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Cylinder Leak-Down Test
04-27-2014, 11:01 PM
Post: #1
Cylinder Leak-Down Test
A "cylinder leak-down test" I've heard of but never done. I have a 2000 Kia Sportage. I'm assuming it has something to do with air into the cylinders through the spark plug holes? I know how to remove the coil packs, plugs, turn the crankshaft bolt with a wrench and place a drinking straw into the spark plug holes to determine what position the pistons are at, I guess???? How is the test done and why. Any help appreciated.
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04-28-2014, 12:08 AM (This post was last modified: 04-28-2014 12:12 AM by Rupe.)
Post: #2
RE: Cylinder Leak-Down Test
Funny you should mention that test as it's one I have never had much use for over 45 years in the business. OTOH, there's people who swear by it and do it all the time.

The short version is this test allows you to determine if you have enough compression leakage to cause an issue and will also tell you if it's the valves or the rings. You basically put air pressure in the cylinder and read how quickly the air goes away. The read out is in percentage.

There are other ways to figure if it's the valves or the rings using compressed air or a "wet & dry" compression test. You can also put air in the cyls and listen at the intake, exhaust, or oil fill. Using this method will not give you a percentage but my figuring is if you can hear the difference you need major work anyway.

The problem with any test where you use shop air in a cylinder is you always wind up with the piston fully down, which is not the worst thing, but it's not always totally conclusive because you have more wear at the end of the stroke anyway. See, the whole point of a decent diagonsis is knowing more then you did when you started and putting all the pieces of the puzzle together to form the big picture. As an example, I have seen people do a leak down test only to figure out the cam belt is off time. Yeah, they thought they needed a valve job because the air got by the valves so they pulled the heads. If they got all their info together they might have only replaced a belt and been done already! Reason being is what's the chances that ALL of the valves are burned or bent? Yeah, you can look up which engines will bend a valve but if that's the case now you have a clue as to why and you have more pieces of the puzzle. IOW, you know what to look for! What I am driving at is a leak down test will not be the first item on a list but rather a tool to use after you have listened to how the engine runs, gone through basic diagnosis of ignition, vac leaks, and other "easier" to trace issues, like a standard compression test. Still in all, not a test to stand on it's own, but another piece of the puzzle.
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