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leak down test?

Old 10-07-2010, 11:26 AM
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what is a leak down test? sorry for my ignorance but looking at getting a boat this weekend so i want to be sure to get as much info i can when down there. saw it on someone post so i thought i asked. thanks

Last edited by hwong601; 10-07-2010 at 11:41 AM.
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Old 10-07-2010, 11:58 AM
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With reference to the lower unit?

Remove upper plug, plumb in an air hose with a gauge, and pressurize the LU to about 10psi. Leave for about 45 minutes to an hour and see if the pressure holds. If it doesn't, it implies that the seals have gone (or the air hose leaks...)

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Old 10-07-2010, 12:01 PM
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Or it could apply to the engine. On the engine it is done both dry and wet.
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Old 10-07-2010, 01:42 PM
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i can only assume this is on a powerhead, as that's likely how it was phrased in the post you saw it in. Basically, lets say you have a 4-cylinder motor. You need to leak-down test all 4 cylinders. start by warming it up. get the motor hot, then you can perform the test. Start with cylinder 1. Take all plugs out. Turn the motor manually until you get #1 cylinder to top dead center. Replace all plugs, except cylinder #1. Then you want to get a leak-down gauge, hook it into the spark plug hole, and put in xxx-psi(not sure how much). Then yourotate the motor maybe 2-3degrees in each direction of top dead center, then wait xxx time(not sure exactly how much) and you figure out how much of your original air pressure you retained. Lets say you put 100 psi in cylinder 1, and after the allotted time, you have 90 PSI remaining. You have a leak-down of 10%. Anywhere up to about 25% is normal, and you want all cylinders within 10% of one another. Then you repeat until all 4 cylinders are done.

It is similar to a compression test, but is a more accurate test to show quality of the cylinder walls and rings on the piston.
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Old 10-07-2010, 03:41 PM
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Originally Posted by Chiefsurfer View Post
i can only assume this is on a powerhead, as that's likely how it was phrased in the post you saw it in. Basically, lets say you have a 4-cylinder motor. You need to leak-down test all 4 cylinders. start by warming it up. get the motor hot, then you can perform the test. Start with cylinder 1. Take all plugs out. Turn the motor manually until you get #1 cylinder to top dead center. Replace all plugs, except cylinder #1. Then you want to get a leak-down gauge, hook it into the spark plug hole, and put in xxx-psi(not sure how much). Then yourotate the motor maybe 2-3degrees in each direction of top dead center, then wait xxx time(not sure exactly how much) and you figure out how much of your original air pressure you retained. Lets say you put 100 psi in cylinder 1, and after the allotted time, you have 90 PSI remaining. You have a leak-down of 10%. Anywhere up to about 25% is normal, and you want all cylinders within 10% of one another. Then you repeat until all 4 cylinders are done.

It is similar to a compression test, but is a more accurate test to show quality of the cylinder walls and rings on the piston.
When you think about it a lot it seems it proves nothing. The cylinder only has to hold pressure for less than a second. I understand Piston rings rotate around cylinder. Depending on ring position you could lose pressure in a second. Also the fuel in the chamber is part of the sealing and cooling for cylinder/piston. No fuel less of a seal.

Whatcha thiink?
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Old 10-07-2010, 03:59 PM
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^I think it is more than 1 second. I am not sure, but I thought it was like 30 seconds.

Yes, if the rings happened to line up when doing the test, that would be bad. The cylinders should be honed, and this should help them rotate opposite eachother, and theoretically should only line up like the same amount we have solar eclipses. If you really want to be sure, do the test, run it for 1 minute, test again, run it for another minute, test it a 3rd time. If all 3 tests come up the same, or very similar, it should be a good representative.

What it SHOULD show is if there is excessive scoring in the cylinder wall, allowing more air through to the crank-case. Any air(which will have fuel in it in a running motor) that gets past the rings, cannot burn, hence leading to decreasing performance.

A perfect example is somebody who was looking for a cat boat to purchase. It had 2 Honda motors. 1 motor had numbers like I would expect, ranging i think from 16% to 27%(within acceptance) and the other had numbers from 23% to 58%. As a test of performance in the real world, he ran the boat WOT on just the port motor, and lets say got 4200 rpm and 20 mph. Then ran it on JUST the starboard motor at WOT and only got 3500 rpm and 13mph. That shows a MUCH weaker engine, which WAS evidenced by the leak-down test. That would signify a tear-down and re-build in a short period of time.
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Old 10-07-2010, 04:24 PM
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In aviation it is called a differential pressure test which I think is a better name. But then I am biased. You are measuring the difference in pressure between two gauges. One is the inlet pressure gauge and the other is a gauge that measures pressure within the cylinder. There is a calibrated orifice (a tiny hole) between the two gauges.

Piston is moved to top dead center on the compression stroke. In a four stroke motor the valves will be closed.

80 psi is applied to the inlet pressure gauge. The other gauge indicates how tightly the cylinder is sealing. If it a perfect seal, the other gauge will also read 80 psi. There is no time limit involved.

But little is perfect. Some air will leak across the piston rings, the valves and their seats or maybe a gasket has a leak in it. Say the one gauge indicates 80 psi but the other reads 70 psi. There is a 10 psi air leak (or differential pressure) in the cylinder that manifests itself in the form of a difference in pressure between one gauge and the other.

The benefit of this test is that if there is a lot of air leakage, you can listen to the intake manifold, the exhaust manifold, the crankcase, or the water jacket to determine what is causing the leak.

Don't be too hung up on values displayed by the gauges. Of the two major airplane piston engine makers, one allows for the cylinder pressure to be as low as 40 psi. The other uses 60 psi as the bottom limit. Even with this much pressure loss, if the cylinder is not scored, the motor is not burning oil, and the motor is making power, then life is good.
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Old 10-07-2010, 04:51 PM
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On a four stroke motor it tells you alot when there is excessive leakage. With air aplied to cylinder via leakdown tester and having excessive leakage you simply listen for hissing. Hiss at exhaust, exhaust valve problem. Hiss at throttle, intake valve problem. Hiss in crankcase, ring problem or cylinder problem. It is basically a more thorough step to pinpoint cause of low compresion. Not sure what someone meant by one second. This test can be done for as long as necesary to find problem.
On a two stoke motor it is of limited value because there are no valves in the head.
At top dead center you are only testing piston, cylinder wall and ring condition. This is basically done by compression test.
So what is this motor anyway?

Last edited by bustedflat; 10-07-2010 at 04:54 PM. Reason: grammar
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Old 10-07-2010, 05:50 PM
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http://www.lovehorsepower.com/MR2_Do...wn_testing.htm
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Old 10-07-2010, 06:28 PM
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I have a leakdown gauge and used to use it all the time...set on TDC for that cylinder, apply 100psi and read leakdown on other gauge...it happen instantly....10% is a tight cylinder...anything more than 25% is getting sketchy....listen at the tailpipe to hear leakage past exhaust valve, open carb throat to hear leakage past intake valve and pull oil fill for leakage past rings....rarely done these days...
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Old 10-08-2010, 03:51 AM
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Get a TDC gauge. It makes finding TDC easy. Make sure the threads are correct for your motor.

http://www.restockit.com/6-25-standa...arisonshopping

You'll also need a leakdown tester and air compressor.

http://www.summitracing.com/parts/SUM-900010/

Any more than 20psi loss and you have a week cylinder.
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