Compression and Leak Down Test

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Compression and Leak Down Test

Postby Larry Jowdy » Mon Oct 30, 2006 8:47 am

Tools needed, spark plug wrench, old style oil can with a long spout filled with engine oil, compression tester and a remote starter button (not necessary if you have someone to help you but very helpful)
Start by removing the spark plugs

If you have a \"screw in\" compression tester, insert the tester in the #1 spark plug hole. If you have a \"push type\" tester, you'll need someone to help you or a remote starter (available at auto parts stores for under $10.00)

If you have the remote starter, plug one wire to the battery 12 volt positive side and the other to the push spade fitting on the starter.

Crank the engine approximately 5 revolutions or 5 to 7 seconds and record the results, then go to #2, #3 and #4 cylinders recording your results

Remove the compression tester and squirt about 2 to 3 \"pumps\" of oil into the #1 cylinder, cover the spark plug hole with a rag and crank the engine about 2 to 3 seconds (this coats the cylinder with oil)

Repeat the above procedure squirting oil into each cylinder prior to the test and record your results.

During the dry test, you should have about 140 psi per cylinder. If, during the wet test, cylinders record and much higher result, you probably have bad rings or bad valves.

If your dry readings are low and your wet readings are low, you probably have bad valves.

If your dry readings are low and the wet readings are higher, you probably have bad rings.

If your wet and dry readings are similar, you're OK.

All cylinders should be within 10% of each other.

Cylinder leak down test:

After doing a compression check, if one or more of the cylinders tests low, then a cylinder leak down test will help you determine the cause of the weak cylinder. Special equipment (leak down gauge) is required which can be purchased at most auto parts stores. With all the spark plugs removed, rotate the engine so that the cylinder to be tested is on top dead center and ready to fire. This will assure that the valves are closed. Install the leak down gauge in the spark plug hole and apply air pressure.

The gauge will register the pressure drop indicating the percentage of leak. In addition, by listening at the exhaust pipe or air intake system or the oil breather you may hear escaping air, which means one of the components, is leaking. Example: air out of the exhaust pipe means the exhaust valve is leaking. If you have a water cooled car, remove the radiator cap and you'll be able to tell if there is a blown head gasket and or a cracked head if there are bubbles
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