Big heater problems, 99 Taurus

My 99 Taurus heater isn't working right. When the selector switch is in one of the two heater options or in defrost, no hot air seems to come from the vents. On the other hand, if the selector is on vent or AC,
heat will come throgh when the temp dial is in the hot position. Given the fact that some heat comes through in some conditons, I don't think the heater core is plugged. The fan motor runs. This is a 99 wagon, 3.0 fi ohv. Any solutions?
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Check the air gates that control flow throughout the ducting network. One may be stuck open or closed. Some vehicles use a vacuum system to control the air gating & the control valve may be faulty or plugged up or one of the vacuum lines may be kinked.
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I suspect problems with air valves also but don't know how to access them. Wouldn't that require removing the dash? The lack of heat seems inconsitent too. It could be that the core is partially blocked. The Haynes book says you can disconnect hoses and flush the core. However, I can't see where water hoses pass through the firewall the way it is pictured in Haynes.
Shawn wrote:

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On my old 97 the hoses go in on the passenger side. Also the vent doors were controlled electricly. I had to replace the motor on the temp part once because the gears stripped on it (cheap plastic). The motor was about 3 inches by 1 inch or so, white in color and mounted under the dash center but a liltle toward the pass side very close to the ashtray. It would make a click sound when you turned it full hot and not go all the way to hot. It has an arm that it pushes/pulls when you go hot to cold. You should be able the see it moving when you change the temp. I think I took the clove box out to make it easier to get to but its been a while and I don't remember for sure.
I've heard about alot of heat problems with the taurus like plugged cores and even a bad water pump (corroded fins on the pump inside). The only ones I had was a stuck open t-stat and that motor under the dash.

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Thanks for the advice. I am suspecting something involving the blender door. I got a flush kit and flushed the cooling system. As far as I can tell, I have been able to flush water through the heater core. On test driving, the heat begins coming through for a few minutes after the engine warms up. Then the air seems to turn cold. There is a fuse to the blender door. It is OK, assuming I found the right one. I also find an opening in an air hose that runs across the firewall, but I don't know the function of that air hose.
ScottM wrote:

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