Leak Down Test Question
09-04-2002 | 10:21 PM
#1
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From: Alexandria, VA
Leak Down Test Question
Can anyone give me an explanation of a leak down test? I currently have a 454 that has little compression on the #7 cylinder. I haven't had a chance to check the actual number so I did the finger test today and it feels really low as compared to the #5 cylinder. This is a fresh rebuild with 25 hours. New pistons and rings and freshened peanut port heads. I am praying for a valve problem and not a cylinder.....
Thanks,
Ken
Thanks,
Ken
09-04-2002 | 10:43 PM
#2
Charter Member #232
Joined: Oct 2000
Posts: 18,417
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From: Carlisle, MA USA
Leak downs are performed by moving the cylinder you are testing to the top of the stoke and then holding the crank in place and filling the cylinder with compressed air. Then you measure the amount of time that it takes to leak down to a certain spot. If you have a bad leak then you will be able to hear the air rush past the valve or rings. Hope that helps
Jon
Jon
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09-05-2002 | 03:58 AM
#4
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Leakdown testing 101
A leakdown test places a gauge in the spark plug hole with the piston at top dead center without moving. The combustion chamber is then pressurized with air from an outside source. With the regulator air pressure as the standard, the test measures the % of the pressure _drop_ through a
particular orifice that results from whatever flowrate leaks out the cylinder being tested. The air flow passes from the regulator through the orifice on its way to the cylinder, and as long as the leakage flowrate out of the cylinder stabilizes, the pressure drop through the orifice will stabilize, regardless of how long you run the test. If air pressure is supplied to the leakdown tester at 100 psi, and the leakage out the cylinders is a stable flowrate through the tester that causes 16 psi of pressure drop through the orifice, the tester reads either 84 psi or 16% leakdown, depending upon how the dial on the second pressure gauge is marked. As with a compression test, the leak down test is repeated for each cylinder. Again, a new engine may lose 5% to 8% on a leakdown test.
The cylinder leakage test is best performed on an engine at operating temperature. Due to the time involved in removing spark plugs, and the hazards of working around a hot engine this may be impractical. The test is generally reliable when done on a cold engine, but if engine damage is suspected, and a cold test shows no problems, then the test should be repeated with the engine warm to verify results.
1. Remove all of the spark plugs and rotate the crankshaft to place the piston in the cylinder being tested at Top Dead Center of the compression stroke.
2. Insert the adaptor hose into the spark plug hole of the test cylinder. Connect this hose to the leakdown tester.
IMPORTANT! To Avoid a sudden engine rotation or
damage to your leakdown tester, make certain the leakdown tester in NOT connected to a compressed air source at this time.
3. On the leakdown tester,unlock & back off (counterclockwise) the regulator knob a few turns & then connect your shop compressed the same procedures. Record the cylinder pressures for
all cylinders and the proceed to interpret your results.
(See chart below.)
Analyzing Test Results
The leakage tester utilizes two gauges so that the cylinders' rate of leakage can be seen as the difference between the test pressure on the primary gauge and the leakage pressure on the secondary gauge. The condition of the cylinder is therefore judged by the comparison of these two pressures, expressed in "percentage of leakage."
The chart below shows the percentage of leakage at three different test pressures. The top row is the percent of leakage based on the pressure observed on the secondary gauge. If you used 100 p.s.i. for the test pressure (primary gauge reading), read the numbers in the "100 p.s.i." row. If you used 90 p.s.i., read the second row, and if you used 80 p.s.i., read the third row. The percent of leakage is shown in the top row above the column containing your observed test pressure. that the test will not only determine how badly a cylinder leaks, but also will help to pinpoint the causes of excessive cylinder leakage. Simply listen and look around the engine to find where the compressed air is escaping. The two primary causes of cylinder leakage are past piston rings and valves. Leakage past the rings causes air to blow into the crankcase. Usually this can be verified by listening at the oil filler cap. Likewise, intake valve leakage can be heard at the carburetor or throttle body, and exhaust leakage can be heard at the tailpipe. Sealing the exiting air by placing your hand over its source often verifies the severity of the leak by causing a pressure rise on the secondary gauge. This technique can help to isolate the worst components that may cause cylinder leakage are cracked castings and blown cylinder head gaskets. Check for air escaping into the coolant and/or excessive
leakage in two adjacent cylinders to verify this diagnosis.
