Which design principle helps prevent vapor lock in fuel lines?

• A. Gentle slopes and avoidance of sharp curves
• B. Relying on higher fuel temperature
• C. Using smaller diameter tubes
• D. Increasing line length

Answer: (B)

Vapor lock happens when fuel in the lines boils and forms gas pockets, interrupting the flow of liquid fuel to the engine. The way to prevent this is to keep the fuel from vaporizing and maintain a steady, continuous liquid column moving toward the engine. Gentle slopes and avoiding sharp curves help accomplish that by reducing turbulence and the chance that air or vapor pockets will form while the fuel is being delivered, especially when the system is warm. Higher fuel temperature would increase vapor pressure, making vapor lock more likely, not less. Smaller diameter tubes, longer line runs, or relying on heat to push fuel along typically introduce more heat transfer or pressure losses that can promote vapor formation rather than prevent it. Keeping heat input to the lines low and ensuring a smooth, straightforward path helps maintain reliable liquid fuel delivery.