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

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

  1. ozmatt

    ozmatt Active Member

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    Hi everyone

    So after cleaning my manifold, cooler and egr valve we still have p0401 on our 2010 :( it's doing my head in lol

    Egr valve operates smoothly by hand, it's easy to open and snaps back to closed quickly and easily without resistance or restriction.

    Our egr system was not plugged to begin with, a bit dirty yeah but not plugged! I had my doubts it was ever the cause of our P0401 but we cleaned everything anyway and added a catch can, it's always been on my mind to do this as preventative maintenance..

    So I drove it most of the day, around 100km then after a full day driving I figured it was all good :) until i tried toyota official diagnostic procedure,, accelerate to 60km/h select B and fuel cut (foot off gas) just like clockwork p0401 popped straight back up.. damn!

    So I am wondering a couple things here

    1. Is the problem more likely to be the stepper motor magnet and spring assembly, valve it's self or perhaps the maf sensor? actually has anyone even had a bad maf cause this?

    2. although I can confirm the whole egr circuit is 100% clean from manifold right through to the cooler, are we risking head gasket failure driving with this code or can I safely drive it while waiting for a solution without causing further issues?

    Trying to source a good used valve or just a working magnet and spring around here seems impossible, if anyone has one they would sell for a reasonable price with postage to australia please let me know!

    Cheers :)

    Matt

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  2. jzchen

    jzchen Senior Member

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    EDIT-. Not sure this is correct my T-SB says USA market and you're in Australia. Best check with a local dealer for revised part #...

    Try the revised EGR part number 04004-58137, 25685-37010 (gasket). I'm afraid it may need a ECU reflash.

    moto g(7) power ?
     
    #2 jzchen, Mar 27, 2021
    Last edited: Mar 27, 2021
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  3. ozmatt

    ozmatt Active Member

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    thanks jzchen, if i can not source a magnet and spring i will look at the new revised part, mainly because i can service the magnet without stripping off the valve again, and im pretty sure the vale its self is okay! .. i think?

    First i have heard about the ecu reflash, is this directly to do with P0401? I need to educate myself on this one
     
  4. jzchen

    jzchen Senior Member

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    We have this here but if you see it says "Market USA"
    JPEG_20210327_181019_1539518004218215785.jpg

    I actually flashed it myself at home even though it is still warrantable because of the T-SB. But this for LHD. Don't know if it's the same part # for you. So I have the old EGR valve but new programming. Probably need the dealer unless you have a compatible VIM and subscribe to Toyota TIS...

    moto g(7) power ?
     
    #4 jzchen, Mar 27, 2021
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  5. ozmatt

    ozmatt Active Member

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    its nice to know about the T-SB and will look in to it, however i have been driving this car without issues for over a year until this happened last month so i cant help but think my main issue has to be something has actually failed in the egr system
     
  6. jzchen

    jzchen Senior Member

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    Mine started making a ticking type noise one very cold morning. (Only that cold morning). It was so prominent I thought there was an animal under the hood. (Back then I didn't know about this T-SB). I opened the hood but no animal was there. We use high grade fuel in ours 91 AKI when the manual asks for 87 AKI, so the anti-knock properties of the fuel should have helped.

    Beyond that I don't know much. Maybe you can call a dealer with your VIN and see if yours is an affected vehicle?

    moto g(7) power ?
     
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  7. ChapmanF

    ChapmanF Senior Member

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    P0401 is set when the ECM tests out the EGR valve during deceleration driving, opens the valve, expecting that to change the manifold absolute pressure reading by more than 1 kPa, and it doesn't.

    Have you done any of the testing shown in the manual or in the valve blockage data thread?

    What are the MAP readings with the EGR open and not open? What's the MAP reading with the engine stopped? Is it plausible?
     
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  8. ozmatt

    ozmatt Active Member

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    Hi ChapmanF, thanks very much for the reply, I had not noticed this thread and looks like it's packed with good information

    The only testing I have done sofar is the road test which results in code P0401 every time

    I will read the whole thread tonight and check the maf and egr readings open and closed, and maf with engine off as suggested.

    Thanks again I will report back with results


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  9. ozmatt

    ozmatt Active Member

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    Hi all, I have read the whole thread mentioned above and it's given me a BIG hint, this could be normal, I don't know.

    When I first fired up my car after cleaning everything I had a very irregular idle much like misfire which cleared out after around one minute of gentle driving, the whole time it STUNK of sulphur ..I didn't think much of it at the time figured it was all the crud clearing out from doing the cleaning, perhaps my valve was actually stuck open at this stage,, does this normally happen?

    When I screwed my magnet back on I simply wound it down gently until it ran out of thread then placed the stepper motor cap back on, as per the youtube video that I had watched! all seemed legit at the time.. did I make a mistake here somehow, is this a hint the magnet is actually sticking and can't self unwind?

    I will post my techstream results on the other thread to keep things neat but I will continue the troubleshooting about my cars problem here on this thread

    do I need to do this in inspection mode, or will it just idle once on the egr monitor screen in techstream



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  10. ChapmanF

    ChapmanF Senior Member

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    I think it's kind of normal to end up with the pintle slightly open after you have reassembled the valve. That gives you a nice opportunity to shove it closed with your thumb, which should feel and sound as if you are pushing a smooth nicely-lubricated spinny thing. It's kind of nice that you get that natural chance to confirm things went right.

    If it isn't quite closed when you start the engine, I think it should be closed very quickly after that. If the waveform info given in the manual is to be believed, the ECM uses about the simplest possible strategy for figuring out where "closed" is: it just keeps trying to step the motor in the closing direction at all times when the engine is idling. That should pretty much end up getting to closed no matter where it started.

