I have a 98 mustang gt, I had a 91 tune added I also had my car filled up with 10% ethanol 91 octane fuel, when I got the 91 tune.. now my car doesn’t run right when I fill up with 100% 91. It’s consistent but I can restart my car an it clears out an will run better. But doesn’t take long for it to start again. It’s also only throwing codes with 100% 91. P0174 an p0171. I can put 91 with 10% an car will run better.