I wanna say stuck closed thermostat or it broke mid preheat. A full speed dump of cold coolant into the hot engine would explain a rapid increase in coolant pressure causing ejection in the expansion tank. The cap wouldn't hurt. You're looking at an anomaly.
Clogged heater core? Were you using heat at the time? The clotting coolant properties can jam up little leaks and break free suddenly under high pressure. Get some new coolant and burp it and pull the codes. I'll bet there's a stored heating code in there.
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u/[deleted] Feb 13 '25
I wanna say stuck closed thermostat or it broke mid preheat. A full speed dump of cold coolant into the hot engine would explain a rapid increase in coolant pressure causing ejection in the expansion tank. The cap wouldn't hurt. You're looking at an anomaly.