I purchaced a compression tester for the sole purpose of validating a 6 cyl Mercury Outboard. The guage reads from 50 - 450 so with some modifications (plumbing) I will use it on the MB's. My Mercury Manual states it's as important to do a leakdown/leakoff test after the initial pressure is achieved - and it should not be less than 20% of specifications. My question, what period of time is adequate to measure leakdown. 5 minutes, 1 hour, a month

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How is the Mercury leak specification written?

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Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)

I don't have the manual in front of me but I can dynamic test placing the piston at TDC on the compression stroke or I can pressurize the combustion chamber with my compressor. Both talk about percentage of leakdown and listinging for escaping air through throttle bodies, exhaust etc and that all understanable. BUT... if I am holding 160# in a cylinder for ( ? ) how many minutes is an acceptable range to measure the percent of leakdown. It's my opinion that a brand new engine will have leakoff after a certain amount of time. Is there a standard?

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An actual leakdown tester will have a flow regulator in it, and the leakdown is the pressure ratio at that flow rate. That's how the specification is formed, not by holding pressure for a certain amount of time, so you may have trouble interpreting a simple pressure drop.

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http://i16.photobucket.com/albums/b1...oD/bioclip.jpg

I starting to undersand. This is not a sophisticated device, just picked it up while browsing the marine salvage yard The example they gave with the documentation was something like 100# with a leak off to 90 # shows a 10% leak rate an is acceptable for (xyz) engine.

Edited:
Please note this gauge was designed for dynamic testing - and the instructions offer suggestions of measuing leak off

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