2011 engine rattles HARD on startup after sitting a couple of days

Discussion in 'Gen 3 Prius Main Forum' started by BlackPrius001, Aug 8, 2025 at 9:08 AM.

  1. BlackPrius001

    BlackPrius001 Junior Member

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    My 2011 Prius has a very hard engine shake / rattle for seconds upon startup after it has sat unused for just a couple of days. When driven daily it starts smoothly and normally. It does seem to "chunk" harder than it used to when going from battery power to starting the engine to recharge (when sitting in Park). The car has about 140K miles. I did the EGR service several years ago, and I also recently did the brake booster / accumulator replacement very recently after that problem popped up. The car does burn some oil, but outside of that and this really aggressive shake / rattle on startup it is running great. Just worried that this is indicative of the next significant problem. Anyone have any idea what might be causing this or whether it is a concern? Alternatively, anyone have any soothing words of solace suggesting it's not a problem at all? Appreciate any insight.
     
    #1 BlackPrius001, Aug 8, 2025 at 9:08 AM
  2. bettergolf

    bettergolf Active Member

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    Likely it is a problem. This is the major symptom of your head gasket leaking coolant into the cylinder ...a known problem on the 2010 through at least the 2014 Prius. it causes this rattle until the coolant is expelled from the cylinder a few moments later. It will likely get worse over time until you must replace the head gasket.
     
    #2 bettergolf, Aug 8, 2025 at 11:19 AM
    Leadfoot J. McCoalroller likes this.
  3. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Probably the head gasket.

    You did "EGR service several years ago": this was DIY? Did the intake manifold with it's EGR passages get cleaned as well? Can you recall the miles on the odometer?

    To check head gasket, on the day after car's been driven, pressurize the engine coolant system (remove reservoir cap and install stopper of some sort and apply sustained air pressure), remove wipers/cowl and spark plugs*, and run borescope into combustion chambers, looking for exceptionally clean piston top(s), coolant weeping from head gasket seam.

    diy coolant pressurizing gizmo:

    IMG_1378.jpeg IMG_1380.jpeg

    * See video #13 here: Nutz About Bolts Prius Maintenance Videos | PriusChat
     
    #3 Mendel Leisk, Aug 8, 2025 at 11:51 AM
    Last edited: Aug 8, 2025 at 12:18 PM
  4. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Sorry for the bad news but yeah, that's a primary symptom of head gasket failure.

    The best test to confirm this is to remove the sparkplugs and use a video borescope to inspect the inside of the cylinders while the cooling system is pressurized with a test pump.

    Good luck!
     
    #4 Leadfoot J. McCoalroller, Aug 8, 2025 at 11:52 AM
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