What does a hissing sound from the exhaust stacks indicate when the propeller is being pulled through manually?

A hissing sound from the exhaust stacks during manual propeller rotation suggests that exhaust gases are escaping past the exhaust valves, indicating that the exhaust valves are not sealing correctly. This phenomenon, known as exhaust valve blow-by, occurs when the combustion chamber is not sealed effectively, allowing gases to leak out rather than being directed through the exhaust system.

This auditory cue can be important for diagnosing potential issues within the engine. Proper sealing of the exhaust valves is crucial for maintaining efficient engine performance, as it ensures that exhaust gases are expelled correctly and not re-entering the combustion chamber, which can lead to decreased efficiency and increased emissions. Recognizing such sounds can lead to timely repairs, helping to maintain optimal engine function and longevity.