Not to be the lone dissenting putz of the group, but I don't instantly agree with the head gasket opinion. The fact that the pressure test is ambiguous leads me to ask a few questions...
- You mention a pressure test. If this is a pressure test of the cooling system alone then you only have half the answer. A proper cylinder compression test should be done to determine if the problem is actually the head gasket or if its something else.
- If the coolant is a correct 50/50 water / ethylene glycol mix and it was a head gasket, would you not smell a sweet note at the exhaust or see a slight amount of moisture in the oil? With that in mind, water spraying into a cylinder would eventually cause a cylinder misfire or a coolant fouled or colored plug.
- To your knowledge has some idiot used Bars Leaks or some other coolant system clogging bullshit in your vehicle? (And has the entire system been properly flushed and refilled per manuf. spec?)
If the engine is operating at correct temperature (which it appears to be..) and all of the other parts are pressure tested, my bet would be on a clogged return line in the coolant system or air bubble that causes the pressure difference that fills the overflow tank when it's not running.
I would approach the problem by getting a compressed air source and "force draining" the system from the highest point. Probably around the thermostat. Some tight rags around an air nozzle until the system is empty. Then do it again a few times with clean water, running it for a few minutes in between cycles..
Refill the system with a 50/50 mix and make sure that there are no "bubbles"... This can take a while as the engine has to get up to running temperature and thermostat full open before all of the air is out of the system. Not real familiar with the 3.0 but I have seen a lot of bleed valves on high spots within the coolant system. If none exist, fill the radiator per the manufacturer spec and warm the engine with the cap off. Once the motor is warm, monitor the radiator level and replace coolant as needed. Make sure to give all the radiator inlet and outlet hoses a squeeze a few times to make sure that there is no air left in the system. Once it stops dropping, seal the new radiator cap.
Take it out and get it hot, run the A/C, etc.... Once home, carefully check the coolant levels. (Even under the cap with a couple of rags..)
If everything looks clean, you should be golden.. If not, I would immediately check for a leak somewhere in the coolant lines that is adding air into the line somewhere... Don't forget about the EGR
system as hot water can sometimes run through that as well....
I was a mechanic a long time ago but gave it up for a cushy desk job with great benefits and a secretary with tits that still make me tear up a little.