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Thread: 2007 JKU 3.8L, Vacuum leak? video link attached.

  1. #1
    Fresh Catch Pixel's Avatar
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    2007 JKU 3.8L, Vacuum leak? video link attached.

    https://www.youtube.com/watch?v=6WF1rKX6rIU

    I have not had a chance to do any DIY smoke machine testing or take it to a shop that has one but I did look for any obvious signs like a hose that popped off and poked around with propane in any areas that seemed like it would be prone to leaking.

    It is very hard to miss the sound I am referring to in this video @ 0:29 this happens every time the engine is turned off. It started as a faint hiss and has gotten much more noticeable lately. I can also faintly hear it from the vehicle cabin while accelerating from a low RPM. It sounds like a release of gas when you open a propane/co2 cylinder up. I want to say it is a cracked exhaust manifold (if you look closely a bolt head popped off the heat shield so I am guessing there is an issue there to be fixed even if this isn't the root cause.

    Anyone else ever experience this?
    - '04 TJ (Sold)
    - '07 JKU (Driving me slowly insane)

  2. #2
    Meme King WJCO's Avatar
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    That is effing weird. I don't think it's exhaust. Sounds like AC system venting or maybe evap system. Any codes? Does it make a difference if AC is on or off? Next time you make it do it, put your hand on low side AC line and see if it feels like's its coming from that system. Also check evap lines for leaks (which would likely have already set a code). Maybe listen around throttle body and air inlet tube as well.

  3. #3
    Fresh Catch Pixel's Avatar
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    Quote Originally Posted by WJCO View Post
    That is effing weird. I don't think it's exhaust. Sounds like AC system venting or maybe evap system. Any codes? Does it make a difference if AC is on or off? Next time you make it do it, put your hand on low side AC line and see if it feels like's its coming from that system. Also check evap lines for leaks (which would likely have already set a code). Maybe listen around throttle body and air inlet tube as well.
    Thanks for the quick reply. There is no code thrown that points to anything like that. I have random codes every now and again that I have just chalked up to gremlins that no one can seem to diagnose that I clear and they don't come back. The noise is still the same with the AC/Heat/Vent on regardless. A few months ago I threw a code for a misfire on #4 ended up being a bad wire and no spark from that particular coil. Replaced it with a screamin demon coil, livewires, and new autolite plugs with a .65 gap. The other day a code popped on for a misfire on #2 -- checked the plugs and wires all were ok. Cleared and never came back

    From inside the cabin it seems most audible from the drivers side. I am almost wondering if a line popped off the back of the climate control in the dash? When turning the knob from Upper vents to upper vents+floor it seems to be difficult to move.
    - '04 TJ (Sold)
    - '07 JKU (Driving me slowly insane)

  4. #4
    Nothing but a Thing black pearl's Avatar
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    brake booster pump maybe? Releases pressure when you lose vacuum pressure like on acceleration or shut down just guessing


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    Nothing but a Thing BananaJeep's Avatar
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    This probably doesn't help at all, and I can't watch the video now because I'm at work, but when I turn my fan off in my TJ (even if the Jeep is off) it makes a noticeable sound like gas or pressure is being released. The Jeep could be off for a few days and I hop in and flip the fan switch from on to off and it'll do it. I always thought it's just how our Jeeps were designed.
    2006 Jeep TJ
    "The Banana Jeep"

  6. #6
    Fresh Catch Pixel's Avatar
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    Quote Originally Posted by black pearl View Post
    brake booster pump maybe? Releases pressure when you lose vacuum pressure like on acceleration or shut down just guessing


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    That is exactly where it sounds like it is coming from but I cannot visually see anything. When I poked around with the propane I could not find anything. It is a pretty audible sound I don't think it is going to be a pinhole sized leak if that is the case no? There is definitely a lag in acceleration on this already lethargic engine. Going to hook up a vacuum pump and see if it holds. Hopefully will have a few mins later today after work.
    - '04 TJ (Sold)
    - '07 JKU (Driving me slowly insane)

  7. #7
    Nothing but a Thing black pearl's Avatar
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    2007 JKU 3.8L, Vacuum leak? video link attached.

    Quote Originally Posted by Pixel View Post
    That is exactly where it sounds like it is coming from but I cannot visually see anything. When I poked around with the propane I could not find anything. It is a pretty audible sound I don't think it is going to be a pinhole sized leak if that is the case no? There is definitely a lag in acceleration on this already lethargic engine. Going to hook up a vacuum pump and see if it holds. Hopefully will have a few mins later today after work.
    It could be something inside the canister itself maybe .... vacuum or smoke test would be easiest way of finding for sure then


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    Knows a Thing or Two madstiles's Avatar
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    Mine has a leak in between the booster and master cylinder it was hard to locate also until I used smoke

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  9. #9
    Knows a Thing or Two Judesign's Avatar
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    2007 JKU 3.8L, Vacuum leak? video link attached.

    Quote Originally Posted by madstiles View Post
    Mine has a leak in between the booster and master cylinder it was hard to locate also until I used smoke

    Sent from my SAMSUNG-SM-G935A using WAYALIFE mobile app
    Same thing happened to me. There is a seal in between the master cylinder and brake booster that sounds like that when it goes. Usually the cause is a leak of brake fluid that eats the seal. Mine was randomly misfiring due to the unmetered air but didn't respond to propane. I replaced the booster and master cylinder and all was well.
    Edit : to test you could vice grip the booster hose when idling and if the sound goes away then that would indicate that seal.

  10. #10
    Hooked Josh27's Avatar
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    I had the o ring between the master cylinder go bad which caused a vacuum leak. But if your brake pedal isn't hard I would think that its not a vacuum leak at the master cylinder or booster. If The hose that connects to the booster is leaking it would also cause the brakes to be hard. My jeep didn't make that noise when I had the bad o ring or the hose disconnected. It was just a constant air sound when the engine was running and stoped when the engine was shut off.

    Edit: sounds kind of like my k&n intake when I shut my engine off except it doesn't last that long.

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    Last edited by Josh27; 04-20-2017 at 11:33 PM.

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