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loss in compression

Posted by bcjjones, Nov 11, 2013

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  1. bcjjones

    bcjjones Proven Member

    Joined Nov 3, 2011
    Austin, Texas
    Does a loss in compression always result in a loss of vacuum?

    I did a compression test and got some pretty nasty numbers. Car seems to run and idle great though. Also at idle, aroind 950, I am pullsing a vacuum between 16-20 in.

    Numbers on two cylinders were 120 and the others were 90

    Possible messed up the compression test I guess, but my real question is will it always loos vacuum with compression loss?
  2. Phantom DSM

    Phantom DSM Proven Member

    Joined Jan 28, 2007
    Dallas, Texas
    When you performed the compression test. Was the motor still cold or after a good drive? All spark plugs out and full throttle while cranking?
  3. boost97gst

    boost97gst Proven Member

    Joined Apr 2, 2012
    st jacob, Illinois
    Cold vs hot makes some difference but not much at all. Try a wet test followed by a leak down to get the real story. Yes, to answer, very low compressions would show less vacuum at idle. Again, not a huge difference but you'll see a bit less vac
  4. bcjjones

    bcjjones Proven Member

    Joined Nov 3, 2011
    Austin, Texas
    Yea wet compression and leakdown are on the to do list when I have time. After those numbers I didn't tgink the car would even be driveable. Like I said though it runs and drives fine. That low of compression should yield some kind of driveability issues right?

    Is it possible to fry the comoression rings and not the oil rings? I haven't burned any oil and it doesn't smoke at any throttle position
  5. BogusSVO

    BogusSVO Proven Member

    Joined Jul 1, 2009
    Pensacola, Florida
    Check your timing belt, if off a tooth, it can cause low compression and low vac.

    If your vac gauge ( or boost gauge w/ vac reading) needle is jumpy/vibrates at idle then you can have a intake valve that is not seating.
  6. bcjjones

    bcjjones Proven Member

    Joined Nov 3, 2011
    Austin, Texas
    Ill check mechanical timing again. My cylinder head is a brand new rebuilt, but the needle is a little jumpy at times. But it is mostly steady. I believe I just have a slight intermediate misfire at idle which is what I think causes the needke to be a little jumpy? Like I said it doesn't always jump a little, only when it fefeels like it has a slight misfire

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