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What is a Compression Test?

By Mike Howells
Updated May 23, 2024
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In diagnosing gasoline engine issues, a common tool used to identify possible problem sources 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 cause to test an engine's compression. Leaks or mechanical problems can cause compression to be diminished in one, or all, of an engine's pistons.

A compression test measures how much air an engine's pistons are compressing, which can be compared against one another, and against manufacturer standards to determine if there is a problem in this area. While a lack of compression is most common, it is also possible for compression to be too strong. Over-compression can result in a problem known as detonation, which can be very damaging to an engine's internal parts.

Compression tests can be performed in one of two ways. There is a manual process that requires only a compression gauge, some hand tools, and general knowledge of an engine layout. An electronic test, that utilizes a special engine analyzer, can also be performed, but is typically reserved only for professional mechanics. Both tests can provide clues as to why an engine may be under-performing.

A manual compression test involves essentially disconnecting the engine itself from the ignition module, making it a giant air pump. For each engine cylinder, the engine is held at wide open throttle for a few seconds, with the cylinder's spark plug removed and the compression gauge inserted. The readings for each piston can then be compared against each other, and against manufacturer specifications, to determine both whether there is a compression problem — and if it is isolated to one piston, or present in all of them. When comparing each cylinder's compression strength against another, a margin of 10% is generally considered acceptable.

The electronic compression test involves a computer that estimates compression strength based on the speed at which the engine cranks. The computer can tell which cylinders are working harder, which indicates greater compression strength. This type of compression test, while just as accurate as the manual version, can be conducted without removing spark plugs or disconnecting other parts, making it faster to conduct.

Possible reasons for decreased compression vary greatly in complexity and severity. If compression is down across all cylinders, it could be that there is too much fuel being introduced into the engine during combustion, which is washing the oil off the cylinder walls and negating the integrity of the seals they can make. Worn piston rings or cylinder walls — or stuck or damaged valves, pistons and other parts — are are all possible causes for poor compression, making it an issue generally best left to professionals to repair.

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