I have a 2006 F250 king ranch with the same problem. It is the 6.0L diesel, a little over 269,000 miles. I purchased this truck back in April of 2017 so I haven't had it very long. Seems to have been taken care of. It has had the EGR delete but they didn't do the head studs and I don't have a tuner for it yet. CEL is on but it's an EGR code. So now that you know a little bit about my truck.

This problem just started today, Since I've been on vacation since Dec 21 the truck has moved since I got home for vacation. Temp gauge has always worked. But today went outside and started the truck about 5:45 am it was 12* outside. Which I know for a lot of you guys that's not cold but for us in Arkansas it's very cold. Let it set warming up while I finished getting ready for work. Went out to the truck to leave and the temp gauge had not moved any at all. Probably ran for 15 minutes by that time. I drove to work which is a 15 mile drive, Temp gauge never moved. The air coming out of the floor registers wasn't cold, but it wasn't Hot like it should be. When I got to work I opened the hood, engine felt warm. Degas bottle is a little low, below minimum. So could that possibly be the problem.

Also the heater control valve, should there be vacuum to it when you have the heat on? If it should have vacuum on it with the heat on then that's my problem. But since the temp gauge never moved I figure it's the thermostat stuck open.

Since I've had heat before today I wouldn't think I'd have a stopped up heater core.

Antifreeze looks good a clean, Person I bought it from said he had flushed it at the same time he had the EGR delete done.

It was a little hard to start today, I blamed it on how cold it was. But this is my first diesel so I really didn't know what to expect on that.

Thanks,
Delos

 

Sounds like a typical stuck open thermostat. It might have done it for a while but the colder weather makes it more obvious.
Replace it with a motorcraft thermostat. Very easy to do.
It's not recommended to drive it this way for very long.

 

If "not heating up" means the entire engine and cooling system there's only one thing that can cause that - bad thermostat. If you're getting different temps at different points (radiator, water pump, heater core, etc.) then it gets a little more complicated.

 

I really agree with getting a genuine ford thermostat.
Many years ago my Ranger pickup would not get warm. I live in Mn. Would take 20 miles to get any warmth in the winter. Installed a “generic” thermostat. Maybe got a bit warmer, but still couldn’t defrost the windshield on a cold day. I flushed heater, installed cardboard in the grill. Froze my butt for 2 winters.
Then on advice of a neighbor I bought a Ford thermostat and 10 minutes later I had heat again. I was mad and happy at the same time. I froze for 2 years because of a simple part.

 

Who makes motorcraft thermostats?

 

Stant makes motorcraft thermostats, purolater (US) Wix (CDN) makes there filters and autolite makes there plugs, Marathon-Ashland (Valvoline) formulates there oil