Flex Fuel Sensor: Restricting Fuel Pressure, How?

The flexible fuel vehicle (FFV) runs on normal gasoline and blends of gasoline and ethanol. The ethanol content in the fuel is determined by a flex fuel sensor, which is placed as close to the injector as possible for an immediate response. The sensor is placed on the feed line of a fuel system to allow for a consistent sampling of real fuel passing through the sensor for an accurate reading. However, if the target power level exceeds a certain limit, the sensor may pose a restriction to the fuel flow. This can be dealt with by either fitting the sensor on the fuel return line or splitting the fuel line into two and running it through the fuel composition sensor and directly to the engine in parallel.

Flex fuel sensors can be placed on the return line to avoid flow restriction

Flex fuel sensors are traditionally placed on the feed line of a fuel system, which is the line going into the engine or the pressure side. This allows a consistent sampling of real fuel to pass through the sensor for an accurate reading. However, some tuners prefer to place the flex fuel sensor in the return line. While this is debated due to the possibility of read errors and sensor bypass, it can be done without causing flow restriction.

For example, one user placed their flex fuel sensor in the return line of their Mitsubishi Eclipse, noting that it is in a spot close to the dash. Another user placed their sensor in the return line of their Camaro and Firebird, specifically on the return port of the fuel pressure regulator, as it was the cleanest and shortest length for their wiring. A different user also placed their sensor in the return line of their Camaro and Firebird, but they noted that the sensor reading fluctuated from 70% to 50% during a track pass, which threw off some of their custom tables.

To avoid read errors and sensor bypass, it is important to consider the plumbing and mounting of the flex fuel sensor when placing it in the return line. Additionally, ensuring that the sensor is under constant pressure and is not a restriction can help maintain accurate readings.

Overall, while it is less common to place a flex fuel sensor in the return line, it can be done without causing flow restriction by considering factors such as plumbing, mounting, and pressure.

They can also be placed on the feed line, but this may cause flow issues

Where to Place the Flex Fuel Sensor?

Flex fuel sensors are traditionally placed on the feed line of a fuel system. This allows for a consistent sampling of real fuel to pass through the sensor for an accurate reading of the fuel. However, some people choose to place the sensor on the return line, although this is debated as it can cause read errors due to less fuel returning than feeding and the possibility of sensor bypass.

Potential Flow Issues with the Flex Fuel Sensor on the Feed Line

While placing the flex fuel sensor on the feed line is a common practice, it is important to consider the potential flow issues that may arise. The sensor has an internal orifice that may restrict fuel flow if the internal diameter is smaller than the fuel system demands. This can result in a loss of flow or the creation of leak points by plumbing Y and adapter fittings.

Solutions to Potential Flow Issues

To address the flow issues, Motion Raceworks has developed a DUAL CHANNEL billet Flex Fuel sensor mount. This mount creates two passages that are greater than the internal diameter of a 10AN fuel hose or fitting. By placing the flex fuel sensor on the pressure/feed side of the fuel system, accurate fuel sampling can be achieved while maintaining sufficient flow to support high horsepower ethanol engines.

Advantages of Placing the Sensor on the Feed Line

Placing the sensor on the feed line offers some potential advantages over placing it on the return line. For reliable and consistent ethanol readings, the sensor must see a constant flow of fuel. If the sensor is placed on the return line, there may be a problem if the fuel pump's flow capability is nearing its limit. In such cases, the amount of fuel flowing through the return line can reduce to a trickle, resulting in erratic readings from the sensor. This can occur under high load and RPM, where a potential lean air-fuel ratio could be dangerous.

Considerations for Fuel System Modifications

When making modifications to the fuel system, it is important to take precautions to ensure the reliability of the connections. A fuel leak can result in catastrophic damage, so it is recommended to leave complex installations to a professional workshop. The fuel composition sensor is designed to work with factory-style fuel lines that clip over the inlet and outlet tubes, providing positive location and preventing the fuel line from coming off. Aftermarket adaptors are also available to convert to motorsport-grade AN fittings.

