Cylinder leakage test
A cylinder leakage test should be performed if a compression test reveals low readings on any individual cylinders. A leak down tests can be performed to measure the percentage of compression lost and also helps to locate the exact source of this leakage. Using the specialized tools available gives you the ability to pinpoint the problem area before you start tearing down the engine. It can also help you determine if the repairs are worth wile on older high mileage engines.
A threaded adapter on the end of the air pressure hose screws directly into the spark plug hole. The source of the compressed air is normally the shops air system but small automotive compressors work well also (see below). A pressure regulator in the tool controls the amount of pressure applied. A gauge registers the percentage of air lost from the cylinder when the compressed air is injected. The scale on the gauge is typically in percentage form.
Cylinder leakage test readings
A 0% reading means that there is no leakage in the cylinder at all. A reading of 100% indicates that the cylinder will not hold any pressure whatsoever. Any reading that is more than 0% indicates some leakage. The location of the compression leak can be found by listening and feeling around various parts of the engine for air leaks. If air is felt or heard leaving the throttle plate assembly a leaking intake valve is indicated at that point. If a bad exhaust valve is responsible for the cylinder leakage air can be felt and heard leaving the exhaust system during the test. If air is heard escaping or leaving from the cylinder head area than an inspection of the head itself must be performed and may indicate a cracked cylinder head.
Head gasket problems cause cylinder leakage
On the subject of head gaskets when you apply compressed air to a cylinder and you hear air leaving the radiator this is a good indication of a faulty head gasket or even a cracked block. You may also notice bubbles in the radiator overflow tank. Compression should be completely sealed from the cooling system and no air should enter any parts of the engine cooling system. Another popular test for leaking head gaskets is the stuff pictured on the right. It changes color if a problem is indicated. If the cylinder leakage test reveals a problem another possible cause could be worn piston rings. In this case air can be heard, rushing down past the Pistons and flowing into the crankcase and in extreme cases, you can feel air leaving the valve cover breather or vent tube.
Internal engine diagnosis using compressed air is accurate and easy. The repairs needed will be the hard part. You will need a automotive compressor. But you do not need a big one. The volume of air required is small because you are only filling the combustion chamber. I own the automotive compressor pictured on the left. It can fill tires and many other things as well. My artistic wife uses it for air brushing as an example. Cylinder testing tipsNote that most engines even new ones will experience some cylinder leakage around the piston rings. And up to 20% is considered acceptable during your cylinder leakage test. This is because when the engine is actually running the rings will seal much better due to the up-and-down movement of the piston and the presence of oil lubricating that cylinder.
Also note that there should be absolutely no leakage into the cooling system and also the cylinder head. The intake and exhaust valves should also seal at 100% with no leakage into the intake or exhaust systems. In order of importance I perform a complete compression test first and then I performed a cylinder leak down tests as described above. These two tests in combination can often point exactly to the engine repair area needing attention.
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