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Thread: Compression Test vs. Leak Down Test

  1. 08-26-2018, 05:37 AM #1
    Boulder Bob
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    Compression Test vs. Leak Down Test

    I own a compression tester but not a leak down tester.

    It would seem to me that if the compression test looks good, the engine is "mechanically healthy" (at least in regard to valves, rings and head gaskets) and there is no need to do a leak down test. If the compression test shows issues, then one can do a leak down test to find out why.

    Agreed?

    Incidentally, my compression test numbers are 118, 118, 114, 115, 120, 115 psi (for a 73 S). Sound OK?
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  2. 08-26-2018, 05:46 AM #2
    Dg567
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    Agreed. Those are strong, consistent numbers, I wouldn't worry about doing a leak down test.
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  3. 08-26-2018, 05:47 AM #3
    Frank Beck
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    Believe or not an engine with good compression can still have an oil burning issue. One of the main reasons we check leak down is to evaluate valve guides... which is a common problem on air cooled engines. If the leak down shows good (1-5%) but the plugs show evidence of oil burning it’s an easy assumption that the guides or valve stem seals are the culprit.

    Go spend the 200 bucks. And remember: A man can’t have too many tools.
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  4. 08-26-2018, 09:54 AM #4
    Boulder Bob
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    Quote Originally Posted by Dg567 View Post
    Agreed. Those are strong, consistent numbers, I wouldn't worry about doing a leak down test.
    Thanks for the confirmation!
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  5. 08-26-2018, 09:55 AM #5
    Boulder Bob
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    Quote Originally Posted by Frank Beck View Post
    Believe or not an engine with good compression can still have an oil burning issue. One of the main reasons we check leak down is to evaluate valve guides... which is a common problem on air cooled engines. If the leak down shows good (1-5%) but the plugs show evidence of oil burning it’s an easy assumption that the guides or valve stem seals are the culprit.

    Go spend the 200 bucks. And remember: A man can’t have too many tools.
    You're right about the tools! Thanks for the response
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  6. 08-30-2018, 12:29 PM #6
    Gearby
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    Unrelated to Porsches but I still have a Saturn that I've had for 18 years. The engines in those cars have a bad habit of sticking their oil rings when the owners don't keep the oil changed. It produces a case exactly as Frank explains. Will still make good compression and run fine but they will burn oil. A leak down tester will do things that a compression tester cannot. It will tell you where the compression is leaking. Each tool has it's own use.
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  7. 08-30-2018, 12:36 PM #7
    Frankr
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    If a car burns much oil you will know it very soon. One way or another.
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  8. 08-30-2018, 12:40 PM #8
    Frank Beck
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    Quote Originally Posted by Frankr View Post
    If a car burns much oil you will know it very soon. One way or another.
    A seller will surely know.

    Not necessarily so for a buyer.
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