How Do Flex Fuel Sensors Work with Ethanol and E85?

How Do Flex Fuel Sensors Work with Ethanol and E85?

In recent years E85 and Ethanol containing fuel has become a popular and viable alternative to high octane race fuels. The automotive industry like any new concept has consistently advanced the technology and it is now used reliably in high horsepower N/A, Turbocharged, and Supercharged combinations.  With the help of Flex Fuel sensors, the fuel variations (actual ethanol content) have made it so that tuners can create a seamless variable tuneup to keep engines safe and maximize power.  

If you stop at 10 different gas stations to get E85, you may get 10 different ethanol contents.  This means that you effectively may have 10 different octane levels.  If an engine depends on that octane, the tuneup can become unsafe in a hurry.  So how does a flex fuel sensor work and what are some common questions?

Where does a flex fuel sensor go?

Flex fuel sensors are traditionally placed on the feed line of a fuel system (line going into the engine or the pressure side).  This allows consistent sampling of real fuel to pass through the sensor for accurate reading of the fuel.  Some tuners will also place the flex fuel sensor in the return line although it is debated that this can cause read errors with less fuel returning than feeding and the possibility of sensor bypass depending on plumbing and mounting.

How does the sensor work?

Flex fuel sensors measure the amount of alcohol (ethanol is a form of alcohol) in the fuel passing through it.  The sensor has a pre calibrated signal that is sent from the sensor in a 0-5v voltage indicating which amount of alcohol is present.  The ECU is calibrated to understand what voltage represents what Ethanol content by means of standard calibration data.

The Flex Fuel sensor internal orifice is too small will not flow what my fuel system demands, what can I do?

We noticed this issue some time ago and developed a DUAL CHANNEL billet Flex Fuel sensor mount that creates two passages that are greater than the internal ID of a 10AN fuel hose or fitting.  This means that you can place a flex fuel sensor on the pressure / feed side of your fuel system without losing flow or having to create a bunch of leak points by plumbing Y and adapter fittings.  Because this is on the pressure side it means that fuel will be accurately sampled while flowing enough to support your high horsepower ethanol engine.  

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What EFI systems will support this functionality?

Many factory ECUs and tuning softwares will have the capability to operate Flex Fuel Tuning as well as Holley EFI, Megasquirt, Haltech, Fueltech, AEM and more! Please verify your ECU is capable through manufacturer before purchasing or attempting to install a flex fuel sensor.   

How do I wire a flex fuel sensor?

Flex Fuel sensors are a 5v reference type of sensor.  This means that it uses a sensor power, a sensor ground, and a signal wire sends data back to the ECU.  Please note sensor power and sensor ground wires must be supplied from the ECU, noise and incorrect signals from power and ground outside of the ECU will create an unreliable signal and false data.  Use the attached diagram from

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