Most carbureted engines have a mechanical in-line fuel pump, which means that the simple mechanical pump is placed on the fuel line, usually next to the engine. The fuel pumps closeness to the engine means that the engines heat causes the fuel in the line to become very hot. When the fuel becomes too hot, it turns into a vapor, just as water begins to turn to steam when it boils. This process is hastened by the vacuum created in the line as the fuel is sucked into the engine.
When the fuel turns into vapor, the mechanical fuel pump can no longer move it along the lines. As a result, some or all of the fuel stops getting into the combustion chambers, and the car either begins to run very roughly or dies completely. If the driver attempts to restart the car, it will probably not start, or will continue having problems.
Vapor lock does not usually happen in fuel-injected engines for several reasons. First of all, most electric fuel pumps are located at or in the fuel tank, which is usually located too far away from the engine to be affected by its heat, so the gasoline is not likely to turn to vapor there. As a result, the fuel pump can push the fuel along without any problems.
Another reason why this problem does not often happen in fuel-injected engines is because the fuel lines are usually pressurized. The high pressure that the fuel is under prevents it from turning into vapor quite as easily, unlike the carbureted system that actually produces negative pressure, also known as vacuum, in the lines.
Fuel today is designed to atomize quicker and more efficiently which means it also turns to vapor easier.
