Define vapor lock and identify common contributing factors.

• A. Vapor lock is air leakage in the intake manifold causing lean conditions.
• B. Vapor lock is a failed fuel pump causing no fuel delivery.
• C. Vapor lock is a sensor fault causing injector misfire.
• D. Vapor lock is fuel vapor in the lines that can block flow; caused by high fuel temperature, heat-soaked components, low vapor pressure, or excessive fuel line length/insulation.

Answer: (D)

Vapor lock happens when fuel in the delivery path forms vapor pockets and can’t flow as liquid to the engine. This blocks fuel delivery and leads to hesitation, stalling, or no-start conditions, especially in hot conditions where vapor forms more readily.

Contributing factors include high fuel temperature or heat soaking of components like the fuel pump or nearby engine parts, which increases fuel volatility; fuels with low vapor pressure or fuels that vaporize easily; and long or poorly insulated fuel lines that absorb more heat and promote vapor formation. In practice, vapor lock is more likely in hot environments and with older systems that have less heat management in the fuel path. Preventive measures involve keeping lines away from heat sources, insulating them, and maintaining proper fuel cooling and pressure.