Reply to "What the heck just happened?"

...So much BS here! IF You had a Blown Head Gasket, You would see Steam Coming out of the TailPipe OR/AND Find Coolant in the Oil Pan!! Excepting the Case where the Gasket is Blown Between Cylinders. Is There water in the Oil?? It would show up as Brown Slug on the Dipstick! You can check for a Blown Head Gasket with a Simple Compression Test. It is My opinion that You Just have a 'Weak' 'Pressure Tank' CAP, or a Cap that does not SEAL to it's Lip, Properly. It is letting the Pressure Release and thus causes Your Overheating. You also need a lesson in Physics!! You never Release the Pressure Cap When Hot or Even Warm!! The suggestion that Your Coolant System was Improperly 'Purged' of ALL Air; was Valid! Also A 50/50 percent Mix of 'Anti-Boil/Freeze' to Water, can make ALL the Difference and may 'Tip the Scales' into Your favor of a Steady Coolant Temperature. You never said where the Steam was coming from? Was it the Pressure Cap?; Or the Side of the Block/Head/Tailpipe? Take it for what it's worth! I've Been there!...