Vapor lock is a problem that mostly affects which engines?

• A. A problem that mostly affects diesel engines.
• B. A problem that is cured by restarting the engine quickly.
• C. A problem that mostly affects gasoline-fueled internal combustion engines, where the liquid fuel changes state to gas in the fuel delivery system.
• D. An electrical ignition failure.

Answer: (C)

Vapor lock is a fuel-delivery problem in gasoline engines caused by the fuel in the lines or pump turning into a gas when it gets too hot. When the fuel vaporizes, vapor bubbles form and block the flow, so the engine can’t receive a steady supply of liquid fuel. Since gasoline is relatively volatile, heat in hot weather, in long fuel lines, or in areas with restricted airflow makes vaporization more likely, especially in older or carbureted systems.

This is why gasoline-fueled engines are the ones affected: diesel fuel is less volatile and diesel engines operate with high-pressure, looped fuel systems that aren’t as prone to vaporizing in the same way, and ignition failures are a separate issue unrelated to fuel phase changes. If vapor lock occurs, symptoms include a loss of power or stalling, and relief often involves cooling the fuel system or allowing it to return to cooler temperatures so liquid fuel can flow again.