Rajan Wilcox
Vapor lock is a problem caused by liquid fuel changing state to vapour while still in the fuel delivery system of gasoline-fuelled internal combustion engines. This disrupts the operation of the fuel pump, causing a loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Vapor lock is caused by heat, which boils the fuel, preventing it from being pumped into the carburettor or fuel injectors before it has a chance to combust. This phenomenon is more common in carburetted vehicles than in fuel-injected engines, and modern diesel engines rarely suffer from it.
| Characteristics | Values |
|---|---|
| Definition | Vapor lock is a problem caused by liquid fuel changing state to vapour while still in the fuel delivery system of gasoline-fuelled internal combustion engines. |
| Occurrence | Vapor lock typically occurs when shutting down an engine, particularly in warm weather. It can also occur when the engine is running, especially at high altitudes. |
| Causes | Common causes of vapor lock include high outside ambient temperatures, extreme engine operating temperatures, fuel lines routed too close to hot vehicle components, using a winter fuel blend in warm weather, engine modifications, and driving at high altitudes. |
| Symptoms | Symptoms of vapor lock include engine stumbling, stalling, rough running, poor acceleration, and engine misfiring. It can also lead to a burned-up fuel pump. |
| Prevention and Solutions | To prevent vapor lock, it is recommended to park in the shade, open the cowling or oil vents, and point the vehicle into the wind to promote cooling. Solutions to vapor lock include adding an electric booster pump near the fuel tank, upgrading to a larger-diameter fuel line, and improving thermal management of exhaust heat. |
| Affected Vehicles | Vapor lock is more common in older vehicles with carbureted engines, but it can also occur in modern fuel-injected engines, especially in aircraft. |
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What You'll Learn
Vapor lock is caused by liquid fuel changing state to vapour
Vapor lock is a problem caused by liquid fuel changing state to vapour while still in the fuel delivery system of gasoline-fuelled internal combustion engines. This disrupts the operation of the fuel pump, causing a loss of feed pressure to the carburetor or fuel injection system, resulting in a transient loss of power or complete stalling. Restarting the engine from this state may be difficult. The fuel can vaporize due to being heated by the engine, by the local climate, or due to a lower boiling point at high altitudes.
Vapor lock typically starts when an engine is shut down. As the engine is shut down, the heat rises up toward the fuel lines. Without air moving through the cowling, the fuel in the lines can start to vaporize. Generally, if you experience vapor lock, it's when you try to start your plane after a quick turn (a shutdown and restart within 30 minutes). But in some rare cases, vapor lock can occur in flight. The higher the volatility of the fuel, the more likely it is that vapor lock will occur.
Vapor lock can also be provoked by engine modifications that push parts of a car's fuel system beyond their design limits. Many vintage cars have barely adequate fuel systems, and heat can push them right up to the limit very quickly. Another cause might be adding freer-breathing induction, headers, or exhaust components.
Vapor lock occurs when the temperature of the fuel causes it to boil, causing fuel pump cavitation and a siphon break in the line feeding fuel to the engine. The higher the volatility of the fuel, the more likely it is that vapor lock will occur. Historically, gasoline was a more volatile distillate than it is now and was more prone to vapor lock. Conversely, diesel fuel is far less volatile than gasoline, so diesel engines almost never suffer from vapor lock.
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Fuel vapour entering the float bowl must be vented
Vapor lock is a problem caused by liquid fuel changing state to vapour while still in the fuel delivery system of gasoline-fuelled internal combustion engines. This disrupts the operation of the fuel pump, causing a loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. The fuel can vaporize due to being heated by the engine, by the local climate, or due to a lower boiling point at high altitudes.
Vapor lock typically starts when you shut down an engine. As you shut down, the heat rises up toward the fuel lines. Without air moving through the cowling, fuel in the lines can start to vaporize. Generally, if you experience vapor lock, it's when you try to start your plane after a quick turn (shut down and restart within ~30 minutes). But in some rare cases, vapor lock can occur in flight. Old cars, new cars, stock cars, and modded cars can all suffer from vapor lock.
Vapor lock has been the cause of forced landings in aircraft. That is why aviation fuel is manufactured with far lower vapour pressure than automotive gasoline (petrol). Aircraft are more susceptible to vapor lock because of their ability to change altitude and associated ambient pressure rapidly. Liquids boil at lower temperatures when in lower-pressure environments.
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High volatility of fuel increases the probability of vapour lock
Vapor lock is a problem caused by liquid fuel changing state to vapour while still in the fuel delivery system of gasoline-fuelled internal combustion engines. This disrupts the operation of the fuel pump, causing a loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. The fuel can vaporize due to being heated by the engine, by the local climate, or due to a lower boiling point at high altitudes.
