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400i Vacuum Leak Test

Discussion in '365 GT4 2+2/400/412' started by Bubba5320, Jun 18, 2024.

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  1. Bubba5320

    Bubba5320 Karting

    Apr 18, 2024
    109
    Phoenix
    Full Name:
    Bubba
    #1 Bubba5320, Jun 18, 2024
    Last edited: Jun 18, 2024
    I believe I have a vacuum leak and wondering thoughts on best ways to test? I can spray carb cleaner, blow propane in any and all places I see. Soap or water? What about brake booster? If smoke test where is the best place to hook up? Should I do tests at idle? Kick up the idle? For which tests? Smoke Pressure test through gas tank a bit worrisome due to old hoses etc. just wondering what folks think as to a good procedure. Thanks much,
    Bubba
     
  2. Rifledriver

    Rifledriver Three Time F1 World Champ

    Apr 29, 2004
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    Brian Crall
    #2 Rifledriver, Jun 19, 2024
    For the brake booster disconnect the hose at the engine end and plug the vacuum port see if the idle changes.

    First thing you need to do is determine which bank is not running correctly. If both banks probably not a vacuum leak.

    Propane while I am watching an exhaust analyzer is the best way in my experience.
     
  3. SouthJersey400i

    SouthJersey400i Formula 3

    Mar 14, 2007
    1,679
    Romulus, NY (Finger Lakes)
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    Ken Battle
    #3 SouthJersey400i, Jun 19, 2024
    What symptoms are telling you that you have a vacuum leak? That should tell you WHERE to test.

    An old school method is to use something like shaving cream on the suspected leak and see if it gets sucked in. I think There is a foam for that. McMaster-Carr has several options: leak detectors | McMaster-Carr
    Ken
     
  4. Bubba5320

    Bubba5320 Karting

    Apr 18, 2024
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    Full Name:
    Bubba
    #4 Bubba5320, Jun 19, 2024
    Last edited: Jun 19, 2024
    Thanks for the replies. After car warms up and is revved up and down idle bounces from 750-1000. If I rev engine and get off throttle fast car will die unless I tickle throttle then it bounces again. My mechanic thinks vacuum but he can’t get me in for a bit. Thoughts? Thanks again everyone.
     
  5. Rifledriver

    Rifledriver Three Time F1 World Champ

    Apr 29, 2004
    37,105
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    Brian Crall
    #5 Rifledriver, Jun 19, 2024
    Common symptom on both BBi, 400 and 412. First off any running issue requires a complete check to be sure its all tuned properly before beating your head into a wall. 400i's have such a well deserved reputation for ignition advance issues if it was sitting in my shop Id be sure of that first. Basically if the fuel mixtures and the air flow ratios of what is coming in past the throttle plate and what is coming in through the air bleeds is not correct it will do that. Do you have a 4 gas analyizer at your disposal? I consider that essential for tuning CIS. You should be able to snap the throttles wide or nearly wide open and have it drop straight to the correct idle RPM.

    Also how many miles since it had a timing chain replaced and the cams properly timed?
     
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  6. Bubba5320

    Bubba5320 Karting

    Apr 18, 2024
    109
    Phoenix
    Full Name:
    Bubba
    #6 Bubba5320, Jun 19, 2024
    No four gas analyzer. About 8k since timing and adjust.
     

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