Where is Psl......and what is your question really ?? In most cases, it is impossible to tell exactly what caused a part to fail.......including head gaskets. Sometimes an episode of overheating causes the heads to warp.
Engine knock, too high compression, running rich, running lean, dirty injector, bad plugs, bad water pump. Has your engine had a rough idle/knocking?
What's the root cause of high compression, carboned-up cylinder heads? Which in turn gets back to the PCV and EGR? Ditto for dirty injectors? And rich/lean? I'm starting to hate those two acronyms...
I'll write that most blown head gaskets occur from overheating....eg driving with low or no coolant or failed water pump. Yes, high compression will blow a head gasket. In a Prius, I am not so sure excess carbon build up causing excess compression would do this, yet these cars have many of their system interwoven mechanically . Also, loosened (or incorrectly torqued) head bolts over time. Broken head bolt (can also occur from corrosion). Failure of the gasket near the coolant journals. So, a warped head being a root issue or a failed gasket....yet what caused those issues? That's the question. Once the head is removed I bet you'll have some leads to probably ascertain the answer. Let us know your findings. Good luck.
Failure to change 'lifetime coolant' every 50k 5 years otherwise coolant turns acidic eating engine parts like water pump and gasket.
