By Mark Sabourin
A compression test should be done at every engine tune-up or whenever the spark plugs are changed. The test is used to find ring sealing, valve sealing, head gasket and cam timing problems. It is probably the most important way for a person, whether the owner or mechanic, to ascertain the condition of the cylinders and valves.
A good quality gauge is essential and the adapter pipe or hose chosen will make a big difference in how easy the test can be performed.
There are two ways to do a compression test. The most common is the "dry" test and if it shows a potential problem then the "wet" test will help to pin-point what the problem is.
Some preliminary work should be done before the test:
Now you can remove all of the wires keeping them in order and all of the spark plugs setting them in order. Inspect the plugs for anything unusual such as heavy carbon build-up, oil or gas fouling or anything else. Record any unusual findings by the cylinder number.
Screw in the gauge hose or press the rubber tip of the steel adaptor against each spark plug hole while an assistant cranks the engine over at least 4 or 5 times, observing the gauge. It shouldn't take more than five turns to reach the maximum pressure.
Look at the gauge as it rises . It should rise quickly and reach maximum by 5 turns. If it rises slowly or takes longer than 5 crank turns there could be a problem. If it leaks down, check the Schrader valve at the end of the tip or at the pressure relief button on a hose.
Pay particular attention to any cylinder that has a questionable plug.
Record each cylinder's measurement and any unusual gauge fluctuations.
If you have any cylinder that is down to 100psi or greater than 20% less than the highest cylinder, then perform a "wet" test by squirting about an ounce of engine oil into those cylinders and check the readings again. If it comes up, you may have a ring problem, if not then it's probably the valves not seating correctly. Although most people don't have the equipment, a cylinder leakage test can be performed at a repair shop to further pinpoint the problem.
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