2012 Prius occasional overheat and no cabin heat for 20 to 30min

Check your fuse for the heating element, as well as figure out how to check the heating element itself...

 

While there is a supplemental electric heat element in there, it tops out around 700 watts, which is scarcely noticeable in comparison to the heat you get from circulating coolant (around 5300 watts worth, when it's up to operating temperature).

In most situations, the 700 watt element never makes enough heat for you to feel a difference, though you can notice that it will reduce the time to defog the windshield. (ECO mode locks out the electric element, so that's an easy way to compare.)

If the issue is being unable to feel much heat when the engine is up to temp, the OP is probably on the right track thinking about the circulating coolant.

 

There isn't any modulating valve that regulates flow through the heater core; it gets flow whenever the coolant is circulating.

I am surprised that you report feeling a bigger difference between supply and return temp when you were not feeling warm air out, and a smaller difference when you were. That's, um, not usually the way the physics works.
But feeling some difference does suggest that there is at least some coolant flowing through the core.

Have you confirmed that the air mix door moves as expected? You can move it around as an Active Test using Techstream, and see its position in the Data List. Without Techstream, I think there is still an actuators-test mode you can get into from the HVAC controls (try holding AUTO and RECIRC simultaneously while turning the car ON). That will also give you some two-digit diagnostic codes on the HVAC display, if there are any.

 

i have the same issue. it overheat intermittently. and it happens more and more often. the turn on & off trick usually works. but sometime it takes more than a few try to make it works.
consulted with someone who used to work with toyota, he suspects it is the computer problem and may cost over a thousand dollars to get the computer replaced.
james, have you fix the problem and how did you fix it?
or anyone has any ideas?

 

The computer has next to no control over any of that, except over how fast the water pump runs and when the cooling fans come on. Everything else in the cooling system is just pipes and hoses and a thermostat based on expanding wax.

Maybe you could capture a data log of the engine coolant temperature, the water pump RPM, and the cooling fan status?

 

what should i use to do the data capture? i have a scangauge only.

talked to another person who works on prius very often (taxi fleet), he said check the water pump. it sound more like the water pump is working sometime and not working sometime.

so i think i will change the water pump and thermostat and see what happen. should i get the factory water pump or any aftermarket water pump is good enough?

 

Techstream is one option that can do data capture. Some searching may turn up others.

Of course, the idea that the water pump might be working intermittently is exactly what you could check by datalogging the water pump RPM. But if you don't have a way to do that, and just want to bet the cost of a water pump on that being the explanation, that can work too.

Reports on aftermarket pumps are not all favorable here.

 

to report back, waterpump is the faulty parts. in the last 2-3 weeks, it always take about 5-10mins to make the warm air come out every morning. once the warm air blow, coolant temperature drop right after. waterpump changed, and change the thermostat as well, coolant temperature back to normal in all conditions.

 

Has the EGR system ever been cleaned? Any chance there’s a stop-leak product in the coolant?