What method checks for internal leakage of a fuel valve without removal?

Monitoring flow to the strainer bowl with the valve in the OFF position is an effective method for checking for internal leakage of a fuel valve without the need to remove it. When a fuel valve is functioning correctly and is in the OFF position, there should be no fuel flowing downstream to the strainer bowl. If there is any flow detected while the valve is supposedly closed, this indicates that the valve is leaking internally. This method provides a practical approach to assess the valve's integrity in situ, ensuring that the system can maintain its required fuel control without physical disassembly.

The other methods, while potentially useful in specific contexts, do not directly confirm internal leakage in the same straightforward manner. Pressure testing the valve assesses its ability to hold pressure but does not specifically indicate flow. Using a vacuum gauge may help identify other issues but doesn't specifically measure the leakage through the fuel valve. Listening for hissing sounds may suggest a leak but can be unreliable and does not provide definitive evidence of internal leakage. Thus, checking the flow to the strainer bowl while the valve is off is the most direct and effective method for assessing internal leakage.