A compression test is a common tool used to diagnose gasoline engine problems. It measures the amount of air an engine’s pistons are compressing and can identify leaks or mechanical problems. Manual and electronic tests can be performed, and a 10% margin is generally considered acceptable. Decreased compression can be caused by various factors, including worn piston rings or cylinder walls, and is best left to professionals to repair.
When diagnosing gasoline engine problems, a common tool used to identify potential sources of problems is a compression test. Strong compression is essential for an engine to run properly and efficiently. A noticeable loss of power or efficiency may be the cause of testing an engine’s compression. Leaks or mechanical problems can cause compression to drop in one or all of an engine’s pistons.
A compression test measures the amount of air an engine’s pistons are compressing, which can be compared to each other and to manufacturer standards to determine if there is a problem in this area. While lack of compression is more common, it is also possible that the compression is too strong. Over-compression can cause a problem known as knocking, which can be very damaging to internal engine parts.
Compression tests can be performed in one of two ways. There’s a manual process that requires just a compression gauge, a few hand tools, and a general knowledge of engine design. An electronic test can also be performed, which uses a special engine analyzer, but is generally reserved for professional mechanics only. Both of these tests can provide clues as to why an engine may be underperforming.
A manual compression test essentially consists of disconnecting the engine from the ignition module, turning it into a giant air pump. For each engine cylinder, the engine is held at wide open throttle for a few seconds, with the cylinder spark plug removed and the compression gauge inserted. The readings from each piston can be compared to each other and to the manufacturer’s specifications to determine if a compression problem exists and if it is isolated to one or all pistons. When comparing the compressive strength of each cylinder to another, a 10% margin is generally considered acceptable.
The electronic compression test involves a computer that estimates the compression force based on the speed at which the engine rotates. The computer can determine which cylinders are working harder, indicating higher compressive strength. This type of compression test, while just as accurate as the manual version, can be performed without removing the spark plugs or disconnecting other parts, making it faster.
The possible reasons for decreased compression vary greatly in complexity and severity. If compression is low on all cylinders, it could be that too much fuel is being drawn into the engine during combustion, which is removing oil from the cylinder walls and negating the integrity of the seals they can make. Worn piston rings or cylinder walls, or seized or damaged valves, pistons, and other parts, are possible causes of poor compression, making it a problem generally best left to the professionals to repair.
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