New Head Gasket!

Discussion in 'Gen 3 Prius Technical Discussion' started by RyanFlorida, Apr 30, 2018.

  1. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

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    Yeah it depends what you're doing.

    For just-in-case, I like to disconnect the 12 volt neg cable before lifting a caliper off its rotor. Otherwise it's way too easy (and a catastrophe) to trigger a pressurizing episode, which'll likely pop out a piston. Also makes it easier to avoid an excess pedal travel warning.
     
  2. johnnychimpo

    johnnychimpo Member

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    Tell me like I'm 5 how does the egr make head gasket faill
     
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  3. cnc97

    cnc97 Senior Member

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    The EGR gasses are supposed to help keep chamber temperatures around 200 degrees lower then they would normally be. When they aren’t there, one or more cylinders run hotter then they are supposed to, and the coolant sensor doesn’t detect the increased temps.
     
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  4. ChapmanF

    ChapmanF Senior Member

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    The members here who believe that it does have seized on the fact that EGR reduces in-cylinder combustion temperature (which is absolutely true, that's what it's for, to keep the temperature cooler than where NOx emissions form), and have pinned onto that true fact the question "ok, so with EGR obstructed, is the un-reduced combustion temperature high enough to murder head gaskets?" and assumed the answer to be yes.

    It's not an insane question, of course, and the answer could be yes, but to date nobody promulgating that theory on PriusChat shows any interest in finding out whether it is or not. It's just stated to be yes. (A little bit implausibly, given that head gaskets are also used on engines that don't use EGR at all.)

    That assumption about raw temperature isn't the only mechanism that could be involved in a connection between EGR obstruction and head gasket issues. Detonation ("pinging") is an established possible effect of insufficient EGR, and that can be hard on the gasket. The ECM automatically retards the ignition timing if it knows the EGR flow is insufficient, in order to ward off pinging. To know whether the flow is insufficient, it has a Manifold Absolute Pressure sensor, and can see the pressure rise when EGR is let in. The engine also has a knock sensor, giving it an additional warning to retard the timing if any pinging starts to happen.

    But there is only one MAP sensor, and there are four separate EGR passages in the manifold, so there could be a case made that if some of those are more obstructed than others, there could be some cylinders at risk of pinging even if the overall EGR flow is judged ok.

    That also is unproven (so far on PriusChat, anyway), but might be a more plausible mechanism than the overall "temperature gets too high" story. It would still need empirical work to find out if it holds up.

    One attractive point about it is that removing just the intake manifold, and checking/cleaning those passages, is way less work than examining the EGR system stem-to-stern. And you can use Techstream and the car's existing sensors to check on the overall EGR flow without having to take stuff apart, leaving those four separate manifold passages as the only bits needing direct examination if the overall flow looks ok (because, again, results from the single MAP sensor can't tell you everything about those).
     
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  5. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

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    It’s also pretty easy to check (for carbon build up) the inside of the stainless steel bent pipe running between intake manifold and the Exhaust Gas Recirculation valve. That’s @NutzAboutBolts video #16 linked in this thread:

    Nutz About Bolts Prius Maintenance Videos | PriusChat

    Videos for manifold cleaning, and full Exhaust Gas Recirculation cleaning are also linked in the same thread.

    When cleaning the intake manifold it’s usually the Exhaust Gas Recirculation passage at cylinder one that’s most clogged. And when head gasket fails (heralded by misfire code), it’s usually cylinder one, or between one and two.

    More info here:

    Bad Flywheel | PriusChat
     
    #65 Mendel Leisk, Jan 13, 2021
    Last edited: Jan 13, 2021
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