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All cooling system hoses / connections and head gasket are fine and I recently installed a new coolant overflow tank / cap and I have 100% water (don't want to waste money on antifreeze yet). PROBLEM - when I drive 10 miles or so, my temp gauge (which works fine) reads midway (like it should) but I'm losing about 2/3 gal water out through the overflow tank's cap. Over the past two years, I've used probably too much Bar's Leaks and my radiator's bottom petcock is plugged, yet when I flush the system (both top-bottom and bottom-top) the hose water is crystal clear.

Any idea what's going on here?

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100% DISTILLED water or tap water? If you are using tap water that will cause lots of corrosion and plug up the radiator.

I question your replacement of a new radiator cap. The radiator cap for an overflow system has two functional seals. One that opens in vacuum and allows coolant to flow from overflow bottle back to engine. The other seal (13 to 16 psi) ensures that there is pressure in the system. At 13 psi coolant doesn't boil until 245 Deg F or so. I suspect your cap isn't sealing at all and your coolant is boiling at much lower temperature.

This is even worse if you live at a high elevation. Coolant will boil at normal operating temp if you are at a high elevation.

Fix: purchase a new radiator cap and ensure it is seated correctly.

Drain all water and replace with correct blend of antifreeze and DISTILLED water

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