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204k miles Gen 3 Idle problem

Discussion in 'Gen 3 Prius Technical Discussion' started by ReadyToRoll, Apr 1, 2022.

  1. ReadyToRoll

    ReadyToRoll Junior Member

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    Getting P0171 lean condition but only seems to present a problem at idle. As soon as the pedal is pressed, I get normal operation it seems. MPG is not off, or just slightly if at all, performance of ICE during acceleration is normal.
    My simple description is while driving and letting off the pedal to coast or stop, idle runs up and down in 3-4 second intervals by probably 300 rpms and will continue until the pedal is depressed and vehicle accelerates normally.

    I can replicate the issue also by going to maintenance mode, where it idles fine at first, then if I rev the engine for 10-15 seconds, then when I let off it goes into this idle cycle. While the up and down cycle is happening, I can hear some valve? engaging, making a brief vacuum noise ( slight mouse squeak), it happens twice within a few seconds, and the idle jumps either up or down, then a pause of 3-4 seconds and the valve noise and cycle repeats. The timing of the cycle is very uniform.

    If I dont touch the pedal (all still in maintenance mode ICE running), the abnormal idle cycle will continue for a minute or two, then return to normal idle.

    I have cleaned the MAF and throttle body recently, several thousand miles ago cleaned the egr cooler and valve ( which had been plugged/restricted for a while) and all seemed well until about 2 weeks ago when this issue started.

    Wondering about the fuel pump but so far not willing to insert a schrader to read pressure. Also thought a bad pump would show up during acceleration or highway driving??

    Thanks for any input, you folks have become experts on these vehicles!
     
  2. Brian1954

    Brian1954 Active Member

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    You might have a vacuum leak. Check the hose from the PCV valve to the intake manifold and the other vacuum hoses on the engine.

    Posted via the PriusChat mobile app.
     
    #2 Brian1954, Apr 1, 2022
    Last edited: Apr 1, 2022
  3. ReadyToRoll

    ReadyToRoll Junior Member

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    That seems the most probable, the first thing I tried was spray with carb cleaner all around to see if I could find a leak. No luck so far.
    Thanks
     
  4. mr_guy_mann

    mr_guy_mann Senior Member

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    Not every vacuum leak can be found with carb cleaner. Could be a crack in the PCV hose, or a bad PCV valve, or maybe the EVAP purge valve is stuck open.

    Posted via the PriusChat mobile app.
     
  5. audiodave

    audiodave Active Member

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    Did you pull the throttle body and clean the back side?
    Posted via the PriusChat mobile app.
     
  6. ReadyToRoll

    ReadyToRoll Junior Member

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    OK, I have replaced the PCV valve last year when I cleaned the EGR system but I will check hoses and look for the EVAP purge and check that as well. Thanks!
     
  7. ReadyToRoll

    ReadyToRoll Junior Member

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    I had it off the manifold last year when I cleaned the EGR and intake manifold, but did not remove it recently with this particular issue happening. It was cleaned earlier, think that would be sufficient? Thanks
     
  8. audiodave

    audiodave Active Member

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    It should be fine after only a year.
    Posted via the PriusChat mobile app.
     
  9. ReadyToRoll

    ReadyToRoll Junior Member

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    I pulled intake, cleaned all ports and replaced gaskets for intake and throttle body. How should I check the purge valve? I blew into it and its not letting any air through but was reluctant to apply voltage to confirm valve function.
    Intake was about as clogged as I expected, not terrible but plenty of buildup in the EGR pathways.

    Already put it back together and while driving and letting off the throttle pedal I still get the high idle and surging as before.

    I am just missing something but just about out of ideas. Looking for an independent with Techstream like diagnostics. Still not able to confirm fuel pressure without diagnostics, my last thought as to the cause.
     
  10. ReadyToRoll

    ReadyToRoll Junior Member

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    Finally did several things that made a difference! I was experiencing racing idle, surging idle, continuing to idle at stop signs, and the terrible rattle when rpms would drop really low. Codes were always P0171 with added P0304 and P3190 at times.

