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Thermostat stuck open?

Discussion in 'Gen 3 Prius Technical Discussion' started by fangtl, Nov 20, 2024 at 3:16 PM.

  1. fangtl

    fangtl Junior Member

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    2012 Prius, 132k miles. Recently, I had our mechanic replace the water pump and thermostat with new OEM units to proactively address a sporadic overheating issue. Now, I'm wondering if I may have purchased a defective thermostat.

    I've been using Hybrid Assistant to monitor the temperatures post-repair and have noticed that the engine isn't reaching the normal operating temperature of 180°F. The highest it has gone is 172°F, but when coasting, it drops all the way down to the mid-140s. In full battery mode, it drops to the mid-130s. On steady highway drives, though, I'm seeing temperatures between 155-170°F.

    I’m in the mid-Atlantic U.S., so it's not cold here yet. It's roughly 60°F during the day and in the low 40s at night. The temperatures mentioned above were recorded with the climate control system off. However, I still get heat with no issues, and my MPG has been the best it's been in a long time, averaging almost 58-60 mpg with normal driving.

    Our mechanic thinks it’s fine, and even if the thermostat is stuck open, it won’t cause any harm. He suggested monitoring the car’s behavior as it gets colder.
     
    #1 fangtl, Nov 20, 2024 at 3:16 PM
  2. Mendel Leisk

    Mendel Leisk EGR Fanatic

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    He's probably right. I was monitoring coolant temp with ScanGuage, and the temp range you mention is similar. The only time I recall seeing it shoot right up to say 190F was climbing a local ski hill.

    One way to test, except you need to extract it: suspend it in a pot of water on the stove, with a thermometer in the water as well, and watch it's performance as the temp rises.

    If you've still got the old one, suspend the two of them side-by-side, watch for differences.
     
    #2 Mendel Leisk, Nov 20, 2024 at 4:42 PM
  3. ASRDogman

    ASRDogman Senior Member

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    When the engine is hot, Carefully, touch the upper radiator hose. and bottom hose.
    They should be hot. And since you are getting heat in the cabin, it's likely working correctly.

    Those temps are fine. Because the NEW coolant pump is functioning correctly.


     
    #3 ASRDogman, Nov 20, 2024 at 5:26 PM
  4. fangtl

    fangtl Junior Member

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    Thank you both for putting my mind at ease. I ran the HA app when it experienced overheating and I was consistently getting 190F on the freeway with spikes to 210F so I am running almost 35-40F degrees cooler now given my current range of 155-170.

    Even though the removal of the thermostat seems fairly simple, I just don't have the time and space for it, although I did feel the radiator hoses with a glove when the car was showing 134F and it was warm to the touch.
     
    #4 fangtl, Nov 20, 2024 at 9:37 PM
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