Random Overheating

Ok, good luck with this!
As you had it in maintenance mode and could observe the process, did the radiator fan come on as it should when the ECU detects the engine temp getting to the upper limit? Because if the fan was operating as it should and not playing a role in these overheating events, then the sticking thermostat is looking more likely. ( I used to take the thermostat and test it by putting it in a pan of water with a thermometer and then visually checking that it opened at the correct water temp, but that was the Good Old Days when cars and airplanes were simple! ) I absolutely agree with going OEM on the new thermostat.

 

Nuts! This is way weird, because I would think that if the water pump had failed, your Prius would continue to overheat and boil over on a repetitive basis as you were driving.
You did check that this fan at the rear of the radiator was properly activated as the ICE got to the 198-200 degrees Fahrenheit range, right? I ask because if the fan did not activate correctly (e.g., faulty triggering circuitry or burned out fan motor) , the car would overheat while standing still, but might still run successfully at highway speeds do to the velocity-induced airflow through the radiator.

 

I had this same issue. Try bypassing the heat exchange. If your water pump is making funny noises at the higher temps it may already be toast. I don't recommend replacing the water pump without first bypassing the heat exchange. You'd just burn up another water pump, I know i've burned up quite a few of them. I bypassed everything but left the old water pump in the mix. Temps still increased at idle in maintenance mode. I'd rev it and stop at around 105c. I put a new water pump w/ heat exchange out of the loop. No heating issues. Raise the reservoir to speed up bleeding the air. Theres some sort of door that will close the flow of coolant from entering the heat exchange but I don't know where and if its accessible. Good luck.

 

Yes thats it. I replaced the actuator where it controls the door for back pressure of the exhaust. But it didn't resolve problem and last time I looked under it was stuck open.

 

Yay! On the new thermostat or its packaging, was a specific temperature to open listed? I have a hazy, 40-year old memory of the opening or fully-open temperature being stamped in the metal ring surrounding the actuating plunger thing, but heaven only knows what they do nowadays!

 

Disclaimer: missed that this is Prius c, my response was for 3rd gen Prius. Might still apply though. And yeah, don't have Prius c Repair Manual:

Thermostat might be rated 180F: "176F to 183F" is mentioned in a test procedure, in the attached, as the point at which the thermostat starts to open.



And this site indicates that the temp rating of a thermostat is the temp it commences opening at:



Stant