Just change the cap you'll be fine. Water boils at 212 as you know. Every cap 'gives' you pound you 3 degrees above 212 before overheat/boil over. For example a 1 pound cap gives you an additional 3 degrees. 15 pound cap= 45 degrees above 212, that gives you 257 degree coolant before you get into trouble. This is why guys get scalded so badly, 257 immediately explodes in the atmosphere to instant 212. Anyway, fill it. bleed it recap it with a vendor supplied car or a correct one you'll be fine. Air is getting in. the water is stuck at 212 and that won't work.
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