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partial or complete interruption of the fuel flow in an internal-combustion engine, caused by the formation of vapour or bubbles of gas in the fuel-feeding system. Vapour forms because of fuel boiling in the fuel lines, usually as a result of excessive heating of the engine in hot weather or operation of the vehicle in areas of high altitude, which lowers the boiling point of the fuel. In some engines the fuel line may be routed too close to the exhaust manifold or other parts of the engine which give off intense heat. Allowing the engine to cool usually enables the vapour to recondense and thereby clears the vapour lock.