I have been through the trials and tribulation of getting the dashboard apart to replace a heater core. As a matter of fact the dash is still out of the truck. I wanted to test before I reassemble.

After the core was installed I ran the truck to get the water hot to test for leaks. No leaks but also no heat at the core. The two heater hoses are firm but just warm from reflected engine heat. The large upper radiator hose is very hot to the touch. I ran the truck with over flow cap off while parked heading up hill to help purge any trapped air. After several cycles still no hot water at the core. I can still touch the upper part of the core and it's just air temperature.

I'm lost as to what could be the problem. Is their a valve between the water pump and the heater core?

97 f150 , Sony Trinitron 4.2L

 

I imagine you checked the thermostat?

The two hoses that connect to the heater core, one side has a flow restrictor or something in it. Is this restrictor clogged?

Just some ideas, hopefully this helps

 

Resloved: It was a bad water pump.
I had the cooling system flushed and the same night I had a coolent leak from the water pump weep hole.

The water pump had deteriorated so bad that all the blades on the impeller were gone. They were erroded away. The heat from the large radiator hose was heated by convection and not pumped.

It was the original water pump with 117K. Never had a bit of over heating trouble. I guess the fan blew hard enough to cool the engine.

This site is great resource for data on repairs. I'm certainly glad I came across it.