Which statement about vapor lock is true?

• A. It always improves performance
• B. It can completely block fuel flow
• C. It only affects diesel fuel
• D. It has no impact on engine operation

Answer: (B)

Vapor lock happens when fuel in the delivery path vaporizes into gas pockets under heat, so the fuel pump ends up pushing vapor instead of liquid. Liquid fuel is needed to maintain a steady flow; when vapor forms, it disrupts that flow and can, in severe cases, completely block it. That blockage stops fuel from reaching the engine, causing a loss of power or a stall. This is why the true statement is that vapor lock can completely block fuel flow. It doesn’t improve performance—hot conditions or high fuel temperatures can actually make the problem worse. It’s more of a gasoline-fuel issue and isn’t typically a concern with diesel, due to diesel’s higher boiling point and different fuel delivery characteristics. And yes, it does impact engine operation, not something to overlook.