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Flex Fuel with MegaSquirt Controller

Many MegaSquirters have expressed an interest in having MegaSquirt® control flexible fuel vehicles, typically E85/gasoline engines. MegaSquirt-II (including MicroSquirt) is the first aftermarket EFI controller with direct flex-fuel support for experimental only testing of ethanol fuels and blends.

Environmental Protection Agency (EPA) laws in the USA dictate that a vehicle labeled as "unleaded fuel only" cannot be converted to another fuel unless they pass certain EPA rules and are certified. However, conversion of earlier vehicles not labeled as such, as well as off-road use, is permitted. Be sure to check local laws first in your area. A vehicle needs to be tested by an EPA-certified testing center in order to be fully legal for on-road use in the USA.

What is "Alcohol"?

In chemistry, an alcohol is any organic compound in which a hydroxyl group (-OH) is bound to a carbon atom of an alkyl or substituted alkyl group. The general formula for a simple acyclic alcohol is CnH2n+1OH. The alcohols commonly mentioned as automotive fuels are:

  • Ethanol: C2H5OH
  • Methanol: CH3OH

Ethanol is the stuff you might drink, methanol is commonly used in windshield wiper fluid.

It's worth noting that ethanol is *much* less corrosive than methanol, though both are alcohols (have oxygen in their structures). While ethanol is slightly acidic, it is generally okay for any road vehicle produced in the last 20 years (in North America, at least). Methanol is the alcohol that eats up fuel systems (and which people sometimes flush with gasoline after running, etc.).

Ethanol (116 (RON+MON)/2 octane rating) is what is blended in gasoline (E10, E85, etc., referring to the percentage of ethanol added), since it is not going to hurt the fuel system of any car made in the last 20-25 years, and its energy density is closer to gasoline (-27%) - so it doesn't lean the mixture as much. The oxygen in the structure reduces carbon monoxide emission - which is why it is often mandated in cold areas in the winter, and NOx emissions are typically reduced as well.

Methanol (113 (R+M)/2) is more corrosive, and less energy dense (-55%) so more fuel is required. This means that usually different materials are needed in the injectors, fuel pump, etc., and higher flow components must be used for the much higher volume. Methanol also produces toxic formaldehyde and formic acid emissions that will make your eyes water!

More information:

E85 is 85% ethanol, 15% gasoline. E85 burns clean, and is a renewable resource. Flexible fuel vehicles (FFV) are being produced by Daimler Chrysler, Ford, General Motors, Isuzu, Mazda, Mercedes, Mercury, and Nissan. You can find out much more about E85 at

Regular unleaded gasoline typically has an octane rating of 87; E85 has an octane rating that ranges from 100-105 that improves engine performance. For example, some Ford flex-fuel vehicles (FFVs) produce about a 5% horsepower gain when using E85 compared to their power output on regular gasoline.

Note: converting to alcohol fuels can damage certain fuel system components like the fuel tank, fuel pump, lines, injectors, etc. If they are not rated for alcohol use and you plan on using greater than 10% alcohol for the long term, these components should be upgraded to items known to withstand higher alcohol concentrations.

E85, like gasoline and diesel, has its composition seasonally adjusted to ensure proper starting and performance in different geographic locations. For example, E85 sold during the winter often contains 70% ethanol and 30% gasoline to help it produce the necessary vapor pressure for easy starting. An E85 fueling gas station usually cannot carry over summer-blend E85, but instead it has to "blend down" any remaining summer fuel to make an "E70" mixture. This can be done by adding additional gasoline to the storage tank. Carrying over winter-blend E70 into summer in not a problem as flexible fuel vehicles can operate on any blend of E85 and gasoline. The result is that the fuel make-up varies, and flex fuel vehicle the need to know what it is burning, and adjust the fuel accordingly. It does this with a fuel composition sensor.

Flexible fuel vehicles use a sensor to determine the percentage of ethanol in the fuel mix (which varies not only with the season and location, but also with the relative amounts of E85 and gasoline in the tank at fill-up) and adjust the fuel injection and timing to suit the fuel. Understanding these sensors is the key to using E85 with MegaSquirt.

The first thing to note is that gasoline and ethanol have different compositions - alcohols, by definition include oxygen as part of their structure, gasoline traditionally does not. Thus, E85 and especially lower ethanol concentration blends are sometimes referred to as 'oxygenated blends' or 'oxygenated gasoline'.

These affect the air/fuel ratios (AFR) required when tuning:

Gas stoich14.680.0681.0001.000
Gas max power rich~12.50.0801.1760.8503
Gas max power lean~13.20.07551.1110.900

E85 stoich9.870.1011.0001.000
E85 max power rich~6.980.14341.400.7143
E85 max power lean~8.470.1181.1530.8673

E100 stoich9.000.1111.0001.000
E100 max power rich~6.430.1551.400.714
E100 max power lean~7.800.1281.1500.870

To determine the amount of "extra" fuel required compared to straight gasoline at stoichiometric mixtures, simply divide the AFRs. For instance, comparing E85 to 100% gasoline, the extra fuel factor is 14.68/9.87 = 1.49. This will require an alteration to the REQ_FUEL value of 149% (less if the gasoline already contained some ethanol or MTBE - most do - so a value of 146% is more typical).

