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  1. #1

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    Compression Test Question

    Which one is more accurate or yields more realistic result:

    1. Remove one spark plug, plug the tester and read...Meaning all cylinders are still sealed
    2. Remove all spark plugs and do a compression test on each cylinder one after another


    I think number one gave me higher number than number two...I thought it should be the other way around...

    This is on a cold engine....

    Thanks guys!


  2. #2
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    Compression Test Question

    Most consistent results will be with a motor at operating temps as well as all plugs out to allow the motor to spin at the correct rpm. Testing a cold motor especially a used one could vary quite a bit.

  3. #3
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    engine will spin faster with the plugs in compression push

  4. #4
    Vern's Avatar
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    Quote Originally Posted by Rocketman View Post
    engine will spin faster with the plugs in compression push
    +1, that is why option 1 above will show higher compression. On the 4-tecs, 2 plugs in shows 10 psi higher than all 3 plugs out.

  5. #5

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    Does that mean it's more accurate this way?
    Quote Originally Posted by Rocketman View Post
    engine will spin faster with the plugs in compression push

  6. #6

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    Even on cold engine I have 200+ psi compression within 10 psi from each other but when I did it with all plugs out I got numbers from 195-210 and it cranked longer to achieve those numbers where when I did it with plugs in with cold engine, I am getting 210-215-215-220 and only few cranks will get me there. Not worried about engine, I just do a compression test everytime I change the plugs and document it. Wanted to know which one would be more accurate, I would think plugs in would be more accurate, isn't it? On warm engine I get 5-10 psi more...
    Quote Originally Posted by FIRE0021 View Post
    Most consistent results will be with a motor at operating temps as well as all plugs out to allow the motor to spin at the correct rpm. Testing a cold motor especially a used one could vary quite a bit.

  7. #7
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    Quote Originally Posted by CaptainReza View Post
    Does that mean it's more accurate this way?
    Quote Originally Posted by Rocketman View Post
    engine will spin faster with the plugs in compression push
    acc would be ur gadge ur way of cking so do it the same all the time for the same result. I like to do it with the plugs in one at a time with batt charger hooked up for more consistent results, leak down is the best test anywho but comp is the first place to start

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  9. #8

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    BTW, I dont have a leak down tester. Anyone any recommendation on something to be used once in a while and not too expensive. It's for personal use not a pro shop..

  10. #9
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    Quote Originally Posted by CaptainReza View Post
    BTW, I dont have a leak down tester. Anyone any recommendation on something to be used once in a while and not too expensive. It's for personal use not a pro shop..
    $58 with free shipping.

    http://www.amazon.com/OTC-5609-Cylin...eakdown+tester

  11. #10
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    Compression Test Question

    Quote Originally Posted by Vman View Post
    Quote Originally Posted by CaptainReza View Post
    BTW, I dont have a leak down tester. Anyone any recommendation on something to be used once in a while and not too expensive. It's for personal use not a pro shop..
    $58 with free shipping.

    http://www.amazon.com/OTC-5609-Cylin...eakdown+tester
    Yep same one I have works great. Same shit as what you get on the Matco truck without the price.

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