The sensors are pass-through, meaning fuel enters one side and exits the other

Flex fuel sensors are traditionally placed on the feed line of a fuel system, which is the line going into the engine or the pressure side. This allows consistent sampling of real fuel to pass through the sensor for an accurate reading of the fuel. The sensors are pass-through, meaning fuel enters one side and exits the other. This is in contrast to return line placement, which can cause read errors due to less fuel returning than feeding, and the possibility of sensor bypass.

The pass-through nature of flex fuel sensors means that they have an inlet and outlet for fuel from the tank to flow through. This fuel passes through the sensor, which detects the ethanol content, and the sensor then communicates this information to the ECU. The ECU can then interpret the signal and adjust the vehicle's calibration in real time to compensate for the fuel type.

The pass-through design of flex fuel sensors means that they can accurately sample fuel while flowing enough to support high-horsepower ethanol engines. This is particularly important when using E85 fuel, which requires a higher volume of fuel to flow through the feed lines to the fuel rails.

The pass-through nature of the sensor also means that it can be placed in the pressure/feed side of the fuel system without losing flow or creating leak points. This ensures that the sensor can provide accurate readings while supporting the engine's fuel demands.

The sensors are designed for stock OE applications, so connections are sized for stock factory fuel systems

Flex fuel sensors are devices that detect and respond to changes in their environment by converting physical changes into electrical signals. They are used to measure the amount of alcohol (ethanol) in the fuel passing through them. These sensors are placed on the feed line of a fuel system, which is the line going into the engine or the pressure side. This allows for a consistent sampling of real fuel to pass through the sensor for an accurate reading of the fuel.

By using these sensors in stock OE applications, users can take advantage of the sensor's ability to provide accurate readings of the ethanol content in the fuel. This information is crucial for tuners, as it allows them to create a seamless variable tune-up to keep engines safe and maximize power. The sensor's compatibility with stock factory fuel systems ensures that it can be easily integrated into the existing fuel system without requiring additional modifications.

It is important to note that the placement of the flex fuel sensor can impact its performance. Some users have reported fluctuations in readings when the sensor is placed in the return line, as there may be less fuel returning than feeding, and the possibility of sensor bypass. Therefore, placing the sensor on the pressure side of the fuel system is generally recommended to ensure accurate sampling and adequate fuel flow.

Overall, the design of the flex fuel sensors for stock OE applications ensures their compatibility and effectiveness in standard factory fuel systems, making them a reliable choice for accurate ethanol content readings.

The sensors can handle up to 10 bar or 145 psi of pressure, so pressure is not usually an issue

The placement of a flex fuel sensor is important for its functionality and the performance of the engine. The sensor measures the amount of alcohol (ethanol) in the fuel passing through it and sends a pre-calibrated signal to the ECU. This helps tuners create a variable tune-up to keep engines safe and maximise power.

The sensor is traditionally placed on the feed line of a fuel system (the line going into the engine or the pressure side). However, some tuners place the sensor in the return line, which can cause read errors due to less fuel returning than feeding and the possibility of sensor bypass.

One issue that may arise with flex fuel sensors is that the internal orifice may be too small, not flowing enough fuel to meet the demands of the fuel system. This can be addressed by creating two passages that are greater than the internal diameter of a fuel hose or fitting, allowing for adequate fuel flow to support high horsepower ethanol engines.

Another consideration is the fuel pressure. While flex fuel sensors are capable of handling pressure, there is a misconception that they are not designed to cope with pressure or that high fuel pressure will affect sensor accuracy. However, flex fuel sensors are used in factory applications and can handle up to 10 bar or 145 psi of pressure, which is typically sufficient for most applications.

It is important to note that for reliable and consistent ethanol readings, the sensor must have a constant flow of fuel. Placing the sensor on the return line can be problematic if the fuel pump's flow capability is nearing its limit. In such cases, the amount of fuel flowing through the return line may reduce to a trickle, resulting in erratic readings from the sensor, especially under high load and RPM where a lean air-fuel ratio could be dangerous.

To summarise, while flex fuel sensors can handle up to 10 bar or 145 psi of pressure, it is crucial to ensure proper placement and adequate fuel flow to obtain reliable and consistent readings, maintaining engine performance and safety.

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