Diagnostic testing procedures in the automotive industry have evolved over the years. One milestone in that industry was the addition of electrical systems and electric starters. With these changes, the industry abandoned the cylinder leak down test and opted to replace this test with the compression test. The compression test is able to identify compression loss caused by other top end problems that are not detected with the leak down test. Examples of what the leak down test can detect would include:
WORN CAMSHAFT LOBES
SCORED CYLINDER BARRELS
BROKEN RINGS
BROKEN VALVE SPRINGS
LOOSE VALVE SEATS
IMPROPER VALVE LASH
LOOSE VALVE GUIDES
BENT CONNECTING RODS
CONNECTING RODS PROBLEMS (ones that are likely to be missed with the leakdown test ,but are often evident as a compression loss with this test)
Another benefit of the leak down test is that it does not need a source of compressed air.
The test is conducted in the following manner:
Warm the engine to insure the metal parts are expanded to their normal running tolerances
Remove the sparkplugs as needed, usually the top ones that are the easiest to reach
Because of the small capacity of an airplane battery, a charger should be connected to the battery to maintain constant voltage for the test.
Connect the adapter to the first cylinder to be tested.
Crank the engine with the starter.
Count the compression “bumps” while watching the gauge, usually four to six bumps is sufficient to recognize that the compression will go no higher.
Record the readings.
The maximum readings will vary from engine to engine depending on the compression ratio of the engine being tested. As was the case in the leak down test, an experienced person will be able to identify causes of compression variations and insure accuracy.