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  • Vacuum Gauge Diagnostics

    Quote Originally Posted by antman
    i acually have never seen a supra pull 20 in.hg of vaccum at idle.
    All of mine have, unless there was an intake manifold leak or damaged rings/valves.

    As a matter of fact a vac gauge is one hell of a diagnostic tool that many people overlook.

    Here are some of the things a vac gauge can tell you:

    • gauge reading steady 17-22 in Hg indicates normal engine in good condition.
    • gauge reading low but steady indicates late ignition or valve timing, low compression, stuck throttle valve, leaking manifold gasket, etc..
    • gauge reading steady but dropping regularly indicates burnt valve or improper valve clearance.
    • gauge reading dropping gradually at idle indicates obstructed exhaust.
    • gauge reading slowly dropping to zero as engine speeds up indicates obtructed exhaust.
    • gauge reading fluctuating between 15 and 20 in Hg at idle indicates stuck valve or ignition miss.
    • gauge reading drifting indicates a minor intake leak at the manifold.
    • gauge reading fluctuating as engine speed increases indicates weak valve springs, worn valve stem guides.
    • gauge reading vibrating excessively at idle but steady as engine speeds up indicates worn valve guides.
    • gauge reading vibrating excessively at all speeds (when not under boost) indicates a blown head gasket.
    • healthy decelerating engine reading should jump to 21 thru 27 in Hg as open throttle released.

    These rules hold true for just about all 4 stroke gasoline engines, regardless of manufacturer or configuration (I4, I6, V6, V8, etc..)
    This article was originally published in forum thread: Boost leak? started by mk3supra View original post
    Comments 1 Comment
    1. Devin LeBlanc's Avatar
      Devin LeBlanc -
      10 in hg vacuum can be 100% normal too at idle. If you have 280 deg cams I personally pull 20 in hg on my motor at idle 650 rpm
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