I recently purchased a 2007 Subaru Outback 2.5i, with approx 146000kms on the clock.

As its a recent purchase, I'm going through preventative maintenance to ensure its running as good as possible.

One of these measures was to get my mechanic to carry out a cylinder compression test and a cylinder leakdown test.

Here are the results of these tests:

Compression test

cylinder # | comp
    1      |  200  
    2      |  200  
    3      |  205  
    4      |  200

Leadown test

cylinder # | leak
    1      |  10%  
    2      |  8%  
    3      |  12%  
    4      |  18%

My mechanic says that all of these results are normal and within specs except for the cylinder 4 leakdown which according to him is leaking into the oil filler tube, which he finds very strange and has no explanation for.

Could you please help me understand these results, in addition to what my mechanic said? What do they mean? Also, do I need to worry about these results or take any additional actions?

Additional information

  • A cooling system pressure test was also carried out and was found to be all ok and within specs.
  • A block chemical test was also carried out and was to be all ok and within specs. I don't understand what a block test actually is so I'm not sure what this means.

Please help.

  • Compression looks good and even across cylinders, I would not worry about the leakdown results. – Moab May 17 '18 at 2:55
  • @Moab thank you for your advice. Could you help me interpret what the leakdown tests mean? How could one cylinder have a higher percentage leak than the rest? And into the oil filler tube? – WeakMech May 17 '18 at 3:51
  • Re exit from oil filler - what other openings are there where air can escape? – Solar Mike May 17 '18 at 6:12
  • @SolarMike my mechanic didn't say anything about hearing leaks from other areas. But if I were to guess based on my knowledge of engines I'd probably say the oil dipstick tube, the PCV valve and the air intake. – WeakMech May 17 '18 at 6:28
  • 1
    @WeakMech leakdown test just shows what compression is leaking past valves or rings, but is a static test and I don't use it to determine engine health when compression is as good as yours, compression is a better indicator.in your situation. – Moab May 17 '18 at 14:49

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