No time to work on it yet. We have winter here and I am a driveway mechanic.
Temps in the teens and 30 mph wind sucks being outdoors (unless you're hunting!)

I also noticed it will start off putting out hot air and then before I get to work (4 miles) cold air is coming out!

 

I took the actuator out and decided to connect the harness to it, turn on the heater, and see what happens. The actuator turned like it should when I rotated the temp control from cold to hot several times. Don't think I need a new actuator. Maybe the door itself is "broke".

Symptoms again: After letting the truck warm up, heat comes out like it should (temp control to highest heat setting), then while driving at any mph cold air will start coming out without touching any control. Then later heat might come out again! Then cold. WTF?!

 

In addition . . . I just manually moved the blend door and it seems to be working fine. I also connected the actuator back up and could see that the "arm" was moving as I rotated the temp control up & down.

What else could cause this??!!

I also was checking coolant reservoir and looked low, as I loosened the cap, the fluid level came up to full from "empty"! Could I have had the cap too tight?

 

If cold air is only coming in while the vehicle is in motion, perhaps we're focusing on the wrong part of the problem here and the fault is not mechanical but structural. Has this vehicle been in any collisions or had any kind of things in the cab slide around and hit the dash area, firewall, or lower glove box area? It sounds from your description like one possible cause could be cold air somehow rushing into the system from outside the truck. At speed, cold air rushing into the ducting would far "outflow" the hot air pushing through the core, regardless of blend door position giving the indication and you have lost heating when maybe you haven't. So:

When the hot air cuts out, does the flow rate increase or change at all?
Is the hot/cold performance of your airflow tied at all to vehicle speed?

Unrelated but I didn't see this in the previous posts, but have you tried flushing the core? I've never heard of an intermittent blockage that returns periodically, but higher speeds = higher rpms = higher water pump rpms = higher coolant flow rate = possibly pushing an obstruction into the core's more narrow passages. When rpms and hence pressure subsides, the obstruction might fall back out into its "on deck" position ready for the next high flow rate to relodge it. Again, very remote scenario but backflushing the core would eliminate that. Normal flow is from the back of the block to the core and out of the core to the water pump's top fitting, so disconnect both hoses and run a garden hose into the water pump hose to the core and catch the contents in a bucket out of the hose previously connected to the back of the passenger side head. See what treasure you get out. If you get something good be sure to post a picture of it.

It sure seems like a long shot but you seem to have exhausted traditional mechanical avenues. Good luck!

 

SOLUTION!![
Sometimes we make a problem harder than it is!!
What a dummas.......... I was a gallon & 1/2 low on coolant!!
Works fine now.

 

Glad you got it and it was an easy one. Nice to catch a break every once in a while!

 

Why were you a gallon and 1/2 low? you might want to find out why...

 

I would investigate the coolant loss also.

 

What they said ^^^^^^^^^^^

What coolant are you using? Have you checked SCA???

 

Uh-oh....how is your engine oil and trans fluid level?

 

Guilty of not checking it more often. That will change!
Prestone.
I would think the oil change place would have checked it last oil change.
7.3's are known for burning water somehow aren't they?

 

No, there's no documented history of these engines consuming coolant. Did you recently have a flush done that perhaps wasn't filled properly? Do you have any signs of contamination in your oil from coolant or the other way around?

Hopefully, it's as simple as a small leak or inadequate fill. The remaining reasons for unexplained coolant loss are considerably more unpleasant.

What oil is the oil change place using, btw? Most everyone here changes our own since oil is so important to these engines. Not getting on your case or anything, I'm just curious what a jiffy lube or the like would if with one of these came through the door.