It didn't get hot during the highway cruising portions?

 

It did not. It got warm enough to unzip the top of my two coats, but nowhere near warm enough to want to turn the heat down, or take a layer off. I dont think anyone removed layers.

 

Oh okay. Definitely something malfunctioning then. The others have got you covered on possible issues. My bet is on plugged heater core.

 

Duplicate. .

 

One of the things that I used to do for customers with a plugged heater core was I offered a last ditch service prior to replacement and that was to connect a garden hose to the outlet side of the heater core, along with an air pressure fitting tee'd into the hose right where it goes into the heater core. Turn on the hose full flow and then blast it through the tee with 150 psi air pressure. You could imagine the soaking that might have happened if the heater core ever blew up inside the housing, but that never happened. What did happen, and I did this dozens of times, was that the crud would loosen up and blow out the other end of the heater core. Never once did I blow up a heater core doing this and I was working with the ones that were plastic capped. End result was the customer came in with no heat at all and left with a big smile and a heater that worked as good as new. Several customers had repeat failures of the same and every fall I would be hired by them to do this service for them and not once did they ever spring a leak. I myself owned a Passat that had the same issue and I had to clear it out twice before it stayed clear.
So basically take an old garden hose and off the end. Put a tee on the end you just cut off. Disconnect both heater hoses from the engine. Connect the tee to one of the hoses and let the other one hang free. Now on the tee, attach an air hose with a valve of your choosing to control air pressure on and off as needed. Turn on the water for the garden hose full force, then apply air pressure in pulses. Start gently, then as the crud comes out, increase the force of the air pressure. I had gotten to the point of being able to fill the heater core with water via the hose, and applying 100% air pressure and the water/crud mix would come blasting out and spraying a wall 20 feet away. Heater cores always held.
This may give you pause about doing such a thing just by reading this, but I had great success at it and quite a few happy customers over it.

 

The only other contributor to poor heat, IMO is a thermostat that won't close all the way. All it takes is a hairpin crack and there's enough circulation in cold weather to prevent the engine from fully warming up. If you block the radiator with cardboard and then it's nice and warm, thermostat is bad. Plus the "modern" engines with electronic controls rely upon the engine getting up to operating temperature before it goes into "closed loop" mode. That's more of a gas engine thing, but the Powerstroke has a few temperature related settings that need the engine to be fully warmed up. So it's important that the thermostat be functioning 100%.

 

Well. That sounds like a fun, and entertaining project during sub zero temps.... 🤣
that said, i halfway did that this summer. I did 8 flushes with a water hose, through the heater core... however, i never approached that pressure level, and it was the correct direction of flow.... so perhaps, a high pressure, reverse flow would be a worthy, as you say, last ditch effort.

 

I can tell you from years of experience that it makes a difference. I used to use two hoses. One for the feed, and the other one to direct the spray away from me, LOL.

 

or someone's idea of a better thermostat.
found this in a customer's 2002 excursion, it wouldn't even get up to operating temperature in the summer.

 

Seems like an aweful lot of holes....