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    kammssss Member

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    Hey People,

    When, in terms of miles, did you change your thermostat? I don't see too many threads about this. I am approaching 200k, so I thought I would change it as a precaution.

    1. Any tips on taking it out? I don't see any nuts/bolts holding it like most cars. Thanks.
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    seilerts Battery Curmudgeon

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    The only time I ever changed a thermostat in one of my Toyotas was to put in a 160F into an 81 Supra that had a tendency to run hot. In 1 million Toyota miles, I have not had one fail on me. In a Prius, it should last the life of the car, or at least the engine. That's why there is scant info here, because it just isn't really necessary.

    Now, should you choose to proceed, combo this with your normal engine coolant change. Otherwise, drain only enough coolant to drop the level below where the radiator hose connects to the block, so as to avoid having to bleed the system. When installing the new thermostat, make sure the "jiggle valve" is on top i.e. 12 o'clock. I can't remember if you need gasket sealer for this particular job. Use only a new thermostat from Toyota, since they never fail.
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    kammssss Member

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    Thanks Boss. I think you are right about Toyotas' thermostat: they don't fail. I won't be changing it at 200k after all.
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    firepa63 Former Prius Owner

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    If it ain't broke, don't fix it.
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    Patrick Wong DIY Enthusiast

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    The engine thermostat cover has two nuts holding it in place. A O-ring fits around the thermostat and no sealant is needed.

    While I agree that the thermostat has a low failure rate, since the car has 200K miles, then I would replace the thermostat and the radiator hoses the next time that the engine coolant is replaced.

    It would be sad to lose an engine due to an easily avoidable cause like a thermostat failing shut.
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    seilerts Battery Curmudgeon

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    If the t-stat is stuck shut, then the car will provide the red triangle and high temp warning light. This sort of thing has DIY disaster written all over it, as in, it is much more likely to cause a problem than to prevent one. Prius t-stats live in a pristine environment of clean, high quality fluid. If there was a failure pattern, such as 04-07 inverter cooling pumps, then it would be a different story obviously.

    Searching Prius chat, the only mention of a t-stat being stuck closed was in the following thread, which also mentions that engine coolant was low, and the technician who reported the stuck shut thermostat did not actually fix the problem with a replacement.

    http://priuschat.com/forums/gen-ii-...hooting/96172-persistent-high-temp-light.html
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    Patrick Wong DIY Enthusiast

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    Hmm, there are many DIY procedures suggested on PC that have the potential to cause disaster if the individual involved is clueless or careless. For example:

    - changing engine oil and oil filter
    - changing transaxle ATF
    - changing inverter coolant
    - changing engine coolant
    - changing serpentine drive belt
    - jump starting the 12V battery

    I would say that changing the Prius engine thermostat is easier than replacing 2G engine coolant.

    In any event, an individual owner can make his/her own decision about whether to replace the thermostat as a preventive move or not.
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    northwichita .

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    I replaced my thermostat with a warmer one at around 120k (88C vs 82C) along with the coolant change. I did have a problem with air in the system with the change, I'd recommend anyone doing this to monitor their engine's temperature with scangauge or equivalent after the change..
    I looked on the web for durability, some said not to change, others------

    quote
    Buy a Toyota thermostat, and replace them at around 100,000 miles or 5 years. Brand X may or may not be as durable, and there is such a thing as false economy. Some say the Toyota thermostats never need replaced, but I talked to an engine rebuilder who says he sometimes gets Toyota heads to rebuild or replace, somewhere around 150,000 miles, because of bad thermostats.

    For a number of reasons, it is not always a good idea to wait until a part fails to replace it, and a thermostat is the best example I can think of, because you just blew $2,000 in repairs when that sucker breaks.
    ---------------------------------------------

    unquote

    .
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    Mendel Leisk Senior Member

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    I changed a thermostat on a previous Accord we had, I think around 150,000 km. The car had started displaying the symptom of engine temp gauge down at the bottom (cold), after extended freeway driving, when cooling is quite efficient.

    If Toyo's thermostats are similar, there is a rubber gasket that can harden and brake over time. Also, without a scangauge or similar, you can't even tell the coolant temp in a Prius. I'm not sure what the internals are, but putting the old and new thermo's together in a pot of water on the stove and bring the temp up: there was a noticeable difference in the opening and closing times.
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    Stev0 Honorary Hong Kong Cavalier

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    I changed it about a week ago. I changed it from 74 to 72.

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