Guys, forget the 555 timer, too old, will drift with temp. What this really needs is a nice little 8 pin AVR or PIC micro to generate the signal, it would cost about $2.50 for all the parts (if that).

Early on when I was playing around with 'fooling' the ECM in to using the ethanol tables for an LPG application I had suggested to the shop that we were testing with that we make a board that simulates the ethanol signal.
For this LPG application it was decided that on the fly variable ethanol % was not needed, I mean, you are on LPG or you are not! And adding in to the custom OS the option to force the ethanol percentage was much easier.

As a project this could go two ways, a simple dial in the car with some markers on a label to show the percentage of ethanol (cheap), or, something with a 7 segment display showing the percentage selected (expensive).

Lets all be thankful that GM left that legacy code to use a real ethanol sensor even though they have never released a car using the E38 and a flex fuel sensor.

Cheers,
Ross