Let it idle a few hours with heater off and monitor the situation. Had a leak from the rear head gasket a few year ago , 200 miles and must ad 1/2 gal coolant . Could not finde it course it evaporate before it could reach the floor.

Watch the reservoir if there is bubbles or level raise much (could be course by a blown gasket/or crack head and get compression in the coolant system.

Dos the radiator fan kick in and off like it shuld do ?

And you are mistaken about the function of the res. bottle.. it has an overflow outlet and can drain down to nearly empty depending on pressure of coolant possibly overfilling it. You are checking coolant at the radiator and NOT at the separate res. bottle correct?

Same system in all gas models 3,3 - 3,8 pre 2008 .

ofcourse the radiator has to be fill with coolant before making the test. if any air pocket in engine block a pressure test is worth less.

If any leak in system it will draw air in when coolant system is cooling down when you leave the car. and not from the reservoir.

A big misstake MOPAR didn't fit a coolant level sensor in the system.

If any compression air is entering the coolant system if will proberly end up in the reservoir tank . if real bad leak it will force the coolant out first .

You could also just leave the radiator cap of and let it idle normal it force a little water out weile heating up, if any air bubbles you got a problem.

The reservoir can offen get black inside if combustion air get in to the system.

Put some of this stuff in it, and hit it with a black light to find the leak.
https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds

BillTheAxe just beat me to it. I saw this idea on tv and was astonished how good this operation was. You'll need to black-out your garage though...
Leedsman.

Hear me out.....
Maybe a head gasket.
A small leak into a cylinder that only draws coolant in on the intake stroke and promptly burns it. It's not impossible. I had a car with disappearing coolant for two whole years! The system passed all the tests you have done. We just shook our heads every week when we topped off the plastic reservoir. One day after a couple years the thing started pumping out lots of white sweet smelling "smoke" from the tailpipe and all the coolant disappeared in minutes. That was the eureka moment. Sure enough, the head gasket had a small failure right next to a cylinder bore. The vacuum generated by the intake stroke was just enough to suck in little bits of coolant that could be burnt off. When it finally failed it was a classic mess with foamy milky oil and all that crap. Up until that point... nothing, just disappearing coolant.

I hear you clearly ;)

Sometimes it just so .... :)

Had a similar with headgasket tiny leak from rear gasket and the leak was so small that nothing was on the pavment . and when it cooldown it draw air in through the gasket. Next day you could hear bubbles in the heater. Took me 6 months to finde the leak :)

In UK I seem to remember an episode of "Wheeler-dealers" on tv where Ed. (the mechanic) put something in the coolant that fluoresces (hope I spelled that ok) under ultra-violet light. So after turning out the workshop lights, he went round with his little lamp until he found the fluorescence.
Hope that helps.
Leedsman.

Ring amongst others - note you want one not all four