Forgot to add the same disclaimer as 357 up there. I am not a mechanic. But do work on my own cars. And I'd rather not pay someone to do something that I could do on my own. Time allowing, that is.
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btw, I'd be interested to know what you find out.
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There is only one real way to fix this

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Hmmm - I actually have a similar problem in no heat unless I'm going above 25/30. So check this out -

The cause is this: The EGR cooler cracked and was replaced but an inline filtration wasn't done properly on removal allowing debris to enter the cooling system and help clog up the oil cooler which itself was most likely partially clogged with sand casting material that wasn't cleaned thoroughly during manufacturing. Since the coolant can't flow through the oil cooler then it can't properly cool the EGR cooler hence the cracking. In addition since it can't flow - it sits in the oil cooling passage ways until it super heats and vaporizes, blowing back into the degas bottle, puking all over the engine. As this happens there isn't enough coolant flow to properly produce enough heat for the cab. The fix is roughly $2600.

So check that out on yours and see if it solves it. :-)


Just joking - only those of us lucky enough to own a Ford 6.0 Diesel motor get to have this much fun.


Ron
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My Dodge Ram does that at odd times. Its a vacuum valve thats screwy....
I can go down the road with just the "vent" on and all the sudden, it goes to the defrost air flow. I let off the gas or take off the cruise control (also vacuum actuated) and it goes back to "vent".

That or the thermostat is sticking... Get a "failsafe" thermostat. If the fail, they fail in the open position; so not to overheat your engine.


Look and see if your water pump is dripping any water out of the bottom of it. When they go bad there is a "weap" hole that water comes out. I had to change mine last summer bc of this.
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