Compression Testing

By Ralph Kalal

A compression test is one of the best ways to check the internal mechanical functioning of an engine. Since an engine’s efficiency is a function of how well it pumps air, good compression in all cylinders is essential. A compression test can not only determine if there is a problem, but help isolate it. Considering the difficulty of removing spark plugs in some modern cars, if you’re going to the trouble of replacing the plugs, you might want to run a compression test, too.

Professional mechanics typically use an electronic engine analyzer to perform this test by measuring cranking speed to estimate compression. But the old-fashioned way to do it—with a compression gauge—is still quite satisfactory. An engine analyzer isn’t a better way, just a quicker one.

A good compression gauge can be had for $40 or less. One end screws into the spark plug hole in the cylinder head. The other end is the gauge. An assistant cranks the engine as you watch the gauge. You repeat this for each cylinder.

To perform the test, first make sure the battery is fully charged. If it’s not, the cranking speed will be reduced, which will lower the compression readings. Then disable the ignition so that no current can flow to the spark plug wires. In a modern car, this is done by disconnecting the electrical connectors to the coil pack. In an older car with only a single coil, disconnect it.

Remove all of the spark plugs as described in the text (except that you’re removing them all, not onebyone). If there is any risk that it could be unclear which plug wire goes to which cylinder, use masking tape to label them with their cylinder numbers.

Open the throttle fully and hold it open by using a length of wire or bent coat hanger to hold it open. If the throttle isn’t open, the engine will be slowed on the intake stroke.

If the car has fuel injection, pull the fuse that controls the fuel pump, so that fuel is not being injected into the cylinders. This avoids spraying fuel into the cylinders, which will dilute the oil on the cylinder walls, and diminish compression.

Install the compression gauge finger-tight and have an assistant crank the engine while you observe the gauge reading. Crank until the gauge stops increasing its reading,but observe the reading at each compression stroke for the cylinder, too. Repeat the process for each cylinder.

The factory shop manual will give you the compression specifications for the engine. Compression below those specifications indicates engine wear. That a high mileage engine should have lower compression than a new one, however, isn’t a surprise and it isn’t necessarily cause for concern. A variation of 10 or 15 psi between cylinders is normal. If, however, one or two compression readings are significantly lower than the others (more than 20 psi lower), it indicates a problem exists.

If one cylinder reads significantly below others, add about a tablespoon of SAE 30 motor oil to the cylinder and repeat the test, which is called a “wet” compression test, to see if compression improves. Here’s what it means:

One cylinder low usually indicates either bad piston rings or leaking valves. If adding oil caused compression to increase, the piston rings are at fault. If adding oil did not increase compression, the valves are leaking.

One cylinder low can also indicate that the head gasket is ruptured—a “blown head gasket”—affecting only that cylinder. If the vehicle has been losing coolant, that diagnosis is likely confirmed, but a cooling system pressure test will provide a sure answer.

Two adjacent cylinders with a low reading suggests a blown head gasket between those two cylinders.

Because head gaskets also seal off coolant passages, you should check for any indication that coolant is getting into the engine oil whenever you suspect a head gasket may have blown. If you find it, it confirms the diagnosis. Coolant mixed with oil takes on a brown, muddy color.

A cylinder that ultimately achieves a reading within tolerances of the other cylinders, but which takes an unusually large number of engine strokes to do so, is likely to have worn piston rings. A healthy cylinder should reach its maximum compression reading in the first two compression strokes. You can verify this by performing a wet compression test on the cylinder.

Compression that is above specifications is generally considered an indication of carbon build-up in the cylinder, cylinder heads, and piston top. It can also indicate that fluid, either coolant or oil, is leaking into the cylinder. In that case, however, you should see smoke from the exhaust when the engine is running.

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