What's a Leakdown Test?
Le Mans Master
Joined: Dec 2003
Posts: 5,561
Likes: 448
From: Atlanta metro Ga.
In a nutshell..
You pressurize the cylinder with air (TDC - compression stroke, intake and exhaust valves closed), with a set of accurate gauges, one measuring the input side from the compressor, the other measuring the leakage into the cylinder (past the valves / rings, etc). Note that a pressure regulator is required to keep the input to the cylinder / output from the compressor at a consistent PSI.
This percentage of "loss" / leaking between the output of the compressor and the input into the cylinder is your "leak-down percentage".
A really tight motor will show less than 5%, anything beyond 8~10% could be cause for alarm.
What's useful about this type of test over a compression reading (spinning motor with a gauge to measure PSI) is that by listening for the sound of the leaking air, you can determine where the issue is, i.e. intake / exhaust / rings, etc.
You pressurize the cylinder with air (TDC - compression stroke, intake and exhaust valves closed), with a set of accurate gauges, one measuring the input side from the compressor, the other measuring the leakage into the cylinder (past the valves / rings, etc). Note that a pressure regulator is required to keep the input to the cylinder / output from the compressor at a consistent PSI.
This percentage of "loss" / leaking between the output of the compressor and the input into the cylinder is your "leak-down percentage".
A really tight motor will show less than 5%, anything beyond 8~10% could be cause for alarm.
What's useful about this type of test over a compression reading (spinning motor with a gauge to measure PSI) is that by listening for the sound of the leaking air, you can determine where the issue is, i.e. intake / exhaust / rings, etc.
Last edited by Dan_the_C5_Man; Mar 1, 2006 at 12:17 AM.
Le Mans Master
Joined: Nov 2005
Posts: 6,993
Likes: 25
Or bubbles in the radiator (leaky head gasket).
Very useful test but not easy on some engines to get to every spark plug hole and getting the cylinder to TDC is interesting sometimes, especially if you can't get to the crankshaft bolt easily to turn the engine by hand. In the "old" days, taking off the valve cover to watch the rocker arms helped you determine when the valves were closed. Getting the valve cover off ain't as easy as it used to be!
Very useful test but not easy on some engines to get to every spark plug hole and getting the cylinder to TDC is interesting sometimes, especially if you can't get to the crankshaft bolt easily to turn the engine by hand. In the "old" days, taking off the valve cover to watch the rocker arms helped you determine when the valves were closed. Getting the valve cover off ain't as easy as it used to be!
