Diagnose EGR system by unplugging it?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Jon Schro, Sep 8, 2021.

  1. Jon Schro

    Jon Schro New Member

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    I'm having the engine shaking/knocking at startup problem and cylinder misfire that seems quite common here. For many people this problem seems to be caused by a clogged EGR system.

    Is it true that you can diagnose whether the EGR system is the problem by unplugging it? (this seems to be suggested in this video:
    )

    So if I unplug the EGR as he does at 0:26 in the video, and the engine knocking still occurs with it unplugged, does that mean that the problem is not due to a clogged EGR system?

    Thanks
     
  2. bisco

    bisco cookie crumbler

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    idk, but if you don't clean it, you're going to blow the head gasket (if you haven't already)

    so why bother with trying to diagnose it?
     
  3. Grit

    Grit Senior Member

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    quote a user here “that’s insanity.”
     
  4. ChapmanF

    ChapmanF Senior Member

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    There are some pretty nutty ideas in circulation about how the EGR system even works.

    Unplugging the wires as a way to check for a clogged EGR system doesn't make any sense, because with the wires unplugged the valve will stay closed, which is the same effect as having a clogged EGR system.

    I've also seen other posts where somebody suspects the valve is stuck open and wants to test that by unplugging the wires. That also doesn't make any sense, because if it is stuck open, unplugging the wires won't magically close it.

    Meanwhile, there are some things that unplugging the wires will change: the ECM can tell when they are unplugged, and it will set a P0403 code, and when it does that, it goes to failsafe values for some other engine parameters like ignition timing. If you unplug your EGR valve wires and you notice some change in how the engine runs, it is probably these other changes that you are noticing.

    There are some methods available that really do help you diagnose the EGR system. You can ask the ECM for its latest self-test result of overall EGR flow, which is a very good first step.

    Because that is only an overall flow test result, it isn't able to tell you about the four separate EGR passages in the intake manifold, if some of them are clogged more than others. So it should be combined with a physical check of the intake manifold where you remove it and inspect/clean those passages. Removing the manifold for that is a smaller job than it sounds; it can seem daunting at first, but is probably no more than 20 minutes with experience.

    You can also use Techstream active tests to move the valve to different positions and confirm that it opens and closes and has the expected effects.
     
  5. rjparker

    rjparker Senior Member

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    Many people are wrong if they think the knocking is caused by the egr. Especially if it runs better in a few seconds. As Chapman indicates, unplugging a closed egr leaves it closed. The normal position of an egr at startup or low speed is closed. It is theoretically possible for the egr valve to be stuck wide open at a coldstart but you would not have blowby yet and resulting misfires (if any) would not go away quickly.

    The knocking caused by a leaking head gasket is not just common here, its a huge problem Toyota mechanics see frequently. Most will suggest a borescope or leakdown test if coolant loss is not already a symptom pointing at a leaking head gasket. Obviously a proper diagnosis is always best but just throwing parts at the easy to access places is costly and inefficient. Especially if most of the parts are replaced with the engine later.

    What confuses most is this Prius hg failure seems to be very slow and self sealing at first. Sometimes days or weeks between knocks. It then gets worse and the coolant reservoir no longer hides the minuscule coolant loss.
     
    #5 rjparker, Sep 9, 2021
    Last edited: Sep 9, 2021
  6. Mendel Leisk

    Mendel Leisk Sand Pounder

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    @Jon Schro : How many miles?

    Exhaust Gas Recirculation cleaning info linked in my signature.
     
  7. Jon Schro

    Jon Schro New Member

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  8. Jon Schro

    Jon Schro New Member

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    @rjparker Thanks for the reply. That makes sense. I've never had to add coolant, but now that I look it is below the LOW line. Haven't been driving since it started misfiring.
     
  9. Jon Schro

    Jon Schro New Member

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    190k miles, low coolant, Knocking at startup and after warming up. P0302 misfire code.

    I'm an amateur at mechanics, but willing to learn and generally a DIY person. What would you guys suggest I start with? Should I take it to a shop to pressure test the head gasket? Should I just go ahead and clean the EGR system? Try the spark plugs and coils?

    Considering the likelihood and and difficulty of the various possibilities, what is the best order of things to try? @rjparker @Mendel Leisk
     
  10. Jon Schro

    Jon Schro New Member

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    update: I removed the EGR pipe & manifold and cleaned it out the best I could with some nylon brushes, a big can of carb cleaner, and then blasting it with a water hose. It definitely had a layer of grime all over the interior, although nothing seemed fully clogged as far as I could tell. I'm going to leave it off overnight in front of a fan to dry out before replacing it tomorrow. I also replaced the PCV valve while I was in there.

    Anything else I should try before I put it all back together? If I still have the engine rattling and misfiring after I put it together, what should I do next?
     
  11. ChapmanF

    ChapmanF Senior Member

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    Misfiring, knock noise, low coolant. Get thee to a leakdown test.

    None of the stuff you did in #10 was bad stuff to do. There is some belief around here that prophylactic EGR cleaning may forestall head gasket damage.

    However, if you have already arrived at head gasket damage (leakdown test will tell you), there is no amount of barn-door closing that is going to put that horse back in the stall. You just have to deal with the situation you now have.
     
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