My heat wasn't working, so I removed and flushed the heater core. After finally flushing all of the rows (even heat, everywhere. I used hot water), I noticed it was leaking. No big deal, new heater core, fill with coolant... Still no coolant circulating through the thing? The other one had an excuse, but this one is brand new.

I pulled 23 inches of mercury on the cooling system - almost all hoses collapsed and the reservoir was starting to collapse. I idled it until warm and still didn't have heat. After I took it for a highway run at 65 mph without overdrive the heat started working. Then, once I got back into town it stopped heating. So I parked it, popped the hood, this time the heater hoses are hot and the hard lines coming off of the heater core are too hot to touch, so why wouldn't the air blowing through the heater core be hot?

The truck doesn't have any cooling issues, my scan tool reads very consistent coolant temps. The heater hose makes a stupidly designed loop that would trap air, but I can't think of a good reason air would still be trapped as well as the heat was just working.

Does anybody know a proven method of bleeding the heating system? All I could find through google was people suggesting taking it for a drive. The heat worked, but it didn't take care of the air problem.

 

I'd let it idle with the cap off of the coolant reservoir.

 

If the heater hoses are both hot, and you have no heat, I would suspect the blend door or the controller for it. The coolant flows through the heater core when the engine is running. The blend door controls whether air can flow through the heater core, the A/C evaporator, ambient air, or none at all.

 

I think the heat was coming from where it had been circulating as well as heat soak. Blend door & controller were both fine, and I could hear the change in tone as the blend door opened and closed, so I took off the water pump for inspection...

I have no idea how the truck wasn't overheating. I just got back from a 200 mile trip, half hauling another F250 which maxed out my trailer (7k load) and about a quarter towing a car (maybe 5.5k load). A friend of mine had a similar issue with a paper thin water pump turbine on his 5.4 F250. I've had my scan tool on the truck reading temperature occasionally. It holds pretty steadily between 188-200. Go figure. I'll call it a well designed cooling system or a bad temperature measurement system.

Do these trucks have cylinder head temp sensors or probes in the water jacket?