Car in sig loses 1 quart of coolant and no more in about 1 hour of driving...

here are the FACTS:

Ebay aluminum radiator
New cap
New thermostat
Coolant jug was filled on bench and held water over night with no leaks
No water in oil
No oil in water
No combustion in cooling system (chemical block test)
Coolant jug is empty all the time after the system loses the quart
NO external coolant leaks of any kind
No coolant burning in exhaust
Car does not overheat
Compression test = fine
Leakdown test= fine

Just to clarify, if I fill the system to the proper amount, and check it again after an hour of run time or 3000 miles the radiator will be exactly a quart low, and with empty coolant jug. It loses a quart and only a quart, no more, and no less, and is accurate pretty much to the drop! I am completely stumped, I have no effing clue what to do next. Any ideas? Thanks in advance!

 

Warm the car up to normal operating temp. Leave it idling and check underneath the motor for water dripping. Check the radiator hoses, the heater and IAC hoses and make sure no water is dripping from the timing cover. Make sure there are no cracks in the radiator leaking coolant. If you find nothing, try having someone give it some gas for a bit while you check for leaks. If that doesn't work, get a UV dye for the coolant system and a UV light.

 

If you can't find it anywhere else, take the timing cover off and check the water pump. Keep in mind that a small leak onto hot engine parts can evaporate before dripping down from the motor.

 

Thanks for the input, I have already done this minus the UV route, there are no external leaks anywhere, and even if there were, doesn't explain why the system will lose only a quart...

 

Unless you drain the entire system and measure the amount of coolant, you can't measure coolant loss beyond an empty reservoir. Once the reservoir is empty, you can continue losing coolant and there's no way to measure how much is in the system without draining. I'm assuming that, "with empty coolant jug" means the reservoir was empty.

 

yes the reservoir is empty, and it takes exactly 1 quart to top off the radiator, that's how I am measuring the loss.

 

take it for a longer drive, if nothing happens then your ok. im thinking since you're not overheating then the cooling system should be good.

 

Ah, I see. Sorry about the misunderstanding. Once enough air is in the system, the pump may no longer circulate coolant because there's no liquid at the pump rotor and it doesn't move air like it can move water. When you use the dye, check the radiator first. The radiator is the most likely place for there to be a leak that is evaporating before dripping to the ground.

 

It can't be air-locking because it holds temp perfectly and doesn't overheat...

 

As air gets into the coolant system it will lower the pressure inside. If the coolant loss actually stops at 1qt then the system is only leaking above a certain pressure. If the loss slows down alot at 1qt rather than stopping then the leak may be somewhere at the top of the coolant system where an air pocket is forming as it loses liquid.