Sylvie Willis
Vapor lock is a problem that occurs when liquid fuel turns into vapour in the fuel delivery system of gasoline-powered 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 of the engine. Diesel fuel is less volatile than gasoline, so diesel engines rarely suffer from vapor lock. However, diesel engine fuel systems are more prone to air locks in their fuel lines due to air leakage or entry from the tank. Modern diesel injection systems have self-bleeding electric pumps that eliminate the air lock issue. Solutions to vapor lock in gasoline engines include installing electric fuel pumps, using return systems, and choosing the right fuel type and delivery methods to minimise fuel vaporisation.
| Characteristics | Values |
|---|---|
| Occurrence in diesel engines | Almost never |
| Occurrence in gasoline engines | More common |
| Cause | Liquid fuel changing state to vapour in the fuel delivery system |
| Consequence | Loss of feed pressure to the carburetor or fuel injection system |
| Result | Transient loss of power or complete stalling |
| Fuel types that reduce the risk of vapor lock | E85, Ethanol, Methanol, Diesel |
| Fuel types that increase the risk of vapor lock | Ethanol blends, Winter blends |
| Conditions that increase the risk of vapor lock | High under-hood temperature, Hot weather, High altitude |
| Solutions | Installing an electric fuel pump, Adding a plastic/phenolic carb spacer, Using a return system, Using black nylon braided fuel hose |
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What You'll Learn
Diesel engines almost never suffer from vapor 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 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 of the engine. Diesel fuel is far less volatile than gasoline, so diesel engines rarely suffer from vapor lock.
Vapor lock can occur when fuel boils in the carburetor or fuel line, creating back pressure in the fuel system and preventing gas from reaching the engine. Modern fuel is designed for fuel-injected engines and electric fuel pumps, which has increased the occurrence of vapor lock in recent decades. However, diesel fuel has a lower volatility than gasoline, making it less prone to vaporization and vapor lock.
Vapor lock is more likely to occur in certain conditions, such as high under-hood temperatures, hot weather, or high altitudes. It can also happen when the engine is stopped while hot and the vehicle is parked for a short period, allowing the fuel in the line near the engine to heat up and vaporize. Diesel fuel, with its lower volatility, is less susceptible to vaporization in these conditions.
While diesel engines rarely experience vapor lock, they are more susceptible to air locks in their fuel lines due to air leaking into the fuel delivery line or entering from the tank. Common causes of air locks include allowing the fuel tank to run dry, changing the fuel filter, or having leaky fuel lines. Air locks can be eliminated by turning the engine over using the starter motor or by bleeding the fuel system. Modern diesel injection systems have self-bleeding electric pumps that prevent air lock issues.
To summarize, diesel engines rarely suffer from vapor lock due to the lower volatility of diesel fuel compared to gasoline. However, diesel engines may experience air locks in the fuel lines, which can be addressed through modern diesel injection systems with self-bleeding electric pumps.
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Air locks are a common problem in diesel engine fuel systems
Airlocks can be addressed by bleeding and properly priming the fuel lines, which can be time-consuming. Modern diesel injection systems have self-bleeding electric pumps that eliminate the air lock problem. AirDog® fuel systems are very effective at addressing air lock issues. The AirDog® fuel system’s first step is to send the fuel through a water separator that eliminates moisture and particulates down to 2 microns. Second, the fuel passes through the AirDog® Fuel Preporator, which removes air from the fuel.
Airlocks can also be prevented by staying away from steel lines, which offer no insulation from heat. Instead, use black nylon braided fuel hose, and be sure to route it away from all heat sources. Additionally, find a gas station that provides ethanol-free blends, as ethanol blends have a lower boiling point and are a major cause of vapor lock problems.
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Fuel vaporisation causes loss of engine power and stalling
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 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. 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 is more likely to occur in fuel injector lines after the flow divider, as fuel is sent to the injector nozzles to be sprayed into the intake manifold. This is because the heat from the engine cylinders and crankcase radiates outwards and upwards toward the fuel injector lines. The higher the volatility of the fuel, the more likely it is that vapor lock will occur. Modern fuel blends can boil at temperatures as low as 100 degrees Fahrenheit, increasing the risk of vapor lock.