Note that the fuel flow will increase by 46% under all conditions with E85 versus gasoline, and your fuel pump and injectors must be capable of the higher flow under high load conditions.

To save a bit of calculation, you can enter your engine's estimated brake horsepower (at the flywheel) and the total number of injectors in the form below and press the "Compute E85 Flowrate" button.

# Injectors
E85 Flowrate (lbs/hr)

There are other issues with E85 such a Reid vapor pressure (RVP), volatility (fuels ability to vaporize), cold starting enrichment, hot restart, and the like. But E85 is a really nice DIY experimenter's paradise.

Finally, when tuning, note that higher concentrations of ethanol increasingly limit the color that the spark plugs will show. The result is that there is less information to tune, and greater observational powers are needed to interpret the spark plugs. In most cases, a properly set-up and calibrated wide band EGO sensor will be more useful than reading plugs for tuning when using E85.

Fuel Composition Sensor Description

The fuel composition sensor (FCS), also known as the flex fuel sensor, measures the ethanol to gasoline ratio of the fuel in the fuel line to the injectors. Flexible fuel vehicles can be operated with a blend of ethanol and gasoline, up to 85 percent ethanol. To optimize the ignition timing and the fuel injection amount, the MegaSquirt® requires information about the percentage of ethanol in the fuel.

The fuel sensor uses a microprocessor to measure the ethanol percentage and fuel temperature, which it uses to produce and output signal. The fuel sensor signal is a square-wave voltage signal. The signal varies in both frequency and pulse width:

You can set the flex fuel sensor parameters in MegaTune under 'Settings/Flex Fuel':

Note that some version of the MS-II code (V2.6+) allow settings for fuel composition based spark control adjustments as well. In these cases, numbers entered indicated advance added, so the default of -13.0 for 100% ethanol represents 13 degrees of retard.

The processor inside the sensor is capable of some self-diagnosis. An output frequency of 170 Hertz indicates either that the fuel is contaminated (or contains methanol which it should not), or that an internal sensor electrical fault has been detected. Certain substances dissolved in the fuel can cause the fuel to be contaminated, raising the output frequency to be higher than the actual ethanol percentage should indicate. These contaminates may commonly be water, salt, methanol, or some other substances.

It should be noted that it is likely that the fuel sensor will indicate a slightly lower ethanol percentage than what is advertised at the fueling station. The reason has to do with government requirements for alcohol-based motor fuels. Government regulations require that alcohol intended for use as motor fuel be 'denatured' (i.e., made unsuitable for human consumption). This means that 100 percent pure ethanol is first denatured with approximately 4% gasoline, before being mixed with the remaining gasoline amounts. So when an ethanol/gasoline mixture is sold as E85, the 85 percent ethanol was denatured before being blended with gasoline, meaning an advertised E85 fuel contains approximately 81% ethanol. The fuel sensor measures the actual percentage of ethanol in the fuel, not the 'advertised' percentage.

Initial testing has been done with the GM fuel composition sensor part number #12570260, which is their latest version of the flexible fuel sensor. The list price is somewhere around $500 (!), a few online parts places has these for $360 or so. However, if you do a search on eBay for "flex fuel", there is a sensor from seller Medimars,who is offering brand new GM sensors (number #12568450, for a 2001 S-15 truck) for $50.00 plus shipping. It operates the same as the other part number sensor. This latter number has been discontinued by GM, replaced by the former number for most vehicles.

Sensor Vehicle Applications

(fleet vehicles only, selected vehicles)

Wiring the Sensor

The fuel sensor uses 'quick-connect' fuel connectors. The connectors have different diameters to prevent incorrect attachment of the fuel lines. All fuel passes through the fuel composition sensor before reaching the fuel rail. The fuel composition sensor measures two fuel related parameters (ethanol percentage and fuel temperature).

The fuel composition sensor has a three-wire electrical harness connector. The three wires provide a ground circuit, a power source, and a signal output to the PCM. The power source is vehicle system voltage, +12 volts), and the ground circuit connects to chassis ground.

The sensor connections, looking into the sensor, from left to right, are:

Note that the output needs to be tied high through a 10K resistor (open-collector). You can use the proto area (on a V3 main board) for the resistor/pull-up circuit. The JP4 jumper is on MS-II itself, and you will need to be able to disconnect the lead to that jumper to remove MS-II.

Note that you can use other DB37 pins:

The electrical connector for this is GM #12126121, with pins GM #12048074, or you can just get the pigtail connector GM #88987992 (or AC Delco PT 1795).

MegaSquirt® and MicroSquirt® controllers are experimental devices intended for educational purposes.
MegaSquirt® and MicroSquirt® controllers are not for sale or use on pollution controlled vehicles. Check the laws that apply in your locality to determine if using a MegaSquirt® or MicroSquirt® controller is legal for your application.
©2005, 2010 Bruce Bowling and Al Grippo. All rights reserved. MegaSquirt® and MicroSquirt® are registered trademarks. This document is solely for the support of MegaSquirt® boards from Bowling and Grippo.