Slider,
More than you wanted to know about leak down testing:
The definitive test is the “leakdown test,” but this is much more involved than the compression test & does require a source of compressed air. This test is done with the cylinder "exactly" on TDC compression stroke. In my experience, a street engine is sour if the test shows more than 10 percent leakage and race engines are often rebuilt if leakage is more than 3 to 5 percent. The test will tell if the problem is intake valves, exhaust valves, rings, cylinder-to-cylinder head gasket leakage, or leakage into the cooling system from cracks or head gasket. A newly built race engine will often be torn down and corrected if leakage is more than 2-3 percent.
The leakdown test is also called a “differential pressure test” in the aviation industry. Piston-type aircraft engines are required to be tested in this manner and to meet certain minimum specifications.
With the commercial tester (differential pressure tester) the pressure is set with the regulator to show a readable pressure on two gages. The first gage reads the applied regulated pressure, and the second gage reads the pressure on the cylinder. For aviation there is a specific calibrated restrictor orifice for each specific engine.
Another variation is the single gage plus regulator tester which is adequate for all non aviation uses.
Basically, the tester is a flow meter that measures the amount of leakage flow in the cylinder. In addition to the leakage readings, the exact cause of the leakage can be determined by listening for air leaks. To do this use a piece of small rubber vacuum hose, and stick one end near the suspected leak and the other end in your ear.
Adjacent cylinder sparkplug holes: An air hiss heard here may indicate a blown head gasket between cylinders or it may be leakage heard through an open valve.
Exhaust pipe: An air hiss heard here may indicate a burnt or stuck exhaust valve.
Carburettor or throttle body: An air hiss heard here may indicate a bent or stuck intake valve.
Oil filler hole: An air hiss heard here may indicate broken rings, worn rings, or a damaged piston.
Radiator filler cap: Bubbles here will indicate a leaking head gasket or cracked head.
Note that a valve leak is sometimes caused by only a small piece of carbon or dirt on the valve seat. This can be corrected by carefully tapping the top of the valve stem to dislodge the carbon.
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