What does a compression test measure in an engine?

A compression test is a diagnostic procedure that specifically measures the pressure created within the engine cylinders during the compression stroke. During this stroke, the piston moves upward, compressing the air-fuel mixture in the cylinder. The test provides valuable information about the health of the engine, as proper compression levels indicate good sealing between the piston rings and cylinder walls, as well as functioning valves. Low compression can signal issues such as worn or damaged rings, leaking valves, or head gasket failures, which can negatively impact engine performance.

The other options do not accurately describe what a compression test assesses. While temperature, oil pressure, and fuel efficiency are essential aspects of engine performance and health, they are not the focus of a compression test. Instead, a compression test zeroes in on the strength of the combustion process by measuring how much pressure builds up in each cylinder, making it a vital tool in diagnosing engine problems.