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Compression versus leak down tests

Discussion in 'XJ Technical Chat' started by Wirehairs, May 18, 2014.

  1. Wirehairs

    Wirehairs Member

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    Most folks here advocate for a compression test on a newly acquired bike to test the health of a motor. However, recently I had a Maxim 400 blow good compression numbers, but later found out that it has oil leaking into the cylinder.

    Anyways, my question is, won't a leak down test tell you everything that a compression will plus? If the leaking is below 20%, am I safe to say that the compression will be within specs?

    Downside to the leak down test appears to be that the testers are a bit more expensive, and you need an air compressor.

    Thanks for the input!
     
  2. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    You can have poor compression and still have whatever compression pressure that has built up "leakdown" thru the rings (or valves). That is the purpose of the leakdown test, to verify if the compression "chamber" is able to hold whatever pressure is built up. It really tells you almost nothing about HOW MUCH pressure the cylinder is capable of building up, which is the purpose of the "compression test".

    Both tests are very useful diagnostic/maintenance procedures, and should be checked (and recorded) periodically.
     
  3. Polock

    Polock Well-Known Member

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    how much pressure is built up is how much pressure your compressor puts in it. the engine isn't used to pump up pressure, it all comes from the compressor.
    a leak down will tell you if it's rings like a regular tester also what valve might be leaking
     
  4. MiCarl

    MiCarl Active Member

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    Also, it takes a lot less equipment to perform a compression test.

    I'm curious about your situation. Are you claiming the rings were shot but oil in the cylinder hid it? If the problem was a head gasket a compression test should have shown that.......
     
  5. Wirehairs

    Wirehairs Member

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    Carl - I'm just trying to understand the diagnostic ability (and limits) of each of these tests.

    I have two current issues and trying to figure out how to use each of these tests to diagnose what's really going on. In one case, I have good compression numbers (on a Maxim 400) but oil leaking into one cylinder. In the other case, I have another Maxim (just picked it up for $150) which I have been unable to start (electrical issues) but still would like to assess the health of the motor.
     
  6. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Compression test tells you a numeric value. Leakdown test tells you where you're losing compression .

    It is a little more involved for bikes, but not by much........its very useful for sleds-----
     
  7. MiCarl

    MiCarl Active Member

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    Bottom line: Compression test tells you if the cylinder and valves are up to snuff. If the compression isn't up to snuff a leak down test helps you diagnose why.

    If the cylinder with oil in it has good compression numbers the oil is likely coming down the valve guides.
     

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