I'm having what seems to be the same issue with a 2007 Ford Explorer XLT AdvanceTrac RSC. Defrost and upper only blow cold air; floor settings blow hot. I get proper airflow to all vents appropriate to the settings. It's not the blend door. The blend door actuator has no stripped gears and operates the flap it's directly connected to; I verified that by inspecting the actuator internals, inspecting the flap movement manually and with the actuator, and when put together, verifying that the little arm on the passenger side is moving. Every vacuum actuator and flap moves for upper, lower, fresh air. The one thing that appears to not move is the flap that is above the blend flap but below the floor flaps. I can make that flap move ~3/4" by pushing on through the floor flap opening or removing the defrost/upper manifold and reaching down inside the upper duct. I cannot find any mechanism to control that flap except that it seems like it has flippers that are intended to be pushed by pegs protruding from a "plastic tray" laying against the "firewall". I don't know what moves that tray but it does move. It just does not move far enough for the pegs to push on the flippers that move the flap. I'm wondering if there were originally some caps on those pegs that made them push more on the flaps flippers. Either that, or that tray isn't extending far enough. I wish I could find some diagram of the HVAC "mixing" box.
All the vacuum lines suck to my finger and back onto their "actuator" when controls are in certain positions. Everyone that I could find made a flap move so those all seem to work fine. Fresh air, defrost, upper, lower, and blend door all respond to control positions. Whatever that flap is immediately behind the floor flaps seems to have no direct control mechanism and manually moving it does change amount of hot air especially when revving engine, which in general produces hotter air presumably due to the increased vacuum.
