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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,274
Originally Posted by mooney265 View Post
I've done compression tests before on anything from lawnmowers, mercury outboards to nissan 2.0 liters... So, I've got the compression test thing figured: Disconnect coil and fuel pumps, pull spark plug, insert compression gauge, turn engine over, read gauge...

The one thing that I can't seem to wrap my head around, however, is how do I pressurize each cylinder for a leakdown? And, how do I align each cylinder for the test?

When folks use numbers like 3% leakdown... What does that mean:

3% leakdown after 1-minute? or,
3% leakdown after 5-minutes? or, 5-seconds???

During a hot or cold compression test you should also hold the throttle wide open while cranking the starter so there is no intake air resistance.

A leakdown test tells a lot more about the motors condition. You use an air compressor with the leakdown tester. There is a regulator on the leakdown tester air inlet and you adjust that to 100psi. The piston is TDC on the compression stroke in it's cylinder or it will be forced down by the air pressure.

There is an air hose from the leakdown tester outlet screwed into the spark plug hole of that cylinder and in the body of the leakdown tester there is a tiny orifice in between the pressure gauge and the leakdown gauge to the right of it that the air has to flow through if any is leaking out of the combustion chamber.

Because of the very small orifice, the right side leakdown gauge which is a 100psi gauge shows how much air is leaking out of the motors combustion chamber if it is leaking out faster than the air can flow through the orifice and replace it.

Being a 100psi gauge on the cylinder side of the orifice, if enough air is leaking out of the combustion chamber to make that gauge read say 93 psi because air can't flow through the small orifice in the leakdown tester body fast enough to replace the air leaking out of the cylinder than thats considered 7% leakdown on that cylinder.

Then you listen for hissing air leak with a hose in your ear and the other end in the throttle body, exhaust header or tailpipe, and oil tank or engine case drain plug to figure out if the leak is piston rings, intake, or exhaust valve.

Compared to that a simple compression test tells little.
Old 06-19-2010, 09:06 AM
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