In a Ford 90's and up this is caused in most every case by a defective air temperature control blend door and the electric actuator that moves the door, the blend door is located deep in the dash between the heater core and the AC core, it mixes or blends the air from the hot heater core with the cool air from the AC or outside air. The blend door is plastic and they get warped, then the actuator that moves it can't move the door due to the warping of the door and it fails from the strain
SOURCE: i have a 1998 ford f-150 and no heat. i have
You just might have an air lock. Try bleeding the system from the heater hose at the firewall. Losen one hose a little, and at operating temp. open slightly to let air out. You will lose some coolant, but just refill the rad. Now, both hoses should get hot.
SOURCE: 2001 FORD EXPEDITION XLT 4.6L V8 BLOWING MILD WARM
You hot air comes from the water circulated through the heater core. It is like a miniture radiator with water flowing thru it constantly. So by opening the "Blend Door" you are permitting so much of this hot air to pass thru.
Seems like for some reason your heater core is plugged up. Just like a radiator can be. So water may be passing through your heater core but not passing through fully. Your heater core gets hot from water passing throught it from the engine. Then your doors open up, in proportion to how much air is permitted to pass over the heater core.
Suggest to remove the heater core and replace it. Seems the blend doors are working properly but again, if you have a non working heater core then no air can pass over the "Hot Core" for you to recieve hot air.
If you are not mechanically inclined, then go to a local parts store and you can buy a car manual, such as Chilton's or Hane's Car manuals, for your year and make to show you how to replace it. Then you can also have it for any other problems that may arise.
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