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Still P0401 after cleaning egr !! should we drive it?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by ozmatt, Mar 27, 2021.

  1. Mendel Leisk

    Mendel Leisk Senior Member

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    My take fwiw: the Toyota engineers are counting on normal Exhaust Gas Recirculation function to keep combustion chamber temperatures in a certain range.

    When the EGR clogs the combustion chambers exceed that range, the engine runs hot.

    When the EGR clogs unevenly, in the intake manifold, there’s also a variation in temp increase. Cylinder one EGR passage tends to completely clog first, and that’s also the usual head gasket failure location being reported.
     
    #141 Mendel Leisk, Apr 14, 2021
    Last edited: Apr 14, 2021
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  2. ChapmanF

    ChapmanF Senior Member

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    Where 'works perfect' means adding £7.99 to the seller's bank account for every unit sold. :)

    It's funny, there are "EGR delete kits" as a popular thing in diesel-vehicle circles, where (because of the way diesels work) EGR helps with emissions but hurts MPG, and the marketing for the kits can appeal to a sort of "don't let the guv'ment force this emiiiiisions thing on me that hurts my MPG" mindset.

    And then you have sellers like the one linked above, marketing the same thing for spark-ignited gasoline engines, where (because of the way SI gasoline engines work) EGR helps with emissions and helps MPG, so the kits have to appeal to the "don't let the guv'ment force this emiiiiisions thing on me that also ... well ... anyway, nyeah nyeah" mindset. I think that means they have a smaller target market.

    This. We do know exactly how the ECM knows how much EGR is flowing, using the (single) MAP sensor that reports how the overall manifold vacuum has been reduced by the overall inflow of EGR. We know the ECM has 110 steps of EGR valve opening to play with to get the overall flow where it wants, and we know it logs P0401 if it detects that it can't within the range of valve adjustment it considers acceptable (as this thread clearly demonstrates). We also know that once the ECM has detected a problem, it applies a bunch of fail-safe measures to protect the engine.

    (Which is what you'll get if you make somebody £7.99 richer and add a blanking plate: an eternal P0401, black tape over your check-engine light, and the ECM eternally running the engine with backed-off, fail-safe parameters.)

    All of that seems to suggest that where the greatest risk of engine damage might be found is in problems of EGR distribution that the ECM hasn't yet detected or can't detect, chiefly between-cylinder differences in manifold clogging. The ECM may still see an overall acceptable flow, but one cylinder may be getting too little and be at risk of knock, and some other is getting too much and at risk of misfire, and the ECM hasn't got per-cylinder sensors to detect that.
     
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  3. StarCaller

    StarCaller Senior Member

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    so if someone would keep the ports in the intake manifold spotless clean, shouldn't that prevent head gasket failure?
    (would be way easier than to take half the car apart to clean the whole egr stuff....)
     
  4. AW82

    AW82 Member

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    Which is maybe is why Toyota has gone through a couple of redesigns of the intake manifold, it seems specifically to how EGR gas is distributed across cylinders. I guess there's a reason they didn't just design it to dump into the manifold without the tiny ports (maybe cyl 4 would then get more than its fair share due to proximity to the inlet?). And in terms of the tiny EGR ports, a design that works optimally when no deposits yet exist probably doesn't work well when deposits start to accumulate, and vice versa. But, maybe the redesigns are better than the original.

    I've not been around long enough to know if there is a correlation between head gasket failure and the model of intake manifold.

    That's where I'm at...maybe we're at the point where we tell people to focus on cleaning the intake manifold if they're overwhelmed by the prospect of cowl removal, coolant lines, etc. Cleaning the EGR valve and cooler is ideal, but perhaps not as critical.
     
  5. ChapmanF

    ChapmanF Senior Member

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    That reasoning does appeal to me. Part of it is that we don't have a way to ask the car how the per-cylinder flow is doing (it has no sensors for that), but the manifold maintenance is easy to do.

    Meanwhile, most of the rest of the system is way harder to clean, but the overall flow is something we have a way to ask about.

