The heat in my 2002 F-150 will only blow warm to slight hot air when driving or if it is revved up to around 1500+rpm. When I let off the accelerator or am stopped the heart goes to warm to almost cold. I have already checked the coolant level, and even burped/purged the air out of the coolant in hopes that would help. So far it has been 3 days since doing this and I am still having the same issue. Any ideas on what could be causing this and how can I go about getting it fixed without having to spend an arm and a leg on repairs? Thanks in advance for any and all advice/help. I forgot to mention that I have checked the blend door and it works properly.

 

Heater core could be plugged up and need back flushing. In my case the impellors on the water pump were eaten away and barely circulating the coolant.

 

I have seen and heard of what JCR is referencing - impellers on the water pump can be worn enough that they don't move coolant much at all at lower RPM.