Ok so i just recently bought a 04 Dakota with the 3.7
I noticed that the temp needle was always running straight up but never really over heating.
So i decided to burp the system. In doing so i noticed someone had already been working on it and had replaced the thermostat and had added a filler cap mod. So I burped it at both the purge screw and the filler cap just to be safe.
Well the temps didn't go down. The temp gauge would come up to straight up and stop there.
Ok so i figured maybe this is just how Dakotas run.

ANYWAY getting to the point....
Today i decided to address the mildew smell coming from the AC so i grabbed some lemon Lysol and set to it.
When i bought the truck the AC was great i mean like FREEZING cold . And the vent was set to recirculate.
Well today i had to select the vent to outside air for the cleaning process,
So after turning on the AC and selecting fresh air i set about spraying the Lysol into the outside intake. I did this for about 3 minutes. which seem to have really corrected the mildew problem.

However i noticed the coolant overflow briefly let out a big belch of coolant, even though the engine was not over heating. and the coolent level in the overflow droped substantially. It did it once and did not repeat the action the rest of the day.

Now here's the weird part....
The engine temp gauge needle now hovers slightly to the left and cooler and stays there.
And the AC doesn't seem quite as freezing as it was but it still blows good and cold.
And now,, when i turn on the AC, i notice the engine temp actually goes down a hair!!
WHAT WEIRDNESS IS THIS!?

I figured i must have caused the heater core to purge some air or something when i was flipping around the vent selector.

What do you think?

 

I think you have 3 different puzzles occurring simultaneously:

1) air pocket in the heater core and/or possibly a slight leak in the heater core allowing coolant to escape out of it (would explain the mildew smell)
2) blend door allowing heater core, which is now filled with warm coolant instead of air, to leak hot air across to the AC side,
3) Cooling system operating more efficiently with air bubble gone and being able to circulate water through the heater core (which is essentially a tiny radiator)

 

Good points!
I can cope with a blender door issue , but i certainly hope the was not a heater core leak.
On the plus side. it didn't smell like that all to familiar "sweet warm coolant" smell that i have smelt in other cars with leaking cores. instead it smelt like old cigarettes and gym shorts Also the windshield didn't have a film on it so im hoping, HOPING, that the smell was just mildew from keeping the ac on recirculation for who knows how many years by the previous owner.

 

Take a pic of the temp gauge and post here. Straight up is in the middle, which sounds normal. To get rid of all the air, point the front end uphill or jack it up, then run with the radiator cap off for about 15 minutes.

As far as the vent odor. I usually hit the button to turn off the AC compressor, while still running the fan on high before I park. Helps to dry out the moisture and avoid smells.