What is a common method for diagnosing engine performance issues?

Conducting a compression test is a fundamental method for diagnosing engine performance issues because it helps assess the internal health of the engine. This test measures the pressure generated in each cylinder during the compression stroke. By analyzing the compression readings, a technician can identify problems such as worn piston rings, leaking valves, or head gasket failures. If any cylinder shows significantly lower compression than the others, it indicates a potential issue affecting engine performance and efficiency.

In contrast, inspecting the color of the vehicle doesn't offer any relevant information about the engine's mechanical condition. Changing the oil is essential for maintenance but does not directly diagnose performance problems; rather, it could be a remedy for issues related to lubrication. Cleaning the air filter, while helpful for maintaining airflow and efficiency, does not diagnose deeper internal issues that could be affecting engine performance.