FAA Powerplant Written Practice Test

What is the best immediate action if a differential compression check reveals a cylinder with air escaping from the exhaust?

1. Adjust the carburetor settings.

2. Replace the piston rings.

3. Inspect for valve leakage.

4. Conduct a routine maintenance check.

The correct answer is: Inspect for valve leakage.

When a differential compression check indicates that air is escaping from the exhaust valve area, the most appropriate immediate action is to inspect for valve leakage. This situation suggests that there may be an issue with the exhaust valve's ability to seal properly, potentially due to wear, carbon buildup, or damage. By inspecting for valve leakage, you can verify whether the exhaust valve is seating correctly during the compression stroke or if it is allowing air to escape. This can be done using techniques such as a visual inspection, using a borescope, or performing a leak-down test, which will help identify the exact nature of the problem. Addressing any valve-related issues early on can prevent further damage to the engine or reduce performance. Other options, such as adjusting carburetor settings or replacing piston rings, do not directly address the symptom of air escaping from the exhaust and may lead to unnecessary work or expense. Conducting a routine maintenance check, while prudent in general, does not specifically target the immediate issue at hand as promptly as inspecting for valve leakage does. Thus, focusing on the valves allows for a more precise diagnosis and resolution of the escape of air.