You would want to check the vac pump to make sure no vanes have stuck for starters. Doing a leak down test is a fairly simple matter.
How To Do A Leakdown Test Mobil
Acceptable leak down test results. Measuring crankcase blowby at 7000 rpm under load will be much more accurate. Above 20 percent loss and it may be time for a teardown and rebuild. The valves rings and head gasket seal the combustion chamber. No engine will have perfect sealing with 0 percent loss. If you have lost ring seal then you will make less power go slower. In simplified terms if air is pumped into the cylinder at 100 psi and the gauge reads 97 psi then the leak down percentage is 3.
Perform a cylinder leakage test by injecting regulated shop air into the cylinder heads spark plug hole then visually searching and listening for a leak. It deals with how well the cylinder is sealing and nothing else. So the great thing about the cylinder leak down test is that. The tester is connected to an air source and is attached to the engine through the spark plug hole. Engine compression atmospheric pressure at sea level is approximately 147 psi. Leak down needs to be taken lightly.
Even for normally aspirated engines respectable leakage numbers would be anywhere from 8 to 12 percent with a variation between cylinders of 4 to 5 percent but its possible the variation could. The cylinder leak down test reading is expressed in a percentage. An engine between 10 and 20 percent can still run ok but itll be time to keep an eye or ear on things. A 91 compression ratio cylinder is compressing the air and fuel mixture to about 132 psi at sea level 9x1471323. To perform the test you must first remove the spark plug on the cylinder youre going to test. One test you could try is to pump a shot or two of engine oil into a cylinder crank the engine over for a dozen rotations or so and try the compression test again.
The procedure is similar to a compression test to a point. I might break out my leak down tester after i find a cylinder with low compression or i might start with a leak down tester when i suspect a mechanical problem with the engine. To get back to your question as long as the cranking compression is relatively equal i wouldnt worry much about the 40 percent leakage. Five to 10 percent loss indicates an engine in great to good running order. The intake valve leads to the intake manifold and the exhaust valve leads to the exhaust tailpipe. Consequently good cylinder leakdown readings should be below 5 8.
Like joe said 15 20 doesnt mean much.
Gallery of Acceptable Leak Down Test Results
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