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?
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.
