Odd Thing I Noticed with Heat @ Idle vs Off Idle

[Silversn95]

So, with the very cold temperatures we have been getting over the last few weeks I have noticed that it seems the temperature of the air coming out of the vents cools down when I'm sitting at idle and warm up when I'm cruising. Never noticed this before but then I haven't seen -25C with this SUV before. Is this normal or a sign something might be about to go south on me?

Conditions:
Explorer warming up almost at normal operating temperature
Outside air temp -25C
HVAC on full hot, Vent upper and lower on

At Idle - if I waited long enough, temperature out of the vent barely warmer than ambient.
While Cruising - temperature at Vent hot enough to make it uncomfortable to keep my hand in front of it.

 

[Turdle]

Check the coolant reservoir and radiator, you have low coolant level.

 

[Silversn95]

Did that and found the recovery bottle empty !! Filled the radiator and bottle back up and let it run a few times and topped-up the bottle to the fill line. I still think it has a temperature variation but nothing like it was before.
I would like to know where the coolant went, there are no leaks that I can see and no smell of leaking coolant. Does it evaporate over time? The cooling system was flushed and refilled three years ago when the rad was changed and this is the first time I've touched it since.

 

[Turdle]

OK, now you need to burp some air out and refill. Parking on an uphill incline will help get the air out of the heater core.

As far as where the coolant went, you will need to be diligent. If you park on a dirt surface place a piece of cardboard under the truck to see if there is a puddle. If not, you may have a leaking head gasket.

 

[Silversn95]

Boy, things change in a day! Cold again this morning and could see vapour coming through the grill when I parked for work. Opened the hood and found coolant puddling in the valley of the engine. Had it checked out and it looks like the thermostat housing is cracked. Sounds like a known issue on the 4.0L. Glad this was caught before I had an overheating incident!