Every month my 5lt AU loses about a header tanks worth of coolant (driven few days a week or even all week) and requires top up and weather does not affect this.
I am currently a 3rd year apprentice mechanic and grown up with cars my whole life so im more than confident when it comes to cars though i have no idea where my coolant is going, I have tested everything, its been under pressure up to almost 20 pounds and no leaks, no leaks inside the car, no leaks or cracks or stains anywhere on the hoses, or stains on motor, it has a new radiator as it was starting to go along with the heater tap, before that the upper rad hose is new due to bypass hose to header tank split off, radiator cap is new (Non-Recovery cap, is the right one and rating) along with a new header tank as the old one brittled away bout 3 years ago. I also have a homemade overflow bottle connected to it to see if its releasing out of the cap and nothing.
It runs beautifully, never blows white smoke/steam, and does not have the cooling system pressurise straight away when its ran from cold, i also here timing cover leaks on them, though no sign or stains once again. All i can think of is a CO2 test but almost positive it'll test positive but it keeps going through my head that the motor may be eating it as a result of a blown head gasket/s.
Any ideas anyone?

don't have much experience with the 5lt but in the 6's the head often goes between 5 and 6 and leaks straight into the bore.

Sometimes it's not enough to be noticeable but it's still happening.

Maybe pull the heads off and check everything if you have the time?

sorry i somehow accidently posted two of the same posts, yeah i know 5 and 6 are bad on them, just rebuilt top half of my old eb that i sold to my mate last weekend, probably the 20th ive done , nah i dont have the time barely have the will, 42 hours a week workin on dunnydoors and rice burners all week makes me feel i dont wanna know haha

its usually the first sign the head gaskets leaking but not too bad at that stage.but it will get worse after time,but hey its a ford they eat water

yeah its killin me

Had a similar problem myself for months, for the life of me could not work out where the coolant was going, found a small corrosion hole in the metal heater pipes that pass under the upper intake manifold, just enough to slowly lose coolant but not enough to be obvious...

if it's been pressure tested with no leaks, radiator/header tank cap.

upper intake manifold? this is on the 5.0lt?

Have heard of that before.. Header tank cap ... wont cost much to at least check....

Sorry, didn't mean to confuse, the big metal box thingy sitting on top of the motor with 5litre V8 on the top

just checked it then, the one where two heater hoses from the tap come form into it and then underneath, cant really see it too well but cant see coolant stains eiether, i have an alloy tank on the way for it as the plastic one fitted is pretty dodgy, gettin old, see how it goes after that, if not it'll be manifold off yeah? cant really get a mirror under there and like i said dont leak under pressure

doubt it will be a head unless ones cracked.
the old 302W is useually preety good at holdin its heads together.