A vapour lock is a condition in an internal-combustion engine where the flow of fuel is partially or completely interrupted due to the formation of vapour or bubbles of gas within the fuel-feeding system. This occurs when liquid fuel turns into a gaseous state, typically due to excessive heat, blocking the fuel lines and preventing liquid fuel from reaching the engine.
How Vapour Lock Occurs
Vapour lock primarily happens when the fuel in the fuel lines or fuel pump heats up excessively, often past its boiling point. Unlike liquids, which are incompressible, vapour or gas bubbles are highly compressible. When these bubbles form, the fuel pump can no longer effectively draw liquid fuel, as it mostly compresses the vapour instead of pushing liquid. This leads to a significant reduction or complete cessation of fuel delivery to the engine's carburettor or fuel injectors.
Key components affected include:
- Fuel Pump: Especially mechanical fuel pumps mounted on the engine, which are susceptible to heat soak from the engine block.
- Fuel Lines: Lines routed too close to hot engine components or exhaust systems.
- Carburettor: The fuel bowl can boil if the engine compartment temperature is too high.
Causes of Vapour Lock
Several factors can contribute to the formation of a vapour lock:
- High Ambient Temperatures: On hot days, the entire engine compartment can reach temperatures sufficient to vaporize fuel.
- Engine Overheating: A malfunctioning cooling system can lead to excessively high engine temperatures, transferring heat to the fuel system components.
- Heat Soak: After a hot engine is turned off, residual heat can radiate and increase the temperature of the fuel lines and pump, leading to vapour formation. This is why vapour lock often occurs when trying to restart a hot engine.
- Low Fuel Level: A low fuel level in the tank exposes more of the fuel pump to ambient heat, and the remaining fuel can heat up more quickly.
- Improper Fuel Line Routing: Fuel lines routed too close to hot exhaust manifolds or other engine components.
- Fuel Volatility: Fuels with a higher Reid Vapour Pressure (RVP) or those designed for colder climates are more prone to vaporizing at lower temperatures.
- Low Fuel System Pressure: A weak fuel pump or restricted fuel lines can cause a drop in fuel pressure, which lowers the boiling point of the fuel, making it more susceptible to vaporization.
Symptoms of Vapour Lock
Identifying a vapour lock involves recognizing the signs of an interrupted fuel supply, especially when the engine is hot.
- Engine Stumbling or Misfiring: The engine may run erratically due to inconsistent fuel delivery.
- Loss of Power: As the fuel supply dwindles, the engine may lose power, particularly under acceleration.
- Difficulty Starting When Hot: This is a classic symptom, as the fuel system may be filled with vapour after the engine has been turned off and subjected to heat soak.
- Engine Stalling: In severe cases, the engine may completely stop running as fuel flow ceases entirely.
- Rough Idling: An engine suffering from vapour lock may idle roughly or surge.
Solutions and Prevention Strategies
Addressing vapour lock typically involves measures to keep the fuel cool and ensure adequate fuel pressure.
Immediate Solutions:
- Allow the Engine to Cool: The most straightforward solution is to let the engine cool down. As temperatures drop, the vapour will condense back into liquid fuel, allowing the system to work normally.
- Pour Cold Water on Fuel Lines/Pump (Caution): In emergencies, carefully pouring cool water over the fuel lines or pump can help condense the vapour, but extreme caution is advised to avoid damaging electrical components.
Preventative Measures:
- Improve Fuel Line Routing and Insulation:
- Reroute fuel lines away from hot engine components like exhaust manifolds.
- Insulate fuel lines with heat-resistant materials.
- Ensure Proper Fuel Pump Function:
- Verify the fuel pump is operating at the correct pressure.
- Consider a fuel pump with a higher flow rate or one mounted closer to the fuel tank (e.g., an in-tank electric pump) which pushes fuel, rather than pulls, making it less prone to vaporization issues.
- Use Appropriate Fuel:
- Utilize fuel with a lower volatility, especially during hot weather. Modern fuel blends are designed to be less prone to vapour lock.
- Maintain Cooling System:
- Ensure the engine's cooling system (radiator, thermostat, fan) is working efficiently to prevent general engine overheating.
- Install a Fuel Cooler (for performance applications):
- In some high-performance or heavily stressed vehicles, a dedicated fuel cooler might be installed to maintain optimal fuel temperature.
Vapour Lock Summary
| Aspect | Description |
|---|---|
| Definition | Partial or complete interruption of fuel flow in an internal-combustion engine. |
| Cause | Formation of vapour or gas bubbles in the fuel-feeding system due to excessive heat. |
| Effect | Prevents liquid fuel from reaching the engine, leading to stumbling, loss of power, or stalling. |
| Primary Factor | Heat (ambient, engine, heat soak) causing fuel vaporization. |
| Prevention | Managing heat, optimizing fuel system design, and using appropriate fuel volatility. |
Understanding vapour lock is crucial for diagnosing fuel delivery issues, particularly in older vehicles with carburettors or mechanical fuel pumps, or in modern vehicles under extreme heat conditions.
