Can anyone help me with step by step on how to check EVAP system for leaks?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by 2GenPrius, Jul 12, 2025.

  1. 2GenPrius

    2GenPrius Member

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    2012 Prius with 216K miles



    I posted a video link on Reddit and didn't get much responses. I unplugged the line that goes to the purge valve and hooked up a smoke machine, had smoke coming out of the gas cap area, took the gas cap off, it rattled, so replaced it with a brand new OEM one, did the smoke test again, smoke kept coming out.

    I had a question, when unscrewing the gas cap, is it supposed to have a vacuum? I remember having it when I first got the car, now I don't have it anymore. As mentioned, cap is brand new OEM from dealer and no rattle.

    When car idles, smell of strong unburnt fuel but no CEL, smells almost like fuel vapor oddly. I thought between no pressure sound from the gas cap that the system is not sealed somewhere. When I hooked up the smoke machine, one of the instructors from the school said its a rather large leak because the PSI meter was really high. Problem is again, no CEL.

    Is there a step by step on how to check the Prius EVAP system on a Gen3? I'm pretty sure my problem lies there, unless someone else has other ideas.
     
    #1 2GenPrius, Jul 12, 2025
  2. Tombukt2

    Tombukt2 Senior Member

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    Get under car to see filler neck hoses etc what do you see from that angle look wet cap o ring lands in neck now maybe notice smokes not from neck o ring and is maybe wafting up from below the neck . Ya see smoke coming from tubing below wer you at now?
     
    #2 Tombukt2, Jul 12, 2025
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  3. BiomedO1

    BiomedO1 Senior Member

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    You need multiple failures of the EVAP self diagnostic before it sets a CEL. If you have a good enough scan tool, you should see pending codes. The simple fact that you can see smoke during a smoke test, tells me you have a large leak someplace. That's assuming you've hooked up the machine properly. The question I have is why is the PSI high? Does that gauge indicate that your generating a lot of smoke, not system pressure? If you have a large leak, shouldn't the pressure be zero since you have a large leak and you see smoke escaping???? Is the purge valve stuck open and the source of your smoke?
    You'll need to jack up the car and either crawl underneath or pull the rear tire and inner fender liner to do a proper inspection with the smoke machine.

    FWIW; that's an automatic fail during a California smog inspection.
     
    #3 BiomedO1, Jul 12, 2025
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  4. 2GenPrius

    2GenPrius Member

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    I hooked up the machine to the line that comes off the purge valve which runs to the back of the car and rest of the evap system. I took the purge valve itself and blew into it and it didn't leak air. So the leak must be from somewhere in the rear.

    When I was using the Snap-on leak detector at school, the instructor said that the smoke machine uses the PSI meter to determine how large the leak is. When we set the knob, it was set to a high psi amount but the machine didn't work consistently.

    For a scan tool, I use Torque Pro and an ELM compatible Bluetooth adapter. How long and how many miles do I need to drive to see the pending codes?
     
    #4 2GenPrius, Jul 12, 2025
  5. Tombukt2

    Tombukt2 Senior Member

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    I think if you look back in the back of the car where the filler neck fuel tube is and all that related nonsense I believe you're leak is back there and it's just wafting up around filler neck . So when you look at the business end of filler neck which is behind sheet metal and liner.
     
    #5 Tombukt2, Jul 12, 2025
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  6. BiomedO1

    BiomedO1 Senior Member

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    Sounds like your unfamiliar with the equipment that your using and/or the equipment your using is defective, or it's simple operator error.
    Find a simple vacuum gauge with a manual pump. Disconnect the same hose you used for the smoke machine test. Hook-up that vacuum with manual pump and draw a vacuum on the system, It shouldn't leak. That's a simpler test of system integrity. If that's your daily driver and you haven't thrown a code in several days of operations - your most likely chasing a ghost........

    Good Luck......
     
    #6 BiomedO1, Jul 14, 2025 at 9:54 AM
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  7. Mendel Leisk

    Mendel Leisk MMX GEN III

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    This might help. if not, let me know, I'll look more.
     

    Attached Files:

    #7 Mendel Leisk, Jul 14, 2025 at 12:22 PM
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  8. 2GenPrius

    2GenPrius Member

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    Thank you both. I'll go get a vacuum tester and try to draw a vacuum out of the line mention, if it doesn't hold (as I suspect it wont), then I'll report back either way.

    I wanted to ask, I saw the diagnostics instructions required Techstream. Is there any version out there that isn't crawling with malware? I checked with my school and unfortunately they don't have it.
     
    #8 2GenPrius, Jul 14, 2025 at 11:45 PM
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  9. 2GenPrius

    2GenPrius Member

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    Hi all,

    Car finally threw a CEL; P0456
    When I check the monitors in TorquePro, it states that the Purge Flow Monitor is "Not Ok".

    Thing is, when I took out my Purge Valve with it out of the car, I did the blow test on it and it held closed, or at least to my opinion. Just got a vacuum pump and will be checking both sides of the valve tomorrow, and will also try to draw vacuum from the purge line coming from the rear tomorrow.
     
    #9 2GenPrius, Jul 20, 2025 at 12:55 AM
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