% of Leakage 6% 7% 8% 9% 10% 11% 12% 13% 14% 15%
100 p.s.i.....94... 93...92...91...90... 89....88....87... 86....85
90 p.s.i...86.5.. 85.. 84...83...82....81....80... 79... 78....77
80p.s.i......75....74...74...73... 72....71....70...70....69....68
Every engine
will exhibit some degree of cylinder leakage. Even in the best of engines you may observe up to about 1 or 2% leakage. As a rule, a good cylinder will exhibit no more than about 5-8% leakage. When an engines' cylinders begin to exhibit leakdown percentages above or about 12 or 15%, the performance and economy of the engine are usually being noticeably compromised. Leakages above 15 to 20% indicate serious enough wear that repair should be made. Note that long stroke or high rpm
engines may tolerate a slightly higher leakage, due to their greater piston speeds. Six and eight cylinder engines respond less noticeably to increased leakage than do four cylinder engines. Small displacement engines(especially single and twin cylinder engines) will suffer greatly with leakages above 12 or 15%. .
particular orifice that results from whatever flowrate leaks out the cylinder being tested. The air flow passes from the regulator through the orifice on its way to the cylinder, and as long as the leakage flowrate out of the cylinder stabilizes, the pressure drop through the orifice will stabilize, regardless of how long you run the test. If air pressure is supplied to the leakdown tester at 100 psi, and the leakage out the cylinders is a stable flowrate through the tester that causes 16 psi of pressure drop through the orifice, the tester reads either 84 psi or 16% leakdown, depending upon how the dial on the second pressure gauge is marked. As with a compression test, the leak down test is repeated for each cylinder. Again, a new engine may lose 5% to 8% on a leakdown test.
The cylinder leakage test is best performed on an engine at operating temperature. Due to the time involved in removing spark plugs, and the hazards of working around a hot engine this may be impractical. The test is generally reliable when done on a cold engine, but if engine damage is suspected, and a cold test shows no problems, then the test should be repeated with the engine warm to verify results.
1. Remove all of the spark plugs and rotate the crankshaft to place the piston in the cylinder being tested at Top Dead Center of the compression stroke.
2. Insert the adaptor hose into the spark plug hole of the test cylinder. Connect this hose to the leakdown tester.
IMPORTANT! To Avoid a sudden engine rotation or
damage to your leakdown tester, make certain the leakdown tester in NOT connected to a compressed air source at this time.
3. On the leakdown tester,unlock & back off (counterclockwise) the regulator knob a few turns & then connect your shop compressed the same procedures. Record the cylinder pressures for
all cylinders and the proceed to interpret your results.
(See chart below.)
Analyzing Test Results
The leakage tester utilizes two gauges so that the cylinders' rate of leakage can be seen as the difference between the test pressure on the primary gauge and the leakage pressure on the secondary gauge. The condition of the cylinder is therefore judged by the comparison of these two pressures, expressed in "percentage of leakage."
The chart below shows the percentage of leakage at three different test pressures. The top row is the percent of leakage based on the pressure observed on the secondary gauge. If you used 100 p.s.i. for the test pressure (primary gauge reading), read the numbers in the "100 p.s.i." row. If you used 90 p.s.i., read the second row, and if you used 80 p.s.i., read the third row. The percent of leakage is shown in the top row above the column containing your observed test pressure. that the test will not only determine how badly a cylinder leaks, but also will help to pinpoint the causes of excessive cylinder leakage. Simply listen and look around the engine to find where the compressed air is escaping. The two primary causes of cylinder leakage are past piston rings and valves. Leakage past the rings causes air to blow into the crankcase. Usually this can be verified by listening at the oil filler cap. Likewise, intake valve leakage can be heard at the carburetor or throttle body, and exhaust leakage can be heard at the tailpipe. Sealing the exiting air by placing your hand over its source often verifies the severity of the leak by causing a pressure rise on the secondary gauge. This technique can help to isolate the worst components that may cause cylinder leakage are cracked castings and blown cylinder head gaskets. Check for air escaping into the coolant and/or excessive
leakage in two adjacent cylinders to verify this diagnosis.
% of Leakage 6% 7% 8% 9% 10% 11% 12% 13% 14% 15%
100 p.s.i.....94... 93...92...91...90... 89....88....87... 86....85
90 p.s.i...86.5.. 85.. 84...83...82....81....80... 79... 78....77
80p.s.i......75....74...74...73... 72....71....70...70....69....68
Every engine
will exhibit some degree of cylinder leakage. Even in the best of engines you may observe up to about 1 or 2% leakage. As a rule, a good cylinder will exhibit no more than about 5-8% leakage. When an engines' cylinders begin to exhibit leakdown percentages above or about 12 or 15%, the performance and economy of the engine are usually being noticeably compromised. Leakages above 15 to 20% indicate serious enough wear that repair should be made. Note that long stroke or high rpm
engines may tolerate a slightly higher leakage, due to their greater piston speeds. Six and eight cylinder engines respond less noticeably to increased leakage than do four cylinder engines. Small displacement engines(especially single and twin cylinder engines) will suffer greatly with leakages above 12 or 15%. .
09-05-2002 | 11:19 PM
#7
Guest
Posts: n/a
lol @ checkmate454mag... and thanks Bigwavz, I enjoy helping when and if I can, but I admit to not having all the answers, and need to learn like everyone sharing these interests. Im sure when I am wrong about something, someone will Slap me, or at least pull the slack outa my chain...lol You guys have some interesting stuff going on and seem like a great bunch of guys... but thats how hotrod boaters are. It's also why the world goes round, and all that other stuff right?
