Syncman

Driving me crazy. No leaks with pressure test, no visible leaks, no gas in radiator, yet using about 1/2 gal per week of antifreeze. Antifreeze is clean, oil is clean. Is it OK to use product like "Bars Leaks"? Mechanic thinks pulling head to investigate is a waste of money on such high mileage engine, and to just top up till engine needs replacing, also that Barsleaks may clog radiator. Someone suggested black pepper in radiator, but that sounds like an old wives tale.

Tech

Well it's going somewhere. If you're positive of no leaks externally then it has to be head gaskets. Just because engine has 240k doesn't mean that it's at it's end of life. I'd be looking at head gaskets. Or you can have them price out cost of labor for repair vs replacement and choose what better suits you ethically

SWHouston

You might try one other thing...
You may be getting a brief boiling/surging when you come to a stop, and getting a overpressure through your Rad Cap. Try putting the overflow tube from your Reservoir into a secondary collection jar (just tape/wire it along side the Reservoir) and see if that may be where the Coolant is going.

IF you do start getting some Coolant in it, you might try replacing the Radiator Cap.