Flex Fuel

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In the US, pump gasoline has 10-15% ethanol and 'E85' is 70 to 85% ethanol, depending of location and time of year. Flex fuel offers the ability to compensate for different fuel ethanol content by using either an ethanol content sensor (S300 V3) or ethanol content analyzer connected to the ECU, which is useful when switching from gasoline to ethanol (and vise verse) or when the E85 ethanol content is seasonally altered.


Analog (Ethanol Content Analyzer)

An ethanol content analyzer converts the output of an ethanol content sensor to an analog voltage proportional to the fuel ethanol content. This analog voltage is then input to the ECU so that it can determine the ethanol content. A suitable ethanol content analyzer is the Zeitronix ECA, which also needs a ethanol content sensor.



Ethanol content analyzer input

Specifies the ECU pin used for the analog input from the ethanol content analyzer.   While only two columns are needed for compensation, 0V and 5V must be present in the table to prevent any problems if the ethanol content analyzer voltage goes outside the normal voltage range.


Digital (s300 v3 only)

With the s300 V3 an ethanol content sensor may be connected directly to the s300 Analog Input 0.  No ethanol content analyzer is needed, as the S300 V3 converts the signal from the ethanol content sensor directly.  Suitable sensors are GM part numbers 13577394, 13577379, and 13577429.  These sensors produce a pulse width modulated output with a variable frequency as well, which ranges from 50 Hz for 0% ethanol to 150Hz for 100% ethanol.




Part numbers:

Ethanol content sensor GM 13577429
Plug & pigtail GM 13352241
3/8" fuel line to -6 adapter Russell 644123 x 2


The sensor should be wired in the return line from the fuel rail to the fuel tank.


Sensor wiring:

'Vcc'  -  power (9-18V)
'Gnd' - ground
'Out' - output to ECU.


Important: A 2.4 k ohm resistor needs to be wired between Out and Vcc.  You can do this anywhere in the wiring between the ECU and sensor.  Do not connect Out and Vcc together; one end of the resistor is wired to Vcc and the other end to Out.




Fuel Compensation




Open and closed loop fuel compensation are used in open and closed loop respectively.  Cranking fuel compensation is applied when starting the engine on top of the fuel compensation from the open loop fuel compensation table.

The exact compensation factors should be checked on a dyno with a wideband lambda meter.


Ignition Compensation




Specifies the extra ignition advance based on the ethanol content for both open and closed loop operation.


Boost Compensation




For boost control, the high and low boost tables can be used based on the ethanol percentage. See Boost Control Parameters.