is the 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 pump 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the more volatile winter grades of gasoline have been replaced by the less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.