blower works great so does changing from floor to vents to defrost...etc... As does fan speeds work great.
3. I changed thermostat. Didn't help.
4. Changed antifreeze...didn't help.
5. Hoses in and out of firewall are hot
6. when switching temp controll knob from cold to hot and back... I can detect the ...a drawdown sound of air under the dash indicating to me the ...a flap is opening and closing...

I have looked online and... saw several posts for causes and solutions...naming a few..
1- flush heater core- how would you know it is clogged..lines get hot..in and out.
2. Door behind glovebox may not be functioning properly or be broken
3. Door at floor console may not be functioning ..what is this four for?
4. Temp controll knob issue..seems to work fine when I switch it hot to cold...5. Temp sensor connected or behind temp co.troll knob may be bad? How do you know...

I think I covered it.... Time to let the doctors evaluate!

[Marktravis]

Shuefly: This sounds as if the blend door is not working. This could be a failure of the blend door actuator, or the blend door itself, or both.

Visit this site to watch process of replacing blend door. Hope this helps. Good Luck.
Heater Treater: Ford Expedition ’97-’02

[drumsdaddy]

With the vehicle running up to temp, have you physically felt the heater core lines to see if they're hot? On the older vehicles with a top radiator cap off, when the thermostat opened you could see fluid moving and know the thermostat was working. If there's an air lock (bubble) trapped in the system the fluid won't move. Remember, you can compress air but not liquids.

Could be the water pump?