About the flex fuel sensor:
I think Sean has already mentioned some of this, but here’s the best description I have seen so far on what the darn thing really sends to the “computer”:
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:
The frequency of the signal indicates the ethanol percentage. The output frequency is linear with respect to the percentage of ethanol content in the fuel. The PCM provides an internal pull-up to five volts on the signal circuit, and the fuel sensor pulls the 5 volts to ground in pulses. The normal range of operating frequency is between 50 and 150 Hertz:
50 Hertz indicates 0% ethanol, and
150 Hertz indicates 100% ethanol.
The pulse width indicates the fuel temperature. The normal pulse width is between 1 and 5 milliseconds:
1 millisecond indicates -40°C (-40°F), and
5 milliseconds indicates 125°C (257°F).
That should be plenty of info for folks who want to defeat/modify it using an Arduino, etc…
But it doesn’t look as though one could simply stick a potentiometer in the line to make changes.