The higher the volatility of the fuel, the more likely it is that vapour lock will occur. Volatility is the measure of how likely a substance is to evaporate at a specific temperature. Gasoline, for instance, has a higher volatility than aviation fuel, which is why vapour lock is a common occurrence in aircraft. Historically, gasoline was a more volatile distillate than it is now, and was more prone to vapour lock.
Vapour lock typically starts when the engine is shut down. As the engine is shut down, the heat rises up towards the fuel lines. Without air moving through the cowling, the fuel in the lines can start to vaporize. This is more likely to happen with higher-volatility fuel. In some rare cases, vapour lock can occur in flight.
Vapour lock can also be caused by engine modifications that push parts of a car's fuel system beyond their design limits. Many vintage cars have barely adequate fuel systems, and heat can push them right up to the limit very quickly. Additionally, in regions where fuels with lower viscosity and lower boiling thresholds are used during the winter to improve engine startup, continued use of these specialized fuels during the summer can cause vapour lock to occur more readily.
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Fuel vaporization can be prevented by cooling
Vapor lock is a problem caused by liquid fuel changing state to vapour while still in the fuel delivery system of gasoline-fuelled internal combustion engines. This disrupts the operation of the fuel pump, causing a loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. The fuel can vaporize due to being heated by the engine, by the local climate, or due to a lower boiling point at high altitudes.
Firstly, it is important to protect your fuel lines from getting hot. One of the easiest solutions is to use a thermal barrier like a heat shield or a heat sleeve. These products are designed to prevent vapor lock by reflecting heat away from the fuel lines, keeping them cool. They can be installed around the fuel lines and are especially useful for protecting the lines that run next to frame rails or floor pans, which are more susceptible to heat. Additionally, sticky heat shields can be used around fuel tanks to protect them from heat, keeping the fuel inside cooler.
Another way to prevent fuel vaporization is to manage the exhaust heat more effectively. This can be done by ensuring that the exhaust system is properly insulated and ventilated, preventing excess heat from radiating towards the fuel lines.
Furthermore, some fuel delivery systems allow fuel vapour to be returned to the fuel tank to be condensed back into the liquid phase. This process removes fuel vapour from the fuel line near the engine, reducing the risk of vapor lock.
Finally, it is recommended to avoid turning off the engine in hot weather or when the vehicle is parked for a short period. The fuel in the lines can heat up and form a vapor lock if the engine is stopped while hot. Keeping the engine running will help maintain the fuel's liquid state and prevent vaporization.
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Fuel injection engines are less prone to vapour lock
Vapor lock is a problem caused by liquid fuel turning into vapour in the fuel delivery system of gasoline-fuelled internal combustion engines. This disrupts the fuel pump, causing a loss of pressure to the carburetor or fuel injection system, resulting in a transient loss of power or complete stalling. The fuel can vaporize due to being heated by the engine, by the local climate, or due to a lower boiling point at high altitudes.
Vapour lock is more likely to occur in carburetor engines as fuel is sucked a long distance from the gas tank, as opposed to being pushed through the fuel line by a tank-mounted pump, as with EFI cars. Carburetor units may not effectively deal with fuel vapour being delivered to the float chamber. Most designs incorporate a pressure-balance duct linking the top of the float bowl with either the intake to the carburetor or the outside air.
Vapour lock was a common occurrence in stock car racing, as the cars traditionally used gasoline and carburetors. With the introduction of the fuel injection requirement for NASCAR-sanctioned events in 2012, vapour lock has been largely eliminated. It is also less common in other motorsports, such as Formula One and IndyCar racing, due to the use of fuel injection and alcohol fuels (ethanol or methanol), which have a lower vapour pressure than gasoline.
However, it is not impossible for fuel injection engines to experience vapour lock. For example, the double Red Bull Racing retirements at the 2022 Bahrain Grand Prix were caused by vapour lock, presumably due to the unusually high temperatures in the fuel system.
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Frequently asked questions
Vapor lock is a problem caused by liquid fuel changing state to vapour while still in the fuel delivery system of gasoline-fuelled internal combustion engines. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling.
Common causes of vapor lock include high outside ambient temperatures, extreme engine operating temperatures, fuel lines routed too close to hot vehicle components, using a winter fuel blend in warm weather, and driving at high altitudes. The higher the volatility of the fuel, the more likely it is that vapor lock will occur.
To solve vapor lock issues, you might need to address other problems in the vehicle such as overheating or misrouted fuel lines. You can also take steps to relieve the fuel system from vapor lock temporarily, such as letting the fuel cool and cranking the engine.
Yes, vapor lock can occur in fuel-injected engines. However, it is less common in modern engines with fuel injection systems.
