P0171 - Recently replaced pistons & rings

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by djimbuser, Oct 27, 2025.

  1. djimbuser

    djimbuser Member

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    Prior to tearing engine down and replacing pistons & rings there was no P0171 code. After getting the engine reassembled I get the P0171 on every second drive (service manual clearly states it's a two-drive code).

    What I can't figure out is why. All hoses connected. Upstream O2 sensor connected. Fittings between the MAF and throttle body have tight clamps.

    I captured the freeze frame data and attached them can anyone single out the issue on the freeze frame data?

    Thank you!
     

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  2. PriusCamper

    PriusCamper Senior Member

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    Wiggle all the vacume hoses... Maybe one of them is damaged?
     
  3. djimbuser

    djimbuser Member

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    Wish it was that simple. All the hoses seem flexible and in good shape. I took off the air cleaner box and noticed that one of the clamps to the duct leading to the throttle body wasn't tight so I tightened that and thought maybe that was the fix but I drove it two more drives and the CEL reappeared :(
     
  4. Brian1954

    Brian1954 Senior Member

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    Check the hose from the PVC valve to the intake manifold. Also, check the two hoses that I wrote about in a different thread two weeks ago which I copied below.
     
    #4 Brian1954, Oct 27, 2025
    Last edited: Oct 27, 2025
  5. djimbuser

    djimbuser Member

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    I have ordered a new upstream O2 sensor I'm going to try that... That said all I did was disconnect those three connections down near bottom of timing of timing cover so I could sling the wiring harness up and out of the way during the engine work days. I'm 100% sure I've reconnected it correctly.
     
  6. djimbuser

    djimbuser Member

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    Oh yes I HATE that hose. Backstory on this car I replaced the head gasket a couple years ago and that hose split at the top so I cut it off and used a hose clamp. I did not have a P0171 CEL for doing that...

    Update/clarification. The hose IS connected to the top engine cover.. it's just that so little of the hose is on the nipple I thought it best to use a clamp to make sure it doesn't fall off.
     
  7. djimbuser

    djimbuser Member

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    I see it now.. it's the "Total FT #1" on image #2. My car doing 39% which is outside of the acceptable high of 35%... Now the question is WHY. I have new Denso Fuel injectors, new Denso coils, new NGK spark plugs, new EGR Valve, and super clean EGR heat exchanger.
     
  8. Brian1954

    Brian1954 Senior Member

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    You have a vacuum leak. Remove the air box and check the three hoses I mentioned in post #4.
     
    #8 Brian1954, Oct 27, 2025
    Last edited: Oct 27, 2025
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  9. Brian1954

    Brian1954 Senior Member

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    That is not one of the three hoses that I was talking about in post #4.
     
  10. BiomedO1

    BiomedO1 Senior Member

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    And it may not be a hose, but a ruptured diaphragm - same as a disconnected hose. You need to use a vacuum pump to check them.o_O:eek:
    That was week of my life that I'll never get back.:cry::whistle:
     
  11. Brian1954

    Brian1954 Senior Member

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    What diaphragm are you talking about on a Gen 3 Prius?
     
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  12. BiomedO1

    BiomedO1 Senior Member

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    Well, if a gen3 Prius doesn't have vacuum controlled diaphragms - forget it. I'm used to working on the really old stuff that use to have those. The rubber would get old and rupture internally; so everything looks good upon visual inspection.
     
    #12 BiomedO1, Oct 27, 2025
    Last edited: Oct 27, 2025
  13. djimbuser

    djimbuser Member

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    Ok.. I'll report back tomorrow!
     
  14. djimbuser

    djimbuser Member

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    The U shaped hose from PCV valve to intake manifold has short splits at both ends. In general the hose seems rather tough and brittle. I have a Gates EMH046 on order to replace it. I removed the PCV valve and it seemed to be malfunctioning as it would allow air flow in both directions so I put a new one in.

    I have also replaced the upstream O2 sensor and the fuel pump. I'm still seeing long range fuel trim of 39% on a short highway test drive and in addition to P0171 I've now got a pending P106A as well.
     
  15. Brian1954

    Brian1954 Senior Member

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    Did you check the other two hoses that I wrote about in post #4?
     
  16. djimbuser

    djimbuser Member

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    Yes. No splits or cracks and still seem nice and rubbery.
     
  17. djimbuser

    djimbuser Member

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    I took the VSV solenoid off and tested it with a 12V power supply it worked fine. I noticed one of the hoses from the VSV was a little loose I had trouble with this hose a couple years ago when I changed the head gasket. I would like to replace both of the hoses the come off the VSV: One goes from the VSV to the top of the engine (see photo) and the other goes from VSV to the intake manifold just under the throttle body.

    Does anyone have the gates part numbers or a method for recreating these hoses?

    Also I tightened the clamp on this hose and drove it again and it would seem like the car isn't "running lean" anymore.
     

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  18. Brian1954

    Brian1954 Senior Member

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    Those are the two hoses that I was talking about in posts #4 & #15. Those two hoses did not have any clamps on them by Toyota. The VSV is sometimes referred to as a purge valve. I do not have the Gates numbers.

    The Toyota part numbers for those hoses are:
    Fuel Vapor Feed Hose #1 23826-37130
    Fuel Vapor Feed Hose #2 23827-37040
     
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  19. djimbuser

    djimbuser Member

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    The only reason I clamped mine is because I cut the 3/4" of the tip off because it split. The ends are permanently shaped to the shape of the fittings they fit onto.
     
  20. djimbuser

    djimbuser Member

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    I have these on order!
     
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