Katerina Swanson
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. Vapor lock is more common in older vehicles with carbureted engines. While a clear plastic fuel filter can help identify vapor lock, it does not cause it. The primary causes of vapor lock are high ambient temperatures, extreme engine temperatures, fuel lines routed too close to hot components, and driving at high altitudes.
| Characteristics | Values |
|---|---|
| Can a clear plastic fuel filter cause vapor lock? | No, but it can be a contributing factor. Clear plastic fuel filters do not fill up completely with fuel when the motor is running. However, vapor lock is primarily caused by a combination of other factors, including high temperatures, fuel line issues, and fuel type. |
| Vapor Lock | Vapor lock is caused by liquid fuel turning into vapour in the fuel delivery system, disrupting the fuel pump and starving the engine of fuel. |
| Causes | High temperatures, fuel line issues (e.g. misrouted or leaky fuel lines), fuel type (e.g. winter fuel blend in warm weather), altitude, and engine overheating. |
| Symptoms | Stalling, hard starting, misfiring, rough running, poor acceleration, and hesitation. |
| Solutions | Address root causes such as overheating or fuel line issues. Relocate the fuel filter to a cooler place, insulate fuel lines, or install an electric fuel pump. |
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What You'll Learn
Fuel filters and vapor 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 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.
Fuel filters are one of the components that should be checked when diagnosing vapor lock. However, the use of a clear plastic fuel filter does not seem to be a direct cause of vapor lock. One user with a clear plastic fuel filter in their Duster noted that the filter had nothing in it when the engine was shut off when it was warm, and that there was very little in it when the engine was running. However, this does not appear to be a unique issue with clear plastic fuel filters, as another user with a fuel filter that was not specified to be clear plastic experienced the same issue.
The primary symptom of vapor lock is hard starting after the car has been sitting hot for a few minutes. Fuel filters do not typically fill completely with fuel when the motor is running, and half-full fuel filters when running are normal and do not necessarily indicate an issue.
To solve vapor lock issues, the underlying cause must be addressed. For example, if vapor lock is occurring due to engine overheating or misrouted fuel lines, repairs must be made to fix the problem permanently. However, there are also steps that can be taken to relieve the fuel system from vapor lock temporarily. This includes letting the fuel cool so that it returns to a liquid state, which can be expedited by parking in the shade and opening the hood. Once the fuel system has cooled, any remaining vapour can be eliminated by pressing the accelerator pedal slightly while cranking the engine, and then holding the accelerator pedal down until the vehicle runs smoothly.
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Fuel starvation and vapor lock
Vapor lock is a problem caused by liquid fuel turning into 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 pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. It is a problem more often found in older vehicles with carbureted engines.
Vapor lock can be caused by high outside 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. It is also more likely to occur when the vehicle is in traffic, as the under-hood temperature rises.
Fuel starvation occurs when the float chamber inside the carburetor is depleted of gasoline. This happens when a vapor lock is drawn into the fuel pump, disrupting the fuel pressure long enough for the float chamber to partially or completely drain.
To prevent vapor lock, the fuel pump can be moved to the interior of the tank, keeping the entire fuel delivery system under positive pressure and the pump running cool. This is why vapor lock is rare in modern fuel systems. Older vehicles with carbureted engines can be retrofitted with an electric fuel pump near the fuel tank.
To address a vapor lock problem, the underlying cause must be identified. For example, if the issue is caused by engine overheating or misrouted fuel lines, repairs will be needed. However, there are temporary measures that can be taken to relieve the fuel system of vapor lock. The fuel must be allowed to cool and return to a liquid state, which can be sped up by parking in the shade and opening the hood. Once the engine starts, hold the accelerator pedal down until the vehicle runs smoothly.
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Fuel bowl venting and vapor 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, the local climate, or a lower boiling point at high altitudes.
Vapor lock is more common in older vehicles with carbureted engines. Modern fuel-injected engines have the fuel pump located in the gas tank, keeping the pump cool and the fuel system under higher pressure, thereby reducing the risk of vapor lock.