    [​IMG]

    If the car itself declares a problem below 1 kPa, maybe you set your personal trigger at "below 5 kPa" or "below 10" and declare an EGR weekend if you get there, and spend your time more enjoyably if you don't.
     
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  6. Mendel Leisk

    Mendel Leisk Senior Member

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    I’ve seen one low res cut away pic of revised IM, and the rev seems to be that it goes from a 4-into-1 to a 4-into-2-into-1 passageway config.
     
  7. AW82

    AW82 Member

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    I need to decide if I want to be a guinea pig for that. I cleaned the intake manifold ports but not the EGR valve or cooler yet. My EGR monitor test is 16 kPa. I feel less urgency to remove/clean the EGR valve and cooler, but I'll sure feel dumb if I wait too long and something bad happens.

    I wish I had looked to see if a model number was stamped on mine when I had it out. I can say that it was NOT a straight shot from the main EGR intake port to the cyl 1 port. There was some sort of baffle or something between cyl 2 and 3. I could snake a flexible straw brush up and over it, though. And the cylinder head ports for cyl 3 and 4 had more carbon deposits than 1 and 2.
     
  8. wheezyglider

    wheezyglider Active Member

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    There's also the notion that partial clogging of the cooler leads to the EGR flow not being actually being cooled. See the reference info in this post by @mjoo for example. The cooler fins get coated and that prevents thermal exchange.

    We don't have data about what kind of temperature difference this leads to, or what the knock-on effects could be. I think the idea of hot gas possibly contributing to OP's valve damage was floated earlier in this thread.

    So it's unknown, but it would lean on the side of full-clean if it matters...
     
  9. AW82

    AW82 Member

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    Maybe, but a LOT of gasoline cars have uncooled EGR systems. But I guess lack of cooling could increase the amount of thermal cycling happening?
     
  10. ozmatt

    ozmatt Active Member

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    Here's something else

    I wreck gen2, I've had about 12 or 14 all up

    Head gasket failure in gen2 used to be relatively rare but in the last couple years I have picked up four gen2 cars with blown head gaskets! all of them were blown at cyl1 just the same as gen3 and all had a bit over 500,000km and we're ex taxi's. Gen2 does not have an egr system.

    To be realistic about it, overheating and heat cycling in general probably just ads up over time, obviously a blocked egr must be a major contributing factor to overheating.

    So the real protection mechanism might be to make sure you aren't ever overheating!.. Good quality oil and strict OCI with a realistic viscosity for your climate (warning minor oil viscosity rant coming up) none of this 0w16 rubbish unless you live in bloody antarctica, seriously!! .. even coolant flushes now and then would be a good idea, I might even try a quality red coolant next time rather than Toyota pink?.. #maintenance

    Posted via the PriusChat mobile app.
     
  11. ChapmanF

    ChapmanF Senior Member

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    I'm really not even remotely prepared to agree with that.

    In a whole lot of the EGR-related talk on PriusChat, there's a recurring confusion being made (in some cases a willful confusion? how many times can a recurring confusion keep recurring after being pointed out, without some level of willfulness involved?) between what EGR does to peak combustion temperatures (which is why it is used), and "overheating" or "how hot the engine runs".

    The combustion temperature during the power stroke follows a curve. Some mixture has been sucked in at some lowish temperature, compressed (raising the temperature), then ignited. As it burns, the temperature rises. At some point it hits a peak. It is also imparting energy to the piston and expanding, and giving heat to the cylinder head and walls, and all of those things bring the temperature down.

    The purpose of the EGR is to get the heat of combustion more slowly and over a longer period (more rotational degrees of the crank) rather than at a higher rate in a shorter time. It looks kinda like this (note this is a rate-of-heat-release graph rather than temperature, for clarity):

    [​IMG]

    (That's a representative image, to give the idea. It's from an article on a hydrogen engine, just because it's a nice clear figure and available under CC-BY license.)

    The left graph is the rate of heat release. At the lowest EGR flow it peaks high and is all done by 20° after TDC. At the highest EGR rate it hasn't even peaked until 30° and is still giving off heat another 50° later.

