Although vapor lock is not often seen in modern cars, it is a common problem with the carbureted cars of past decades. Vapor lock causes a car to stop running when the fuel lines overheat; cars sitting on the side of the road with their hoods raised used to be a common sight, particularly on hot days.

Vapor locking primarily happens in cars with carbureted engines, but since electronic fuel injection replaced carburetors in the 1980s, most car owners don’t have to deal with vapor lock anymore. Carburetors and electronic fuel injection are two different methods of delivering the proper amounts of fuel to the engine. Fuel injection is more advanced, and requires a computer to tell the injectors how much gasoline to squirt into the engine. A carburetor, on the other hand, is a mechanical device that uses the engine’s natural vacuum, allowing specific amounts of fuel to be sucked into the combustion chambers. The amount of fuel that is delivered to the engine can be changed using simple mechanical adjustments on the carburetor.

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 mechanical fuel pump’s proximity to the engine means that the engine’s 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. This is known as vapor lock.

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. Because the fuel tank is usually located too far away from the engine to be affected by its heat, gasoline is not likely to turn to vapor there. As a result, the fuel pump can push the fuel along without any problems, avoiding one of the major causes of vapor lock.

Another reason why vapor lock 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.

More efficient engine cooling systems also contribute to the decreased likelihood of vapor lock occurring in modern cars. The cooling fans in older cars usually ran off of the momentum of the engine, which meant that when cars were at a disadvantage when they sat idling in traffic for long periods of time: The lack of motion meant less air flowing through the engine compartment, and the fan — stuck at the idling speed of the engine — was unable to cool the engine sufficiently. As a result, the engines in older cars tend to run hotter in heavy traffic, making vapor lock more likely to occur.

Modern cars, on the other hand, typically have electric cooling fans. These fans are linked to a sensor that detects the engine’s temperature, and tells the fan whether to speed up or slow down. As a result, the internal combustion engines in modern cars are less likely to overheat than those in older models, making vapor lock much less common than it used to be.

All of the problems that cause vapor lock are easily corrected, even on older cars. Older cars can often be retrofitted with the newer technology that reduces the likelihood of the fuel lines overheating. For instance, even on a carbureted engine, a low-pressure electric fuel pump can be installed near the fuel tank, which will keep the fuel moving along the lines even if it starts to heat up in the engine compartment.

Another easy remedy for vapor lock in older cars is to install an electric fan in the engine compartment. Many auto parts stores carry aftermarket cooling fans, which can be installed on any car. Some cooling fans can be connected to the existing engine temperature sensor, mimicking the behavior of the cooling fans in modern cars.

Finally, vapor lock can be prevented in any car by protecting the fuel lines from the heat of the engine. There are two ways to do this. One way is to install heat shields between the engine and the fuel lines, which will divert most of the heat away from the lines. Fuel lines can also be insulated to prevent the fuel from getting to the temperatures at which vapor lock occurs.