Engine knocking and carbon buildup

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by DrowningInteger, Dec 2, 2025 at 8:22 PM.

  1. DrowningInteger

    DrowningInteger New Member

    Joined:
    Jun 23, 2025
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    Location:
    Arlington WA
    Vehicle:
    2006 Prius
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    N/A
    I've been hearing some knocking from the car when the engine's running, so I replaced the spark plugs, coils, PCV valve, changed the oil, and had the injectors professionally cleaned. None of that changed the noise, but I took a look inside the cylinders and I'm wondering if all the carbon buildup might be causing the knocking noise. Also the spark plugs only have about 1500 miles on them and they already look kinda terrible. I'm not sure if the blackness is from running rich or oil fouling or something else.

    I have a couple cans of Berryman B12, and I was planning on doing a piston soak and seeing if that does anything. Does this sound reasonable? I'm trying all the cheap and easy stuff before I put new rod bearings in it or replace the engine. I have no engine codes and I can't seem to get a good recording of the noise.

    pistons.jpg spark-plugs.jpg
     
    #1 DrowningInteger, Dec 2, 2025 at 8:22 PM
    Last edited: Dec 2, 2025 at 8:29 PM
  2. PriusCamper

    PriusCamper Senior Member

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    Location:
    Pacific Northwest, USA
    Vehicle:
    2007 Prius
    Model:
    Two
    Keep in mind that a piston soak introduces solvents that can deteriorate/damage some parts inside the engine and should be done only after you've exhausted less invasive options. If I were you I'd read all you can on here about engine knocking and learn as much as possible about how to eliminate engine knocking.

    I know there's a knock sensor that you might look into. Also disconnecting the 12v for few minutes so you get a factory reset of settings.

    In general, I experienced alot of knocking challenges in my old Subaru, but I've never had that problem with my Prius after 200K miles, nor with all the Prius I've worked on. And based on that, I submit that maybe its not knocking, but a different noise?
     
    #2 PriusCamper, Dec 2, 2025 at 9:16 PM
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