Hissing sound after turning engine off

Discussion in 'High Tech Retrofits' started by 75Maro, May 5, 2019.

  1. 75Maro

    75Maro Veteran Member

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    engine had 67k on it when it was pulled from the donor. Wouldn't a leaking intake valve show other signs as well? I will swap out the booster check valve first since its the cheapest. Thanks for the input.
     
  2. 75Maro

    75Maro Veteran Member

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    brake booster vacuum hose .jpg pcv piping.jpg
     
  3. mrfleet12

    mrfleet12 Veteran Member

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    I agree with Supwicha, mine does the same thing only a shorter duration.
    Car runs great, no issues.
    Others have said it's most likely the IAC.
     
  4. 75Maro

    75Maro Veteran Member

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    Ok I’ll check IAC second. Thanks for the input. And yes the car runs great no issues.
     
  5. supwicha

    supwicha Veteran Member

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    Yes, a leaking intake valve would most likley have other symptoms and driveability issues which is why it's last. Another test, with the engine running, remove your oil cap, and shut down the engine. I'm curious if your block is vacuum balanced or in a vacuum. The oil cap being removed would ensure the block is ventilated. You can also cap off your hose to the booster temporarily. Also, a couple ways you can test the check valve. You can (suck/blow) on the valve (I know that sounds bad), you should allow you to either suck on it (engine side), but not blow through it. If you can do both, it is not working. You can also start the engine for several seconds, shut down, and remove the valve from the booster which will result in an obvious release of vacuum from the booster. You don't need to arbitrarily replace the check valve prior to these tests. Also, do you have an external vacuum reservoir for your dash controls? Or did you plug your dash control vacuum source line direct to the engine? There is a check valve built into the vacuum reservoir that prevents vacuum bleed back that can fail. Or if you are not using a reservoir, it could be your blend door controls bleeding vacuum off after shutdown due to not having a check. Admittedly, the long duration that is demonstrated in your video is odd/not typical, but probably of no harm. As it relates to the IAC (if equipped), after shut down (key word after), the IAC pintle will do one of few things. Nothing at all and remain at the current position, or it will open a small percentage for the next start, or during the next key cycle, it will open a small percentage during start. This is why EFI motors tend to start at above normal idle, and settle down after a few seconds. If your vehicle starts normally, and idles properly, and no codes for IAC related faults or position codes, then changing the IAC is not going to fix this as it is doing what is commanded by the PCM.
     
    Last edited: May 21, 2019
  6. 75Maro

    75Maro Veteran Member

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    No vac reservoir. No vac dash controls. I have tried the oil cap test. With the cap removed and engine started, engine runs rough and when I turn it off no hissing sound. If I start the engine and then try and remove the oil cap it’s almost impossible.
     
  7. supwicha

    supwicha Veteran Member

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    Hmmm, with the oil cap off, does it idle higher than normal? How is your PCV system set up?
     
  8. 75Maro

    75Maro Veteran Member

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    With the oil cap off it runs very rough I’m assuming due to the extreme vacuum leak it’s creating
     
  9. 75Maro

    75Maro Veteran Member

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    See post #12 for PCV hose routing
     
  10. badazz81z28

    badazz81z28 Veteran Member

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    Looks like there is not a hose connected to the throttle body???
     

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