I have 2002 Ford F150 with a 5.4 Triton I flushed my cooling system and replaced the coolant and water mixture as per instructions everything was fine but I noticed the heater was getting cooler as time went on and no loss of coolant and the thermostat is working fine so I thought the heater core was stopped up, I flushed ou the heater core it had some restriction but not much so when finished I had full flow I put everything back together and the heater worked fine for about two weeks then slowly started cooling down until I had no heat at all and the coolant is full. Does anyone have any idea what could cause this? I repeated the process of flushing ou the heater core and had the same results.

 

The blend door works because when I flush the core and the heat is there the blend door works fine as far as the hoses one is hotter than the other but neither is cold.