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Compression test questions

This is a discussion on Compression test questions within the Internal Engine forums, part of the LSx Technical Help Section category; I bought a '98 LS1 from a guy on Craigslist last week. I spent a few days stripping the accessories ...

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    Compression test questions

    I bought a '98 LS1 from a guy on Craigslist last week. I spent a few days stripping the accessories and crud off and ended up with a decent-looking long block on an engine stand.

    I decided to do a compression test before going any further. Spinning the motor over by hand, all cylinders came up between 50-60 psi after three full compression cycles.

    Cylinder #1 was the slowest to get there. Also, if I put my hand over the intake port as it was compressing, I could feel a puff of air blasting back at me.

    I've never done a compression test on a non-running engine before. I don't expect to see numbers in the 150 psi range when cranking slowly by hand, but I'm still wondering if my readings are "normal" and whether there's something wrong with the valve or seat in cylinder #1 (and potentially others).

    Just looking for opinions. Should I get the heads redone? Should I hook up the starter and battery and get new readings? Should I stop worrying and get my car on the road?

    Advice would be appreciated. Thanks in advance.

    Eric

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    The only way to get a good test reading is to spin the engine with the starter. Rotating the crank by hand is simply too slow and all the compression bleeds off.

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    Just a note. Make sure all the plugs are removed and crank with the starter. If your feeling a puff of air on compression the valve seats are leaking on number 1.
    Last edited by GBurke; 05-09-2011 at 01:20 PM.

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    OK, I'll run the test again tonight. Thanks.

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    Quote Originally Posted by GBurke View Post
    Just a note. Make sure all the plugs are removed and crank with the starter. If your feeling a puff of air on compression the valve seats are leaking on number 1.

    Good point. OP: If you have a cylinder with low compression, add a teaspoon or so of oil down the plug hole and try it again. If the compression pops up to where it should be you have a ring sealing issue. If the oil results in no change it has to be the valves... or a hole in the piston.

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