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P0171 P0441 P0455 | System too lean AND Evap leak detected

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by anonymous, Apr 15, 2018.

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  1. anonymous

    anonymous Member

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    Check engine light came on; had the codes read at auto parts store:
    IMG_0799.JPG

    They suggest it's a bad MAF sensor, but how would that also cause a large leak to be detected in the EVAP system?? I've already cleaned the MAF sensor (which didn't appear to be very dirty) by rinsing it with isopropyl alcohol. And yes, the gas cap was checked first, and last time we had an issue that seemed to be solved by tightening the cap, it did not include P0171.

    Based on the limited informated I found about this combination, I suspect it's a bad EVAP purge valve or a bad PCV valve.
    I cannot find any information about replacing the EVAP purge valve in a Prius, and couldn't find where it's located. But there's plenty of information about replacing the PCV valve, so maybe that is more likely to be the problem?

    What do you think the problem is?
     
  2. Raytheeagle

    Raytheeagle Senior Member

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    Have you had any maintenance performed recently?
    Any other odd occurrences in your Priuses most recent history?
    Did you notice any strange behavior with the codes being there?
    Is the gas cap o-ring in good condition?
     
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  3. NutzAboutBolts

    NutzAboutBolts Senior Member

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    I would check the pcv valve first since that’s part of the evap system. Also, the system is running lean so check for any loose hose or vacuum leaks around the evap system.
     
  4. anonymous

    anonymous Member

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    No

    Not that I'm aware of, other than a mouse living behind the driver-side hood hinge (i saw it (or its tail at least) pop out and back in from under the hood, after starting a travel, which was intriguing)... and some rodent making noises somewhere in the underside near the rear... but there are rodents crawling throughout it all the time... of course I first suspected that it was caused by rodent gnawing, but I haven't ever find gnawing damage anywhere, even with all these years of rodent traffic, so it seems they just don't gnaw... (they do, however, frequently tear up soft stuff to make nests, most recently someone tore up that soft pad that goes on top of the spare tire, along with an envelop with a spare headlight bulb, to make a nest there :mad: ) still might be caused by that... would probably need a smoke test if that's the case... but it must be somewhere that would cause both problems

    Occasionally the engine behaves strangely, running when it shouldn't be and/or reving up and down repeatedly; just like it did the last time there was an EVAP leak.
    A few times it was stuttering on acceleration, making a pulsating vibration...
    But most of the time it has been working fine.

    Looks fine; smooth and rubbery, seat is also smooth
     
  5. ChapmanF

    ChapmanF Senior Member

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    I don't believe Toyota would consider the PCV valve to be part of the evap system, no.

    The evap system is what is in place to capture vapors from the fuel in the fuel tank, and then draw them into the engine for burning later, so they don't escape unburned into the atmosphere.

    It's one of the fiddliest systems in the car to get your head around. It routinely tests itself for airtightness, during times when the car's not in use, using a little pump back by the fuel tank. (This is a Gen 3 detail; not all gens have worked the same way.)

    The diagrams and theory of operation in the New Car Features manual will be invaluable to you if you want to troubleshoot it.

    The auto-store printout is clearly giving you canned advice from their code-reading system: "hey, there's a system too lean code? throw in the boilerplate language for MAF sensor and PCV valve!"

    The system is evidently not sophisticated enough to see that the P0171 and P0455 on the same report and think "hmm, now do you think a gross air leak into the intake might somehow be connected to a system too lean?"

    So, probably, getting the evap leak fixed will resolve the lean burn code.

    -Chap

    Edit: a couple more things about testing the EVAP system.

    • Pressure testing can be useful, but requires a special regulator for very low testing pressures (no more than 1/2 psi or so, there should be a big warning label next to the test port); the regulator has to be one made for the purpose that won't exceed such a pressure under any circumstances.
    • The test gas has to be inert, typically nitrogen. Adding oxygen-containing air under pressure to a system containing fuel vapor really isn't an option.
     
    #5 ChapmanF, Apr 16, 2018
    Last edited: Apr 16, 2018
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  6. NutzAboutBolts

    NutzAboutBolts Senior Member

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    You’re right chap, I was thinking of a different part. Whoops.
     
  7. anonymous

    anonymous Member

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    I think I found the problem!

    So I was trying to improvise a smoke test using incense sticks and whatever I had but then I thought I'd try something simpler first. I taped a vacuum cleaner hose to the evap hose (the one that connects to the purge valve, which I discovered was under the air box) and plugged the other end into the air output. First thing I looked at was the gas cap, and guess what, air was coming out!

    It is weird, but apparently the rubber seal leaks, even though it looks perfectly fine. The only reason I could think of is that the seal has shrunk or softened and isn't providing enough pressure.
    Guess we'll be getting a new cap, but in the meantime I might have fixed the issue; I stretched an o-ring around the cap's ring, into the groove, to thicken the seal. Now it tightens up and clicks before it bottoms out; before it did the click at the bottom of the thread. I did that after putting things back so I didn't test it.

    upload_2018-4-18_21-12-22.png
     
  8. Raytheeagle

    Raytheeagle Senior Member

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    Glad it was the o-ring after all;).

    Thanks for stopping back and sharing the solution (y).
     
  9. ChapmanF

    ChapmanF Senior Member

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    I'm glad you found the problem!

