Flexfuel - E85 - How does the ECM know?

[gmh308]

Hi team,

I have not managed to find the answer to this by searching, but was hoping someone may have some insight into how the E38 ECM (for example), knows when you have filled the tank with E85 or other E% fuel?

Is there a switch or a dial, or a sensor that figures this one out?

Thanks All.

[kbracing96]

I know the older E85 equipped vehicles have a fuel composition sensor, that the fuel passes though and it can tell how much ethanol content it has. But I have heard that GM did away with that last year and now it basically does it with "smart, learning" fuel trims and relearns them every time it sees you refuel the vehicle. It sees how far off the fuel is right after you refuel and then readjust to get the fuel back in line based off the o2's and from that determines the amount of ethanol is in the fuel and adjust the timing and stuff accordingly. That's my understanding of it anyhow.

[gmh308]

Thanks guys for your input on this question.