Which of the following is not a cause of fuel vapor lock?

• A. Low fuel pressure.
• B. High fuel temperature.
• C. Excessive fuel turbulence.
• D. Proper air-fuel ratio.

Answer: (D)

Vapor lock happens when fuel in the lines or near the intake boils and forms vapor pockets, interrupting the steady flow of liquid fuel to the engine. Low fuel pressure can’t keep the fuel in liquid form under hot conditions, so some of it boils in the lines and creates vapor that blocks flow. High fuel temperature raises the fuel’s tendency to vaporize, increasing vapor formation in the lines or carburetor, which fosters vapor lock. Excessive turbulence in the fuel flow disturbs the liquid column and helps generate vapor pockets that disrupt delivery to the engine. A proper air-fuel ratio, on the other hand, relates to combustion balance inside the engine and does not by itself cause vapor lock; maintaining correct mixture avoids unrelated combustion issues but won’t create vapor in the fuel supply.