Vapor lock occurs in a gasoline engine when some of the gasoline boils in the fuel-supply system. Excessive heating of the engine may cause boiling, or vaporization, of the fuel. This reduces the amount of fuel pumped to the engine, because vapor takes up more space than liquid. The engine then runs erratically or stops until the vaporized gasoline cools and turns to liquid. Vapor locks occur most often during long, steep climbs on hot days, or when slowing suddenly after a hard drive.
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