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Thread: cause of low compression

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  1. #1

    cause of low compression

    Trying help out a guy with a 7g whose car won't run. Engine was just above an idle when the timing belt broke. Belt replaced but car won't start. Part of the work up on the car since then included an engine compression test. All 4 cylinders now registering 60psi (Normal value is 192 psi, minimum spec is 145psi). Could blowing a timing belt bend all the valves and cause all of cylinders to have such a low compression?

    Piston rings are the first thing that comes to mind for me but it seems unusual that all four cylinders would show the same pressure. Looking for other thoughts on what would cause such a low compression reading across all four cylinders. thanks, jjj

  2. #2
    Experienced TGC Member
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    05-25-2006
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    recheck timing to make sure it is correct and to find where the leak is do a leakdown test. To do a leakdown test bring the cylinder to tdc and put in compressed air threw the spark plug hole with the leakdown guage. If air comes out of intake mani you have bent intake valves, threw the muffler is bent exhaust valves, threw the dipstick is worn rings. Hope that helped.
    ASE Certified Master Automotive Tech
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  3. #3
    Anything is possible but I would definitely do the leakdown test like Sleeper suggested, it helped me a lot when I busted my timing belt.

  4. #4
    Senior Moderator DOHCstunr's Avatar
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    12-17-2004
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    Louisville, KY
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    4,328
    if you have low compression after a tiing belt failure.
    10 times out of 10 its your valves are bent.

    plan on getting a new, rebuilt or used head to replace it.

    you coudl buy just the number of valves and guides that were damaged and rebuild it yoruself,

    but undoubtedly you will be able to snag a decent head for cheap.
    ______________________________

    1994 Galant GS-Turbo

  5. #5
    yhanks for the advice. Leak down test sounds like the right thing to do. I'll pass it on. jjj

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