Explain the procedure for conducting a leak test on fuel lines.

Conducting a leak test on fuel lines primarily involves applying pressure to the system and checking for signs of leaks. This method is essential because it allows for the detection of even minor leaks that may not be visible through other means. By pressurizing the lines, any compromised areas can be identified through the escape of fuel or air, which is evident in the form of bubbling, wet spots, or changes in pressure readings.

The other methods mentioned do not effectively ensure that the fuel lines are sealed properly. Submerging the lines in water might reveal larger leaks but is impractical for on-site inspection of fuel systems. Heating the lines to expand them does not apply to leak detection, as it may not accurately represent the conditions under which the fuel lines operate. Observing fuel gauge fluctuations could indicate a problem but is not a direct method for verifying the integrity of the lines. Thus, applying pressure and checking for visible signs are critical for conducting a thorough leak test on fuel lines.