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Strange cycling sound

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Slopoke, Nov 2, 2020.

  1. Slopoke

    Slopoke Junior Member

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    We are on a long road trip, and in the middle of it, we stopped at a stop sign, and I heard this cycling sound coming from under the hood. Took a video of it, hope I can upload it. (File too large, will upload after we get home).

    Have heard the sound one more time since then. Sounds like someone sawing wood with an old fashioned saw. Engine was running when I first heard it, then continued with the engine off. It seemed to last about 5 or 6 times, then repeats.

    Any ideas?
     
    #1 Slopoke, Nov 2, 2020
  2. rjparker

    rjparker Tu Humilde Sirviente

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    Could be the brake booster pump at the master cylinder. Usually it runs when you open the driver's door after sitting overnight but will run at stop signs and other times with brake actuation. Normally you don't hear it while driving because of road and engine noise.
     
    #2 rjparker, Nov 2, 2020
  3. ChapmanF

    ChapmanF Senior Member

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    If it is a Gen 2 Prius, and it is a going-back-and-forth kind of a sound, you might listen to the coolant control valve, near the thermos under the left fender. The engine computer wants to send the valve to a certain position; the valve's position sensor gets old and flaky and sends back wrong jumping-around signals for where the valve is, so the computer keeps trying to send it "back" to the right place.

    The brake pressure pump does not make a going-back-and-forth noise. It just pumps in one direction, and sounds like a pump. Or like one of those prank handshake buzzers.
     
    #3 ChapmanF, Nov 2, 2020
  4. Slopoke

    Slopoke Junior Member

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    Thanks for your reply.

    I have not heard the sound since, but have experienced other issues. I have made another post about them.

    If I do hear this sound again, I will pay more attention to where it's coming from.

    George
     
    #4 Slopoke, Nov 20, 2020
  5. Alty

    Alty Junior Member

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    I have the cycling sound & it seems to be emitting from under the fuse block near left fender. Should the coolant valve be replaced? Will it prevent engine coolant flow?
     
    #5 Alty, Jan 30, 2022
  6. ChapmanF

    ChapmanF Senior Member

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    Often, the cause of the problem is just the position sensor inside the valve. The rest is mechanically ok, but the computer gets flaky information about the valve position, so it keeps sending the valve back and forth trying to find it.

    You could follow this thread about the position sensor (a simple potentiometer) as an alternative to replacing the whole valve assembly.

    P1121 and inside the coolant valve | PriusChat

    [​IMG]
     
    #6 ChapmanF, Jan 31, 2022
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