    [​IMG]

    The total travel between fully closed and fully open is said to be 110 steps (111? not remembering exactly). How fast it covers that distance depends on just what they're calling a 'step'. The wiring diagram and waveform show that they're using the "two phase on" strategy described here, and if they're using the terminology the same way, one 'step' would be what happens each time one phase is switched. The waveform shows that happens every 50 ms, which would mean a stepping rate of 20 steps a second, meaning it could get from full open to full closed in 5½ seconds (if nothing is stuck). Surely yours was nowhere near fully open no matter how hastily you put it together.

    [​IMG]

    That manual excerpt says you should see the waveform when "idling with warm engine". Does that mean they aren't sending "close" steps right away from engine start? That seems like a weird choice to me, but maybe that could explain your minute of weirdness. Or the weirdness could have been something else and the "with warm engine" could be a goof in the manual.

    Sounds like another question for a sunny day and an oscilloscope.

    It is easy to mix up MAF and MAP. The sensor that matters for EGR flow checking is the MAP.

    I don't remember whether I was using inspection mode when I played with that last. If you don't, and the engine shuts off while you're playing, then sure, inspection mode will prevent that. I don't think the ECM usually changes behavior much just because you have an active test window open in Techstream. The only behaviors result from the active test commands you actually send.
     
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  11. ozmatt

    ozmatt Active Member

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    Thanks so much for the effort helping me with this ChapmanF, not everyone likes to share their knowledge, much appreciated ..

    having studied the above its still not overly clear to me if i have made a mistake with the stepper or its just failed, i just spent some time in the car with the laptop and have some data

    I found inspection mode in the hybrid control part of TS its not in engine and ecu section durr ts

    engine temp 85-90c ambient temp 14c
    998 -1050 rpm

    map 4psi @ step 0 to 24 engine runs normally without any noticeable change (yeah 24!)
    map 5psi @ step 25 engine starts to hesitate
    map 7psi @ step 27 engine starts to run poor
    map 8psi @ step 28-29 engine hates life ..
    did not go to step 30 as its not recommended
     
  12. ChapmanF

    ChapmanF Senior Member

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    Almost makes me think the valve is opening/closing as commanded, but somewhere in the EGR flow path there is still an undiscovered severe restriction....
     
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  13. ozmatt

    ozmatt Active Member

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    Hmm, I noticed the test value column is very low compared to others in the data thread, like it's under 1 where most others are 18/20 ish, do you happen to know what that column represents

    when I first took the car apart I noticed the gasket joining the cooler to the exhaust not on properly, maybe it started to plug up inside the exhaust
    IMG_20210227_144333_638.jpeg

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  14. Rocky Mountain Priusman

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    You know I wondered the same thing. If people are cleaning the EGR system what about the exaust pipe beyond it? Looking at it with a flashlight there was some build up in there, but it didnt look major.

    It would be a pain (and maybe not fix your problem), but I guess you could take off the cooler and put a borescope down the exhaust? Perhaps the clog actually would be quite close and visible via flashlight?

    And after putting that gasget on myself I can see how someone in a rush could make that mistake. Was having a lot of time getting both the bolts to thread through the hole in the gasget and into the cooler. I worked for about 30 minutes in a very awkward position before I got it correct.
     
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  15. jzchen

    jzchen Senior Member

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    Not everyone knows the answer, like me. Glad you're in good hands...


    moto g(7) power ?
     
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  16. ozmatt

    ozmatt Active Member

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    Hi Rmpm :)

    Yes, the idea of taking it all apart again and potentially removing the front half of exhaust has me in a less than ideal mood could we say lol!! Thing is I know the rest of the egr system is spotless so it's all pointing to the exhaust, I inspected the pipe under torch light while cooler was off the car and it did not look restricted at all, there was no buildup on the misaligned gasket or inside the exhaust pipe so I didn't think any more of it..

    The gasket between cooler and exhaust is actually relatively easy if you tape the gasket to the cooler (on the mating surface of the cooler, outside of the studs) once cooler is in place carefully fit the studs loosely then sneak the tape off, takes less than a minute doing it this way

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    #16 ozmatt, Mar 29, 2021
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  17. ozmatt

    ozmatt Active Member

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    Nah I know, I wasn't having a go at anyone btw just wanted to thank Chapman for sharing his knowledge!
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  18. ChapmanF

    ChapmanF Senior Member

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    The test value is the amount the manifold absolute pressure changes when the ECM opens the EGR valve by a certain amount to let gas flow in. It's essentially the same test you were using Active Test to do, only done from time to time behind your back automatically by the ECM during driving while you are decelerating.
     
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  19. wheezyglider

    wheezyglider Active Member

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    If the "minute of roughness' only happened the one time, it's a commonly reported thing for the first run after an EGR cleanout. It happened to me on our 2010, which ran scary rough for what felt like a lifetime on the first start and threw a code. That went away and didn't come back. On our 2012 I made extra sure to air out the carb cleaner, and it started up without a fuss. So no worries there unless it keeps happening.

    Given the symptoms of an apparently operational EGR valve (demonstrated by Techstream-commanded steps) and the P0401 by the fuel-cut operation trigger, I'm wondering if you have an EGR leak due to a missing gasket. That would explain those symptoms IMHO. The valve could be working fine, there could be no restriction, but the expected EGR is blowing out the leaking gasket. The computer says "hey where's my pressure?" P0401.
     
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  20. ChapmanF

    ChapmanF Senior Member

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    Well, keep in mind that the "A" in "MAP sensor" stands for "absolute".

    When you see those MAP values of "4 psi" during testing when the EGR is closed, that's 10 or 11 psi below atmospheric—a decent vacuum.

    If there's a leak, when you open that valve, you'd probably still see that MAP reading go up, whether it's the exhaust gas you were expecting or just the surrounding atmosphere rushing in through your leak.
     
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