Intermittent High Coolant Temp Message

You should check your oil level, it might be out of oil.

You should also check the RADIATOR, when the car is cool. The reservoir level is not a good indicator of Radiator Fluid level.

You have to remove the front black radiator cover, and you'll see the radiator cap. If you need to add fluid, it just means something is leaking fluid. Example.....radiator cap, water pump, etc

 

Belt fell off water pump or engine ran out of oil and blew the head gasket.

 

With your high mileage I would do a radiator flush and new coolant while you are replacing all the other stuff.

The Prius is touchy about trapped air, there are YouTube videos about removing trapped air, I don't know if Gen 3 Prii are as touchy as Gen 2 or not.

Worst case a blown head gasket.

 

I am sure that permanent damage was caused to the engine since you continued to drive the vehicle knowing the engine had overheated. The drop in temp was probably due to to engine coolant leaking and no longer reaching the engine temp sensor. The actual engine temp remained overheated. Normal operating temp is 185 degrees F.

1. What was the fluid level in the radiator?
2. Your question about what could cause coolant to come out of the reservoir: how about excessive pressure in the system due to the engine overheating and fluid exiting via the radiator cap. The issue is not necessarily that the radiator cap is bad - rather the issue could be that the pressure in the system exceeded the normal values and the cap allowed fluid to exit as it is supposed to.
3. Your observation that the cabin heater was not working well for several months prior to this episode is further evidence pointing to air in the engine coolant loop. It would be reasonable to say that the engine was near overheating all of that time.
4. I suggest that you plan to replace the engine if you wish to keep the car. Or else tow the car to the salvage yard, 380K miles is an extraordinary service life.

Here is my theory about what happened to your car:

a. The engine coolant pump developed a leak at the pump bearing. This is very normal after 80K miles of service life.
b. Eventually, sufficient coolant leaked out so that the cabin heater operation was impaired.
c. Because air entered the coolant system via the pump bearing as fluid leaked out, the reservoir did not replenish the radiator and the radiator level continued to drop.
d. You and your wife continued to drive the car and the radiator level continued to drop.
e. Finally your wife noticed an overheating indication which showed the coolant level was at a seriously low level. IF at that time you had replenished the radiator coolant, noticed the coolant pump was failing and replaced it, you would be good.
f. Instead you and she ignored the overheating, which led to your current situation.

Good luck with your decision about what to do with the engine repair.

 

It's a leaking water pump that's causing the splash. Change the water pump and belt

 

for the next 3 days, every morning, check the radiator coolant level. Fill until full. It takes a few days for all the air to go away.

You probably don't have a bad head gasket. You may want to change the radiator cap if you haven't done so already

 

Looks like you dodged a bullet, kudos to you.