Which tool is essential for measuring alcohol content in engine fuel?

The essential tool for measuring alcohol content in engine fuel is an alcohol tester. This device is specifically designed to determine the percentage of alcohol present in a fuel mixture, which is crucial for ensuring that the fuel complies with engine specifications and performance requirements. Alcohol content in fuel, especially when used in small engines, can affect combustion, engine performance, and emissions.

Understanding the alcohol content helps technicians adjust the fuel mixture appropriately for different engine types and operating conditions, ensuring optimal performance and longevity of the engine. Other tools mentioned, such as a compression gauge, crankcase vacuum gauge, and ring compressor, serve different functions related to engine diagnostics and maintenance, but they do not provide a means to measure the alcohol percentage in fuel.