    Flattening that peak is what makes all the difference in the world to NOx emissions.

    But the total heat given off is the area under that rate-of-release curve. It just doesn't change all that much. You're still getting the amount of energy you get from burning that amount of fuel. Quickly or slowly, the head and block are still getting the heat. The cooling system is still having to carry a bunch of it away. It still can. Its thermostat still regulates the temperature.

    What EGR does to the peak temperature during a handful of degrees of crank rotation should never be confused with how much heat the engine is dealing with, or whether it is "running hot" or overheating.
     
    #151 ChapmanF, Apr 14, 2021
    Last edited: Apr 14, 2021
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  12. AW82

    AW82 Member

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    I've thought about this more and I don't have a Chapman quality answer, but here are a few points...
    • EGR gas lowers combustion temps because it's inert, not because it's cooled. But cooling makes it denser, which improves it's NOx reduction affect and increases mpg.
    • Uncooled exhaust gas is around 800 degrees F, depending. Temperatures in the combustion chamber can measure in the thousands of degrees.
    • The melt point of the intake manifold is likely well below 800 degrees so I think we'd be hearing about melted manifolds if uncooled EGR flow was an issue.
     
  13. wheezyglider

    wheezyglider Active Member

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    Full marks on point 1. (After the gen3 more than killed all regulatory requirements on NOx, Toyota still doubled down on EGR in gen4 for the additional fuel efficiency improvement.)
     
  14. ozmatt

    ozmatt Active Member

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    Seems i spent too much time reading about cleaning the egr and adding the catch can, and not enough time researching exactly what the egr is really doing and how it does it

    Really thought i had read everything about this, maybe i have and just still don't get it!!?? .. to save me asking a million dumb questions here or reading days of crook information could someone please care to point me in the right direction for the most informative & reliable thread about egr its being related to head gasket failure
     
  15. Mendel Leisk

    Mendel Leisk Senior Member

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    I’ll just continue to keep on top of Exhaust Gas Recirculation and intake cleaning, see how that goes. :)

    Our automotive usage is at a ludicrous new low btw: 1855 kms in the last 6 months.
     
  16. wheezyglider

    wheezyglider Active Member

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    Yeah, I mean the headgasket / EGR connection arose because gen3s are seeing early HG failures and we don't really know why. Gen2 taxis all blowing HGs at cylinder #1 at 500,000km is interesting. Wish we could count on our gen3s lasting at least that long.

    So it's at least reasonable (dare I say not at all "insane") to notice the gen3 switch to external cooled EGR, which feeds the cylinder head, clogs at mileages correlated with early HG failures, isn't properly detected, has no maintenance guidance, and think it prudent to at least remove that as a possible cause just by cleaning it.

    Meanwhile we also keep hacking away at the theory and digesting real world experiences. We can do both...
     
  17. AW82

    AW82 Member

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    Not to mention oil consumption that seems worse in comparison with Gen 2s. More oil getting burned, more crap going into EGR; lower oil levels and increased fuel dilution, increased friction, etc.

    I wish I would've spent all this time here learning about all this before I bought the stupid thing...well, hopefully, I can stay ahead of the game.
     
  18. Mendel Leisk

    Mendel Leisk Senior Member

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    Someone here reported a car having had at least two head gasket failures in a row, maybe 10k miles apart, and:

    after the first HG replacement they didn’t do anything to the EGR; hadn’t heard about it.

    maybe that constitutes causality?

    They were going for HG repair number 3, WITH EGR cleaning, last I heard.
     
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  19. wheezyglider

    wheezyglider Active Member

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    Yikes. At least you can safely rule out thermal fatigue on that second one!

    In the "good old days", the rule was if you blew a HG you were supposed to find out why so it didn't just happen again. But these aren't happening after an obvious overheating event or a confessed drag race. "There's gotta be some kind of design flaw" is where I'm starting.
     
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  20. StarCaller

    StarCaller Senior Member

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    if it's like that, it's very likely not an easy fix for the end user /