Only Cylinder 4 misfires

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by johnnychimpo, Feb 12, 2021.

  1. johnnychimpo

    johnnychimpo Member

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    After some hard work head gasket has been replaced successfully.
    along with fuel injectors plugs and coils.
    egr circuit cleaned along with cleaning intake and throttle body and replaced intake exhaust and EGR gaskets including injector seals.

    A pressure test has been done twice by separate mechanics both find compression is good on all four cylinders
    Car has no codes

    The car misfires on deceleration and only on number four. we thinks its a leak on the intake but wouldn't that affect all four instead of just number four.

    Any thoughts?
     
  2. tankyuong

    tankyuong Senior Member

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    Sound like your rods are bent
     
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  3. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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    Maybe just one of them - but wouldn't that have shown up during compression testing?
    I'd swap some of the obvious components (coils, plugs, injectors) from #4 to #1 or #2.

    Fairly easy to do and definitive in ruling out those components.
     
  4. Mendel Leisk

    Mendel Leisk Sand Pounder

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    It’s usually piston one or two that gets bent piston arm.
     
  5. johnnychimpo

    johnnychimpo Member

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    All components replaced. New sparks coils and injectors. Two mechanics confirm compression is good. What is the tool used to measure depth of piston? I think this will for sure reveal the answer. Also car only went around 100 miles with bad head gasket and 2/3 rds of coolant. Then gasket replaced.
     
  6. wheezyglider

    wheezyglider Active Member

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    The only way I can imagine an intake leak affecting only #4 would be damage to the #4 part of the IM gasket, which would be easy to check. (That orange silicone gasket between the IM and head has a ring for each cylinder's intake.) I can't speak to the likelihood of that being the actual problem though.
     
  7. rjparker

    rjparker Senior Member

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    I don't understand how deceleration impacts the engine which should be off. Are you driving in "B" mode?

    Maybe a shudder is felt when the ICE is cycling off or on. Another engine running problem may be causing the misfire code. An engine misfire code would normally occur when it is running or starting.
     
    #7 rjparker, Feb 16, 2021
    Last edited: Feb 16, 2021
  8. johnnychimpo

    johnnychimpo Member

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    No I'm not driving. This is the progress the mechanic has made he has explained that the engine misfires on number four but only when decelerating. Since compression is good I advised him to measure the depths on the Pistons to see if one is different than the others. I suspect if and big if cylinder number four has a bent rod the compression would be less and the rest correct?
     
  9. rjparker

    rjparker Senior Member

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    Again, a Prius engine is normally off when decelerating. Bent rod or not. Ask your guy to clarify.
     
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  10. mr_guy_mann

    mr_guy_mann Active Member

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    "Good" is not a useful test result. If someone performed a compression test, then please post the actual resulting values for each cylinder.

    Yes, if a connecting rod is bent then compression on that cylinder should be lower than the others. (At least until the bent rod failes completely)

    What is your symptom? I agree that misfire on decel makes no sense as typically fuel is cut so there is no combustion. If there are no codes then what is pointing to cylinder #4?

    If there is an intake manifold or lower injector leak that "only" affects cylinder #4, it would show up as higher than normal fuel trim numbers- especially at idle but less so at higher rpm and load. Again if it is a single cylinder misfire, but has poor fuel trim under all conditions then you might have a (new) bad injector- but that's another guess without any test data to support it.

    Posted via the PriusChat mobile app.
     
    #10 mr_guy_mann, Feb 16, 2021
    Last edited: Feb 16, 2021
  11. johnnychimpo

    johnnychimpo Member

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    First test gave 160 psi on all four and second test yielded 150 to 155 each. I suspect difference in tools but still with in specs. The behaviour was there prior to replacement sparks and cops. I can ask hom to replace them again.
     
  12. rjparker

    rjparker Senior Member

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    Your mechanic knows to swap parts to other cylinders. I hope he has already done that.

    C5AC978F-9512-4255-9218-C74B61759467.jpeg
     
  13. mr_guy_mann

    mr_guy_mann Active Member

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    Again

    Assuming that your mechanic did in fact replace ignition components (and that the parts he installed weren't defective in the exact same fashion as the previous parts), and your "behaviour" hasn't changed, then what would be accomplished by replacing those parts again?
     
  14. johnnychimpo

    johnnychimpo Member

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    yea I don't know at this point. I was unaware the engine was not firing on Decel so I am at a loss as to what is wrong. If he can figure it out by Friday I will be taking it to the dealer.
     
  15. johnnychimpo

    johnnychimpo Member

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    Tech has explained only miss fires at 40 MPH and all components swapped issue persists on #4. He said may be clogged cat or mechanical issue compression is good on all four so I think maybe weak valve spring not closing. anyone have any other sugjestions?
     
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