    Earlier post shows what I had already done so I decided to clean the EGR cooler and do a piston soak. I did not get any response to a post about how long the piston should be able to hold my cleaning fluid so not sure it that made a real difference, but I did find a loose connection in the EGR system.

    I guess the most obvious answer to P0171 lean condition is exactly that, if leaking exhause gas out of the EGR system would cause it. The connection between the cast curved section and the valve itself had a loose bolt and spraying carb cleaner did not produce consistent results so I did not find it until disassembly to clean.

    ICE is running as well as I have ever seen and cutting off when I let off throttle. Cold startup is smooth and no check engine light, so I am calling it a success.

    Thanks again to all the info in this forum!
     
    audiodave, xliderider and ASRDogman like this.
  11. xliderider

    xliderider Senior Member

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    Been there, done that... it's easy to forget to tighten all of the many bolts down, many in out of the way, difficult to see or get to locations.

    Posted via the PriusChat mobile app.
     
  12. ChapmanF

    ChapmanF Senior Member

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    Yeah, the pressure at that location is intake manifold vacuum—below atmospheric.

    So you weren't leaking exhaust gas out; you were leaking unmetered fresh air in. "Unmetered", because that's downstream of the MAF sensor, so the ECM doesn't know about it, and calculates the fuel mixture without it. So that's exactly what a P0171 lean condition is about.
     
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  13. ReadyToRoll

    ReadyToRoll Junior Member

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    Maybe that could cause the surging, when the EGR valve was closed, it would draw fresh air triggering the lean condition, when open it would almost be normal with maybe some exhaust escaping?? Do not know the EGR cycle time but early on I noticed a regular surge of about 5 second intervals in maintenance mode idle, and I could hear some valve actuating with each idle change. Just guessing but it makes sense to me.
     
  14. ChapmanF

    ChapmanF Senior Member

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    I suspect you pretty much never get exhaust gas escaping at that point. That's a wide-open pipe elbow connected to the intake manifold vacuum, and the EGR valve opens by very small steps to meter the exhaust flow. So I'd expect pressure at that junction to be below atmospheric pretty much all the time.

    But I haven't instrumented it there to check....
     
  15. CR94

    CR94 Senior Member

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    Right. Otherwise, there wouldn't be much EGR flow.
     
  16. onabuc

    onabuc Junior Member

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    Have u notice if you get oil on the intake manifold under the throtle body?
    Posted via the PriusChat mobile app.
     
  17. ReadyToRoll

    ReadyToRoll Junior Member

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    I always see oil in the intake below the throttle body, even after installing a catch can. I have to believe the can has improved on the situation as I have emptied it with a fair amount of oil and water. I did not measure the amount but at least it's that much less in the intake. It is not a complete answer for the blow by issue but just one step of several to improve the situation most of us have.
     
  18. ReadyToRoll

    ReadyToRoll Junior Member

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    Probably correct, I only mentioned exhaust leaking because I saw the carbon buildup or pathway across the gasket but it would not take much for that to happen. Beyond my capability to instrument but these DIY diagnostic efforts would be much better with more skills!
    Thanks
     
  19. ReadyToRoll

    ReadyToRoll Junior Member

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    xliderider, I noticed on another post you use a Scangauge II on your Prius, will that device report fuel pressure for my Gen 3? This is the only way I could figure out to get a message to you. Thanks
     
  20. ChapmanF

    ChapmanF Senior Member

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    There isn't a built-in fuel pressure sensor, so there's no chance of just asking the car about the pressure. The car can be asked about other things that may suggest whether mixture is or isn't a problem.

    If it becomes a matter of reading the fuel pressure, that has to be done by plumbing a pressure gauge into the system. Even at a dealership they'll just unhook the hose at the firewall and MacGyver something up with a tee and fittings. They have part numbers if you want to do the same 'official' MacGyverage that they would.

    [​IMG]
     
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