To promote easy starting, all gasolines contain volatile
constituents that under some conditions, such as high ambient temperature, tend to produce more vapor than the fuel-system vents can handle. The action of an engine-mounted fuel pump, in decreasing the pressure at its inlet, tends to vaporize the fuel. If the vapor forms faster than the pump can draw it from the fuel line, the flow of fuel to a carburetor
is effectively stopped and the engine stalls. Vapor lock is much less likely on a fuel-injected engine with an electric pump in the fuel tank. However, an engine with port fuel injection may experience vapor lock if the injector
or fuel overheats, or if the Reid vapor pressure of the fuel is too high.
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