Marginal heat 5.4 f150 Supercrew. I have replaced the thermostat and flushed the heater core both ways till it ran clear (not much rust came out. I can hear the change in air flow so I know the blend door is moving.the hose coming from the front of the intake is very hot. the hose from the rear of the intake that comes from that small tube is not too hot. Which way does the coolant flow through the heater core?

 

No you don't. Those are airflow diverter doors, not the temperature blend door.

From the hotter side to the cooler side.


You still have inadequate flow through the heater core due either to a restriction, air bubble, or a weak pump.

 

How can I tell if the temperature blend door is working on my 2002 F350?

 

If you can control the temperature of the outlet air with the temperature control, the blend door is working properly.

 

My coolant overflow tank was dry and all my windows are fogging up when I turn the heater control **** into the red zone - so I'm pretty sure I need a heater core.

I am going to start searching, but if anyone knows of a good step by step guide to replacing a heater core, please reply or send me a PM.

 

There's more than a few YouTUbe videos about heater core replacement in the F-Series--give that a shot. Here's one to get you started: