I brought my car to the dealer on Friday for its 10k miles service. When I got the car back, I noticed that they had reset everything, indicating that they had the battery disconnected.
One thing that having the battery disconnected does is reset the ECU. Everything that the ECU "learned" about running on E85 (short term and long term fuel trims, in particular) was lost.
Seeing as I was running at 38% ethanol, I was worried that I'd get a CEL.
Well, on the way to work today, I got one!
Damn! This is a setback. Luckily, I'm within a day or so of a new tank. But I'll have to back off the ethanol content down to 30%. This is going to set me back weeks.
On the positive side, this shows that the ECU can adapt to higher ethanol content as long as you increase the ethanol slowly and give it time to learn and adapt.
Also, I think that this shows that the CEL is set when the difference between STFT and LTFT becomes too great. Again, by slowly increasing the amount of ethanol in the tank, you can overcome this problem.