A couple of things: it sounds as if a lot of crud broke free and was floating around in your cooling system, probably blocking the flow in several places which would help cause the higher temps. Also, bear in mind that running a modern system without the proper pressure (no cap, cap loose, pinholes in the system, etc.) will cause the coolant to boil out at (relatively) low temps. The boiling point of a system rises 3 degrees for every pound of pressure: a 7 pound cap will raise the boiling point to 233 deg, for instance. I think that, had your system been airtight, even with the blockage, your engine would not have boiled over. It's the combination that did you in. It sounds like you've covered all of the bases, though. Good luck!
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