You weren't running pure E85 but a mix of 4 gallons of of E85 to a tank that was otherwise full of gas? Is that correct? Was the regular gas Ethanol Blend (E10) or straight gas?
Assuming 17 gallon tank (I think that's what all the T-birds have right?) 4 gallons of E85 in a tank of normal gas would be 20% ethanol. 4 gallons of E85 in a tank of E10 would be 28% Ethanol. Either way, stoic for E85 is 9.8:1 compared to 14.7:1 of normal gas. E10 isn't enough to make you run lean because the car would run a bit rich from the factory anyway. But E20 or E28 or how ever much higher Ethanol content whatever you were running if you didn't have a full tank could be enough to cause a bit of a lean condition.
Former owner of 94 Bird with DOHC 4.6L swapped from a mark VIII
2011 CTS-V Wagon
1968 Torino was an original 289, C4, fully documented numbers matching car. Now: DOHC 4.6L/T-56 swapped, Cobra intake, IMRC delete, BBK long tubes and H pipe. Dynotech Aluminum Driveshaft, 9" rear with TrueTrac and 4.11's.