I have a 97 f-150 with a 4.6L v-8. Last year I had problems with heating in the cab. Checked it out Verified I had flow, Blend door worked and heater core was not restricted after back flush. I did however bypassed the heater hoses through a couple of "t"s and installed an in line filter to the heater core.

The other day I turned on the heat and nothing. It blows but no heat. So I start over again to check the system...Water flows through the heater core, Blend door opens and closes and I found out that A/C compressor is on in all positions except "panel". It does come on during "panel & floor". So I turn to "panel" and it gets hot! Just what i want. However if I turn to any other positions it warms for a short time then gets cold but not as much if blend door is set to cold.

Any Ideas? Oh yea, Door goes full range on blend door.

 

The a/c is only on to dehumidify the air itself.. Either you have poor flow thru the heater core or you have too much flow. Is the water restrictor still installed in the hose?