    I'm also glad you thought better of the incense stick idea. :eek:

    I would be even gladder if you had found a source of nitrogen rather than using the vacuum cleaner to force-feed oxygen-bearing air in there with the fuel. But then, all things considered, I'm glad you're uninjured, and able to make the followup report.

    Note to future readers of thread: please be safe.

    -Chap
     
  10. tvpierce

    tvpierce Senior Member

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    Not trying to be argumentative -- just asking because I'd like to learn more about it:
    - Isn't atmospheric air always present in the system?
    - If the system self-checks periodically by activating a pump, isn't that pump doing effectively the same thing as the vacuum output by adding pressurized atmospheric air to the system?

    Again, not trying to be a smart aleck. Thanks in advance for sharing your knowledge.
     
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  11. anonymous

    anonymous Member

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    Not sure if that problem has been solved or there is a separate problem, but, as I'm told, this morning, black smoke was coming out of the tail pipe. That means too much fuel is being burned, which probably relates to the lean condition.

    Considering that last time we had an EVAP leak caused by the gas cap, there was only those two codes, if it's also the gas cap this time, then why is there also P0171? Looks like there are two separate problems...

    I think we'll be getting an ODB tool to aid in troubleshooting.
     
    #11 anonymous, Apr 19, 2018
    Last edited: Apr 19, 2018
  12. NutzAboutBolts

    NutzAboutBolts Senior Member

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    Black smokes mean it’s burning rich due to the p0171 code. The ecm/pcm is reading that the vehicle is running lean due to a vacuum leak somewhere, so the ecm/pcm compensate by telling the fuel injectors to spray more fuel, which causes the engine to run rich and cause the black smoke in your tail pipe.

    Do you still have the code P0171? How long did the black smoke last?
     
  13. anonymous

    anonymous Member

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    Will need to find out what the codes are now (if that's the only code left then indeed they are separate problems).
    So what happened was, a motorcyclist notified the driver (not me) that black smoke was coming from the vehicle, especially during accelerations. After that the driver noticed the smell but couldn't see it. Later today nothing was noticed. Then I tested it myself, by reving up the engine while in park, but didn't see anything coming out of the tail pipe.
     
  14. NutzAboutBolts

    NutzAboutBolts Senior Member

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    It’s hard to rev the Prius while park because the hybrid side is active, you can find out by driving it on the highway and have someone look in the back to see if there’s any black smoke.
     
  15. anonymous

    anonymous Member

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    Haven't noticed any black smoke since.

    I got a basic ODB reader which can view freeze frame data; here is what it said:
    1. DTCFRZF : P0171
    2. FUELSYS1 : CL
    3. FUELSYS2 : N/A
    4. LOAD_PCT (%) : 28.6
    5. ETC (˚F) : 189
    6. SHRTFT1 (%) : 20.3
    7. LONGFT1 (%) : 28.9
    8. MAP (inHg) : 11.5
    9. RPM (/min) : 1002
    10. VSS (mph) : 5
    11. SPARKADV (˚) : 6
    12. IAT (˚F) : 81
    13. MAF (lb/min) : 0.217
    14. TP (%) : 14.9
    15. 02B1S2 (V) : 0.015
    16. RUNTM (sec) : 1293
    17. EQ_RATB1S1 : 1.232
    18. O2B1S1 (V) : 4.474
    19. EGR_PTC (%) : 0.0
    20. EVAP_PCT (%) : 0.0
    21. WARM_UPS : 255
    22. CLR_DIST (mile) : 19081
    23. BARO (inHg) : 29.2
    24. EQ_RAT11 : 1.232
    25. O2S11 (mA) : 0.438
    26. CATEMP11 (˚F) : 997.52
    27. CATEMP12 (˚F) : 848.66
    28. VPWR (V) : 13.535
    29. LOAD_ABS (%) : 10.2
    30. EQ_RAT : 0.993
    31. TP_R (%) : 0.0
    32. TP_B (%) : 46.3
    33. TAC_PCT (%) : 14.9
    34. CLR_TIME : 599h:45m

    The barometic pressure appears to be correct so it might not be the MAF. I see fuel trims are up but I already know that. Does anything else appear to be wrong?
     
  16. anonymous

    anonymous Member

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    I had cleared the codes. After a day full of driving I checked it again and found P0441 & P0455 pending! I did a quick EVAP test again, and gas cap is still leaking. So my fix didn't work—need to buy new gas cap.
    P0171 isn't pending, yet.
     
  17. anonymous

    anonymous Member

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    Well more than a week and thousand miles after clearing the codes, the light did not come back and there are no pending codes. My fix worked.

    Also I can't edit my own posts on this forum??
     
  18. anonymous

    anonymous Member

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    Update Jan/Feb 2019 for anyone finding this thread via search: I believe this issue was caused by a vacuum leak caused by the EVAP purge valve randomly binding and being stuck open, as I diagnosed in this thread. The solution is to replace the EVAP purge valve.
     
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  19. testriderchuck

    testriderchuck New Member

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    You're not a smart alek, you're asking a very legitimate question that few analyze. The reason cars get away with that kind of thing is simply that the air/fuel ratio in the tank is WAAAYY richer than what will burn. probably like 1:1, maybe richer even. Have you ever looked at a fuel sender? It lives in either liquid or vaporized fuel. It ALSO has bare wires rubbing together (can you say sparks). The first time I saw this it was HOLY SH!!!!! but upon thinking about it, DUUHH, too rich!