To prevent vapor lock, it is important to ensure that the fuel bowl is adequately vented. This can be done by incorporating a pressure-balance duct linking the top of the float bowl with either the intake to the carburetor or the outside air. Alternatively, some fuel delivery systems allow fuel vapour to be returned to the fuel tank to be condensed back into a liquid or use an active carbon-filled canister where fuel vapour is absorbed.
In addition to adequate fuel bowl venting, there are other measures that can be taken to prevent vapor lock. These include:
- Using an electric pump and return-style fuel filter or EFI
- Moving the filter to a cooler place and insulating the fuel lines
- Using a metal fuel filter instead of a glass or plastic bowl fuel filter
- Installing a thicker gasket spacer under the carburetor to minimize the chance of vapor lock/heat soak/fuel boiling away in the bowls
- Using a fuel with a higher boiling point, such as diesel, or a fuel blend with multiple boiling points for summertime operation
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Fuel cap and vapor lock
Vapor lock is a problem more often found in older vehicles. It occurs when liquid fuel becomes hot enough to vaporize, creating a lean condition (not enough liquid fuel) that prevents the fuel system from delivering the correct amount of fuel to the engine. This can lead to stalling, hard starting, misfiring, rough running, and the engine accelerating poorly or hesitating.
Vented gas caps are designed to let air in to prevent a vacuum condition, not to let fumes out, which is the job of the canister. Drilling a hole in the gas cap would work to let air in, but in the middle of summer, the rear end will constantly smell of gas as the pressurized vapors escape through the hole instead of getting absorbed by the canister.
True vapor lock usually comes from heat soak conditions. For example, on a hot day, you run the car hard and then stop for 20-45 minutes. You come back out, the car starts fine, but then you go about a block and the car stalls and won't restart. The fuel in the pump has vaporized from the heat soak, so the pump won't pump fuel. The engine may also be difficult to restart once it's warmed up.
To solve vapor lock issues, you might need to address other problems in the vehicle, such as overheating or misrouted fuel lines. Automotive technology has improved, and today's fuel-injected engines have the fuel pump located in the gas tank, which keeps the pump running cool and the entire fuel system under higher pressure, reducing the risk of vapor lock.
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Fuel type and vapor lock
Vapor lock is a problem caused by liquid fuel turning into 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 more common in older vehicles that have a carburetor and a mechanical, engine-driven fuel pump. This type of setup, which operates at relatively low pressure and locates the fuel pump a long way from the gas tank, increases the likelihood of a vapor lock between the tank and pump. The fuel pump runs cooler when located inside the tank, reducing the risk of vapor lock.
Fuel type also plays a role in the occurrence of vapor lock. The greater the volatility of the fuel, the higher the probability of vapor lock. For example, 100LL is less volatile than the gasoline used in cars, but it can still easily vaporize when subjected to high heat. The use of winter fuel blends in warm weather can also cause vapor lock to occur more readily.
To prevent vapor lock, fuel lines should be routed away from hot vehicle components, such as exhaust parts and heater hoses. Heat shields can also be used to protect fuel lines from heat. Moving the fuel filter to a cooler place can also help, although this will only make a marginal difference compared to other solutions.
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Frequently asked questions
Vapor lock is a problem caused by liquid fuel turning into vapour while still in the fuel delivery system of a gasoline-fuelled internal combustion engine. This disrupts the fuel pump, causing a loss of pressure to the carburetor or fuel injection system, resulting in a loss of power or engine stalling.
Vapor lock is caused by fuel vapour forming in the fuel line, which reduces the pressure that normally moves the fuel from the tank to the carburetor. Common causes include high ambient temperatures, extreme engine temperatures, fuel lines being too close to hot components, and using a winter fuel blend in warm weather.
Vapor lock can cause a variety of issues, including engine stalling, hard starting, misfiring, rough running, and poor acceleration or hesitation.
To fix vapor lock, you must first let the fuel cool so it returns to a liquid state. You can do this by parking in the shade and opening the hood. Once the fuel has cooled, you can help eliminate any remaining vapour by pressing the accelerator pedal slightly while cranking the engine.
While a clear plastic fuel filter itself may not directly cause vapor lock, it is more common in older vehicles with carbureted fuel systems. Plastic fuel filters are also more prone to causing fires.
