Vapor lock happens, as was mentioned above (in a now-baleeted writeup), by fuel boiling in the fuel lines. Technically, the correct and rigorous thermodynamic phrasing is that the vapor pressure of the fuel has been raised past the surrounding pressure of the fuel lines, causing the fuel to vaporize. What's the easiest way to raise the vapor pressure of the fuel? Heat it. This longwinded technical explanation was brought to you by Chemistry, Thermodynamics and Heat Transfer.

The usual symptoms are a sudden loss in power and the vehicle cutting off, not the engine siezing (which would be caused by a lack of oil and lead to catastrophic failure). The vehicle will be unable to start and run as long as the fuel in the line is a vapor. Fuel pumps were made to push liquid fuel, not a gas, and are fairly useless until the fuel condenses back into a liquid. If a vehicle is suffering from vapor lock, open the hood and let the fuel lines cool off.

Most modern-day vehicles are relatively immune to vapor lock due to advances in fuel pumps. Vehicles of old were equipped with positive displacement fuel pumps placed near the engine that sucked fuel all the way from the fuel tank and were sometimes powered by the rotary motion of the crankshaft. Since it sucked the fuel to the engine, the pressure in the fuel lines was necessarially lower than atmospheric, which could make the fuel vaporize easier. Modern vehicles (both fuel injected and carbureted) have fuel pumps in or near the fuel tank which pushes fuel under high pressure to the engine. Besides this high pressure delivery, these vehicles also have a return fuel line to the tank, so fuel is supplied fresh and cool from the tank at a constant high pressure.

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