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What is a Leak Down Test?


SmilinSam

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I know you pressurize the cylinder and see how quickly the pressure falls off, but how and what does it tell you?
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If it leaks down quickly, you add oil to cylinder . If it still leaks at same rate , you have bad valves or head gasket. If leakage stops ,you have worn rings. At least that is what was told to me a long time ago.
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A cylinder leak tester, or leak down tester as they are some times called , is a tester for checking the sealing qualities or the parts of the engine cylinder with the engine at rest. It has a pressure gauge, an adjustable air regulator, and a fixed orfice as it's main componets. In use the pressure gauge is reading pressure drop across the fixed orfice to give you a relitive number for how large the air flow is past the rings,valves, and or gaskets. The units are quite simple to make from items found arround the shop. If there is any interest in the subject I will be glad to write an article on how to build and use one.
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quote:
Originally posted by slin
A cylinder leak tester, or leak down tester as they are some times called , is a tester for checking the sealing qualities or the parts of the engine cylinder with the engine at rest. It has a pressure gauge, an adjustable air regulator, and a fixed orfice as it's main componets. In use the pressure gauge is reading pressure drop across the fixed orfice to give you a relitive number for how large the air flow is past the rings,valves, and or gaskets. The units are quite simple to make from items found arround the shop. If there is any interest in the subject I will be glad to write an article on how to build and use one.
Sounds like a good idea...
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Sam, AS has been explained, this a calibrated air supply with two pressur guages, one on the input to the orfice and one on the "out" side of the orfice. You put the piston at Top Dead Center on the compression stroke and lock the flywheel. Next you connect the hose to the spark plug hole. Remove the air cleaner and dip stick. Open the throttle wide open and connect the air supply to the tester. Adjust the regulator to the "Set Line" on the input pressure guage. Then read the guage of the other side of the calibrated orfice. If the engine is in good condition this guage will read in the green. If it reads low, you then listen to the muffler to hear exhaust valve leakage, the carb for intake valve leakage and the dipstick hole to hear the leakage to the crankcase. This very quickly determines where the problems are. Normally one would first do a compression check, then a crankcase vacuum check and then a leakdown test. By using these 3 checks and interpolating the results, you can usually quite accurately evaluate an engine. If tahe compression is low and the crankcase vac is normal, I would suspect an valve or head gasket problem, the leakdow test will verify and isolate this. If the crankcase vac is low and the compression marginal, I would suspect cylinder leakage or a bad breather valve. The leakdown test will pin point the severity of the compression leakage and help you to decide if the cylinder condition of the beather is the most likely cause. Al Eden
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quote:
Originally posted by SmilinSam
How is the crankcase vacuum check done?
Generally with a water manometer or vacuum gauge thru dipstick hole. Tecumseh is the only one that I know that published that value. I would imagine that all the air cooleds would register the same if you new the value of one. I'll have to look in my older repair books. Don't recall seeing that value in older newer books. Onan at one time required a crankcase vacuum check on new applications to insure engines were not being worked over 85% of their given rating to insure engine life.
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