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Thread: Deciphering Compression Readings

  1. #1
    Registered User Threehz's Avatar
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    Default Deciphering Compression Readings

    Finally got around to compression testing my engine. Did the compression test manually (engine is out of car on stand). Squirted some gear oil in the spark plugs holes a few days ago. Here are the results:

    1 - 125 > 90
    2 - 132 > 90
    3 - 10 > 30 > 125 (squirted oil in)
    4 - 67 > 65 > 150 (squirted oil in)
    5 - 215
    6 - 210

    The > sign just means I redid the test on that cylinder. Anyways, it doesn't look good. My friend says that the improvement after adding oil means it's likely the rings, pistons, or block and not the valves or head that has an issue. Makes sense to me.

    Car had sat 8 years when I got her, engine has been out another 3 years since then. I was planning to have everything put back together in the next month or so and see if I can get her to start. Where should I go from here? Tear into the engine while it's out? Put it back in and see if I can loosen stuck rings?
    Different Strokes for Different Folks
    1991 BMW 318is - DD
    1977 Datsun 280Z - Resto

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    By manually you mean with a wrench on the damper bolt? I wouldn't trust those numbers, even on a freshly run engine. You'll get some leakage past the rings when things are moving that slowly. If you've adjusted lash on these engines you'll know the sound. But beyond that, 11 years of not running will almost certainly allow some rust in the cylinders and valve seating surfaces.

    Might be worthwhile to mount the transmission and starter and give it a good spinning with the plugs out, then re-test. I don't think that your results are very informative.

    Do you know anything about the engine? 200 psi+ is well over the pressure you would see on a stock engine. The 5 and 6 numbers are strange.
    1976 280Z, with some minor modifications

  3. #3
    Registered User Threehz's Avatar
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    Yep, put a break bar on that big bolt and cranked her until pressure stopped building. I've looked up her skirt and down her shirt (oil pan and valve cover) and the internals look great for an engine with 177K. Nothing looked odd or out of place or overly worn to my admittedly novice eyes.

    I can say that the first two cylinders took a good amount of strength to push through, the middle two were pretty easy until I squirted fresh oil in, and the last two took all of my might.

    I believe the engine is stock, never rebuilt, but I don't know for sure. The high readings on the last two cylinders are probably from being recently oiled.
    Different Strokes for Different Folks
    1991 BMW 318is - DD
    1977 Datsun 280Z - Resto

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    Crumudgeon
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    As Zed said, you need to spin the engine with the starter.

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    Registered User Threehz's Avatar
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    Is there an easy way to do that without connecting the starter up to the regular wiring? For instance, could I just connect the starter up to a 12V battery?
    Different Strokes for Different Folks
    1991 BMW 318is - DD
    1977 Datsun 280Z - Resto

  6. #6
    Crumudgeon
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    Yes. You'll also need to ground the starter to the engine and have a small wire to send 12v to the starter solenoid to trigger the starter. And make sure the engine is on a stand or securely mounted to something.

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