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

Discussion in 'Gen 3 Prius Technical Discussion' started by JamesLee, Dec 30, 2020.

  1. JamesLee

    JamesLee New Member

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    2012 Prius
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    III
    over 200k miles. I have been observing ECT with an obd2 scanner. The temperature rises normally. I think the coolant thermostat is OK. The radiator fan run when ECT reaches above about 185F. No noticeable heat for about 20 minutes. Although I feel slight warmth where the cabin filter is located. I detected the blend door actuator is working. Once it reaches about 185F, if I turn full heat on and off repeatedly, I could get the good heat sooner. I removed windshield wipers to access the heat core. I do feel warmth on both hoses when there is good heat. I feel less heat on the hoses at first when there is not heat. As unexperienced person, it is somewhat inconclusive for me as far as the heat core hoses are concerned. Another related problem is that I am getting overheat warning light when I am going uphill on freeway. When I get cabin heat, ECT comes down dramatically for the obvious reason. After reading online, I thought there is air in the coolant system. I took the car to a dealer and had it flushed and coolant refilled. It did get better. But, pretty much the same problem. I thought about replacing the water pump. But I am not sure. It seem more likely that there is still air trapped somewhere. I already tried removing the ECT sensor hoping to get rid of any possible air. Also, I also tried raising the reservoir tank with its lid opened while doing the maintenance mode. Could someone guide me to resolve this issue? Or maybe this is normal give the age of the car?
    Thanks
     
  2. PriusCamper

    PriusCamper Senior Member

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

    ChapmanF Senior Member

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    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.
     
  4. JamesLee

    JamesLee New Member

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    What controls the coolant flow to the heat core? Or is the coolant always flows in and out the heat core? I set heat to high and with maximum fan speed and observed ECT rising to about 200F, blowing cold air all along. I stopped and checked the two hoses connected the heat core. One was relative hot while the other (out) was relatively cool. Then, I turned off the heat control and immediately turned heater back to high and full fan speed. I get the heat then and also observed two hoses both were relatively warm. Do I have somewhat restricted heat core? Or is something is controlling the coolant flow to the heat core?
     
  5. ChapmanF

    ChapmanF Senior Member

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    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.
     
  6. JamesLee

    JamesLee New Member

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    I definitely felt noticeable difference in temperature between the two hoses. The coolant was not circulating while ECT was rising to 200F. I stopped the car because I knew the temp would rise above 212F and would have overheat warning and also loose some coolant. Only way to avoid the situation is to either try not to engage engine by slowing down or turn heater off and on. And most of times, I get heat and ECT would come down rapidly by doing the turning heater on and off trick. So, I thought I might have a problem with the blend door actuator. While getting no heat, I removed the passenger side filter and observed two separate moving part working when I toggle between A/C to heat mode. The blend door actuator seems to work because I am getting cold air when on AC mode and slightly warm air when on heat mode. I guess should check if an electric heater is working or not. But I am not sure if 2012 Prius has an electric heater to assist heating and plus I feel it insignificant whether works or not. But I am not sure of anything at this point. Very weird. Currently, I just drive the car locally and constantly observing ECT.
     
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