Vapor lock can also develop when the engine is stopped while hot and the vehicle is parked for a short period. The fuel in the line near the engine does not move and can thus heat up sufficiently to form a vapor lock. The problem is more likely in hot weather or high altitudes. Even temporary disruption of the fuel supply into the float chamber can cause issues, as most carburetors are designed to run at a fixed level of fuel in the float bowl, and reducing the level will reduce the fuel-to-air mixture delivered to the engine.
Vapor lock can be prevented by proper fuel system maintenance, rerouting fuel lines, and using cooling techniques. If vapor lock occurs, using the electric boost pump or allowing the engine to cool can help clear the vaporized fuel. Some fuel delivery systems allow fuel vapour to be returned to the fuel tank to be condensed back into the liquid phase, or use an active carbon-filled canister where fuel vapour is absorbed.
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Modern engines are less prone to vapor lock
Vapor lock is a problem caused by liquid fuel turning to vapour 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 higher the volatility of the fuel, the more likely it is that vapor lock will occur.
Vapor lock was far more common in older gasoline-fuel systems that incorporated a low-pressure mechanical fuel pump driven by the engine, located in the engine compartment, and feeding a carburetor. Modern fuel blends can boil at temperatures as low as 100 degrees Fahrenheit, which is lower than the temperature of the fuel line in a car with an ambient temperature of 70 degrees Fahrenheit.
Vapor lock can be prevented by selecting the right fuel, improving fuel system insulation, and parking facing into the wind to help cool the engine.
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Solutions to vapor lock include installing an electric fuel pump
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. Diesel engines, on the other hand, almost never suffer from vapor lock due to the lower volatility of diesel fuel compared to gasoline. However, diesel engine fuel systems are more prone to air locks in their fuel lines due to the reliance of standard diesel fuel injection pumps on non-compressible fuel.
Modern diesel injection systems have self-bleeding electric pumps that eliminate the problem of air locks. Moving the fuel pump inside the tank helps prevent vapor lock as the entire fuel delivery system is under positive pressure, and the fuel pump runs cooler compared to when it is located in the engine compartment. This is the main reason why vapor lock rarely occurs in modern fuel systems.
To solve vapor lock problems, one solution is to install an electric fuel pump near the fuel tank. This can be done by retrofitting carbureted engines with an electric fuel pump. Another option is to install an inline electric pump to pressurize the fuel up to the inlet side of the mechanical pump. However, this approach may not improve the low-pressure side of the pump, and returning excess fuel to the tank is still necessary to prevent heat absorption before reaching the carburetor.
Another solution is to use a return system, such as the Aeromotive system, which returns excess fuel to the fuel tank to cool it down. This system also prepares the engine for a future conversion to fuel injection. Additionally, using black nylon braided fuel hoses instead of steel lines can help minimize heat absorption and reduce the risk of vapor lock.
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Frequently asked questions
Vapor lock occurs when liquid fuel changes state to vapour while still in the fuel delivery system of gasoline-fuelled internal combustion engines. This disrupts the operation of the fuel pump and causes a loss of feed pressure to the carburetor or fuel injection system.
Vapor lock is caused by an increase in temperature, which can be due to the local climate, the engine, or a lower boiling point at high altitudes. It is more likely to occur in hot weather or when the vehicle is in traffic, causing the under-hood temperature to rise.
Vapor lock can cause the engine to stall or run roughly in warm temperatures. The engine may also need to sit for several hours before starting again after being shut off.
There are several ways to prevent vapor lock, including installing an electric fuel pump near the fuel tank, adding a plastic carb spacer, or using a return system that returns excess fuel back to the fuel tank to cool down.
Diesel fuel is less volatile than gasoline, so diesel engines rarely suffer from vapor lock. However, diesel engines are more susceptible to air locks in their fuel lines due to air leaking into the fuel delivery line or entering from the tank.
