What does a compression test measure in an engine?

A compression test is a diagnostic procedure used to measure the sealing ability of the combustion chamber in an engine. Specifically, it assesses how well the valves, piston rings, and cylinder head seal against the combustion pressure. This is crucial for engine performance, as proper compression is necessary for efficient combustion and power generation.

If the compression is low, it often indicates that there could be an issue with valve seat sealing, piston rings, or the head gasket. Therefore, the result of the compression test helps in diagnosing problems related to valve operation and overall engine health.

The other options do not accurately represent the focus of a compression test. Fuel efficiency relates to how effectively an engine uses fuel, power output is a function of peak engine performance, and oil quality pertains to lubrication and not directly to engine compression.