I am baffled with my heater. I have 2002 4.7L 1500 and I am having problems with poor heat. I bought the truck used and knew when I bought it that I had to change the heater core. I changed it last summer and had the cooling system flushed by a dealer . He noticed that my rad was cool in spots and he suspected that some kind of sealer was used in the cooling system. I changed the rad as well and had good heat for a while. Slowly the heat got poorer and would only put out heat with the blower on low speed. I felt the hoses on the heater core and both were hot but had no flow in them. this was weird so I put a hose to the outlet of the heater core and ran the truck thinking I would see flow going into a bucket but the flow was zero at idle speed and weak at about 2000rpm. I am wondering if it is possible that my waterpump is somehow slipping or the impeller is worn badly.