Leak Down Test

By Daniel Dillon, April 15, 2008

The purpose of a Leak Down Test is to evaluate the sealing ability of the Pistons, Piston Rings, Valves, Cylinder Walls and Head Gasket. It is commonly used as a Diagnostic Test to determine the cause of a weak Compression Test reading in 1 or more Cylinders. The Leak Down Test is performed by first positioning the Piston for the Cylinder to be tested in the top dead center position of the Compression Stroke. Compressed air is then pumped into the Cylinder through the Spark Plug hole by a special tool that contains a gauge that measures the percentage of air that escapes from the Cylinder. If 20% or more of the air 'leaks' from the Cylinder, then that Cylinder is in questionable condition.


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