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Thread: Different weird compression test results - diagnosis?

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    Default Different weird compression test results - diagnosis?

    My high school auto mechanics is pretty rusty...

    I have been working on this 8/70 Z that has been sitting since 87. I've gone through all the basics, hoses, belts, gas tank, all fluids, rebuilt the carbs, etc. The last thing I did was rebuild the carbs. The front 3 carbs were getting no gas to the engine - I adjusted the float and then I heard what sounded like a loud exhaust leak somewhere towards the front of the motor. I ended up taking off the intake and exhaust manifolds and was ready to start putting it back together when I thought - wait what if it wasn't an exhaust leak but something worse? So I got a compression tester from Autozone and here are my results:

    1-50, 2-60, 3-105, 4-130, 5-55, 6-110

    I didn't take all the plugs out (mistake maybe). This is the 2nd gauge I used - the first one showed no compression at all on 3 of them so I figured it had to be bad. I'm pretty sure the gauge is OK.

    I have no antifreeze in the oil, had no blue smoke when it was running. Is this still a valid test with both manifolds off? I can't think it would be that big of a deal. I'm thinking if the bottom end of the motor was bad, I'd hear different things or see blue smoke. Should I pull the head off and get into that whole bucket of worms or am I getting paranoid?

    Any words of wisdom for this amateur mechanic would be appreciated...
    72 240Z desert survivor white original but interior gutted
    70 240Z HLS3007969 Series 1 8/70 earliest known Automatic being restored

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    Registered User LeonV's Avatar
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    Take all plugs out and do it again. Crank enough times to stabilize the reading.
    2/74 260Z

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    OK, did that. Higher but still bad.

    1-70, 2-85, 3-120, 4-135, 5-70, 6-125

    Head gasket?
    72 240Z desert survivor white original but interior gutted
    70 240Z HLS3007969 Series 1 8/70 earliest known Automatic being restored

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    Registered User LeonV's Avatar
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    Before jumping to any conclusions, have the valves recently been adjusted? If not, do so and run another comp check.
    2/74 260Z

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    Yeah if it has an E31 head or brass seats then they may be pushed up into the head making it difficult to set the lash and seal. If this is the case,you will need to:

    1. Do a valve job and use shorter lash pads
    2. New seats and valve job
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    Have you done a wet compression test? And it sounds like you can't run the engine to operating temp before the test either. How much have you been able to run the engine since you began reviving it? To get useful results from a compression test you may need to run your engine for a while to get things freed up if it's been sitting for 20 + years. The advice about valve adjustment should be heeded as well. A lot of times an engine that has been sitting for a long time will "repair" itself if driven gently for a few hundred miles. And if not you are going to have to rebuild it anyway.
    http://www.car-techie.com/compression.php?003

    Steve

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    I can't get it up to temp since the manifolds are off. I don't know what a "wet" compression test means. I'll have to adjust the valves cold - should I still use .012 exhaust and .010 intake when it's cold? That loud exhaust leak type of noise from the front part of the motor could very well have been coming from the head gasket and not the manifold since they're so close and the compression is low on those front 2 cylinders. I took the gauge back to Autozone already so I guess I'll adjust the valves and go back and get it when I'm done and re-test.
    72 240Z desert survivor white original but interior gutted
    70 240Z HLS3007969 Series 1 8/70 earliest known Automatic being restored

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    Registered User doradox's Avatar
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    Quote Originally Posted by kurtwwalters View Post
    I can't get it up to temp since the manifolds are off. I don't know what a "wet" compression test means. I'll have to adjust the valves cold - should I still use .012 exhaust and .010 intake when it's cold? That loud exhaust leak type of noise from the front part of the motor could very well have been coming from the head gasket and not the manifold since they're so close and the compression is low on those front 2 cylinders. I took the gauge back to Autozone already so I guess I'll adjust the valves and go back and get it when I'm done and re-test.
    The link I provided explains a wet compression test.

    Steve

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