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Thread: High RPM rev fall vibration/noise

  1. #1
    Join Date
    Aug 2004
    Location
    Va
    Posts
    483
    My Cars
    96 328i

    High RPM rev fall vibration/noise

    So, I have an issue with an n54 engine with auto trans. At low rpm, it sounds just fine. But when you rev it up to ~ 5k and let it fall, there is a drone?/vibration? from under the center of the dash/center console. When I'm driving and keep it in a higher gear and keep the rpms up, while engine braking, I hear it and can sort of see the dash vibrating at high frequency. When I apply some throttle it goes away. It is VERY noticeable. I checked the alternator pulley, it still spins in 1 direction freely. I can just start replacing pulleys and whatnot, but seeing if anyone had any insight first. Did a quick look over the vacuum lines, etc and didn't see anything obvious. I can't pinpoint it to the turbos either. There is some wastegate rattle. Any input is appreciated. Thank you!

  2. #2
    Join Date
    Aug 2004
    Location
    Va
    Posts
    483
    My Cars
    96 328i
    Ok. Found a vacuum leak on the line going to the brake booster. There is another line coming off of it that I think goes to the exhaust flap? Car now holes vacuum once you turn it off.

    Next question. I noticed when I press the brakes, the vacuum falls, then slowly recovers. It does the same thing when I release the pedal. If I repeatedly pump the brakes, I can get the vacuum level close to 0. Maybe -3 inhg. Is this normal? How long should it take to recover back to -18-23 inhg? 1 second? 5 seconds? Near instantly? Should the brake booster be taking vacuum when I release the pedal? Thanks!

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