Originally Posted by jdstrickland

I'm confused. How is it possible that it leaks AND holds pressure well enough that the needle stays in the middle of the scale?

If it leaks, then it has to NOT hold pressure, that's why it is leaking. I can understand that you can't find where it leaks from, but I cannot understand the objection to a leak-down test. I have to admit, I'm not sure what such a test will tell you, but it should tell you that the system has lost integrity and does not hold coolant any longer.

Coolant is collected at the bottom of the radiator and fed into the engine where it circulates, then is expelled into the top of the radiator where it falls to the bottom and cools, then starts over again. Just because you have lost a quart of coolant, the coolant that remains should still flow to the heater to heat the cabin.

My guess is that you have, among other issues, a clogged radiator, although this should also cause over heating troubles. Restricted coolant flow can affect coolant that goes to the heater core, and this will result in cold temps in the cabin. A clogged heater core will also give poor heating performance inside the car, but not affect cooling of the engine.

My point is, the loss of a quart of coolant should not be the reason why the heater blows cold. And, the repeated loss of coolant is either onto the ground, or out the tailpipe. Your description of the symptom set is not clear that the coolant is in a puddle under the car, or simply missing into thin air.