vapor lock - disruption of fuel movement to a gasoline engine caused by excessive vaporization of gasoline. Vapor lock occurs when the fuel pump, which is designed to pump liquid, loses suction as it tries to move fuel vapor. The engine will usually stall, but in less severe cases may accelerate sluggishly or knock due to an excessively lean fuel mixture. Factors favoring vapor lock are high ambient temperatures, low ambient pressure, volatile gasoline, and vehicle designs where heat from the engine can create high fuel line temperatures. Automotive engines are more likely to experience vapor lock during an acceleration that follows a short shutdown period. Vapor lock problems are most likely to occur in the late spring on unseasonably warm days, before the more volatile winter grades of gasoline have been replaced by the less volatile spring and summer grades (see volatility). Vapor lock can also occur in other types of pumping systems where volatile liquids are being handled.