Properly Checking and Testing Engine Compression

A compression check indicates the condition of the top end of your engine, consisting of the pistons, rings, valves and head gaskets. More specifically, a compression check can help you narrow down a loss of compression due to worn-out piston rings, defective valves or a blown head gasket. The engine should be warmed up to the normal operating temperature to check compression. The battery should also be fully charged to perform the tests.

Clean the area around the spark plugs very thoroughly before removing them from the engine. Compressed air is best for this, if available, if not a small brush, or bicycle pump will get the job done as well. Make sure you use eye protection whenever using compressed air. The idea is to get these areas as clean as possible, preventing any dirt from getting down into the cylinders when removing the spark plugs, or performing the compression test.

Remove all the spark plugs from the engine, and set them aside out of your way. Carefully, block the throttle wide open. There are many ways to achieve this, just be careful not to damage anything in the process. You will need to disconnect the ignition system, the easiest way is to unplug the primary wires from the coil or coil packs. Also, disconnect the fuel injection system by unplugging the connector to the injector wiring harness.

Usually, you will start with number one cylinder, so first determine which spark plug hole is in fact the number one cylinder. Install the compression gauge in the number one spark plug hole. Crank the engine over, at least seven compression strokes, watching the compression gauge as you crank the engine over. The engine should build up compression rather quickly. If you notice low compression on the first stroke, and then a gradual increase in pressure upon the strokes following, this would indicate worn-out piston rings. If you notice a low compression reading on the first stroke, and it doesn't build up as you continue to turn the engine over, this indicates that the valves are leaking or you have a blown head gasket, or possibly the cylinder head may be cracked. Carbon build up on the underside of the valve face can also cause a low compression reading. You will want to note the highest reading on the compression gauge.

You will want to repeat this process for each cylinder your engine has. After you complete the compression test, and have recorded your results, compare them to the specifications for your vehicle. This information may be obtained at the car dealership, some auto parts stores, or you can look this information up at the library.

If the compression readings are below normal add some engine oil to the cylinders from a plunger type oil can. Usually, about three squirts is sufficient. You will want to add this oil through the spark plug holes, and repeat the compression test. If the compression goes up after adding the oil, the piston rings are definitely worn out. If the compression does not increase, the leak is occurring at the valves or the head gasket. Compression leaks past the valves because of a few different reasons; burnt valve seats, or the faces of the valves are burnt, warped, bent, or cracked valves.

If two cylinders next to one another have the same low compression reading, there is a strong possibility that the head gasket is blown out between them. If you notice coolant in the combustion chambers or the crankcase this will also verify that the head gasket is blown. You may also want to pull the dipstick to see if you can detect any coolant with the oil.

If low compression is isolated to just one cylinder, accompanied by a rough idle, a worn exhaust lobe on the camshaft may be the cause.

If the compression is unusually high, the combustion chambers are most likely coated with carbon. In this situation the cylinder heads will need to be removed so that you can clean the carbon out. Always replace the head gaskets when you remove a cylinder head. If the compression is very low, or you have a great variance from one cylinder to another, you may want to take it into a garage where they can perform a leak down test. The leak down test will help pinpoint the problem and also determine the severity of it.