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I understand that both cylinder leak down and compression tests are used to test for cylinder head, cracks, warps, and gasket leaks. I understand the difference between how each works: compression uses engine compression and leak-down uses an external air supply.

My question is: Are there differences in the results and what each of the two are testing? Are they used for different kinds of test and is one more appropriate than the other in some scenarios?

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Compression test is more to test if there is a problem. If your compression is low, it could be a number of different issues. You can test the rings by adding oil to the cylinder and see if the compression is higher. Other than rings it can be difficult to determine the source of the issue.

A leakdown test will tell you where the problem is. By watching and listening with a stethoscope, you can determine if the leak is a stuck valve, or a blown head gasket.

Also, a compression tests a moving engine, including opening and closing the valves. This can be useful if there is a cam/rocker issue.

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    if the vehicle overheats, is one of the two a better test than the other, assuming there is nothing wrong with the coolant system (chemical coolant leak test and coolant pressure test turn negative) ? – amphibient Jun 17 '15 at 17:57
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    a compression test will quickly tell you if any individual cylinder has low compression. a leakdown test is a little more involved and will help pinpoint where you're losing compression. but if the engine is overheating you should really check the entire cooling system (radiator fans, thermostat, water pump, etc) before either test – user4546 Jun 18 '15 at 3:00
  • Agreed, If the engine is overheating, very likely a coolant system issue. Sometimes you can't tell a water pump has failed unless you remove it. If you want to test for damage cause by overheating, a leakdown is a more thorough test. I will typically do a compression test, and if that is low, do a leakdown. – rpmerf Jun 18 '15 at 10:55
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    so, would you say that, if a compression test turns negative (the pressure remains constant), there is no point in performing a leak-down test because a leak-down test only finds which cylinder is leaking ? Another way to ask is, if a compression test is positive (there is a leak), could that leak be somewhere outside the cylinders so that, even though there is a leak, no cylinder leak down test turns positive ? – amphibient Jun 30 '15 at 20:54
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    Yes. A leakdown test used to find the source of the leak. If the compression is good, there is no leak to search for. If there is a leak, it is something in the cylinder - rings, head gasket, valves, or a cracked block (unlikely). – rpmerf Jul 1 '15 at 12:13

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