...or something like that.

I let a friend take it for a day and he out of habit put e85 fuel in it on the way back home to me because it is what he uses in his very tuned 335i. The car was barely over half a tank when he took it, and about 5/8 when he brought it back. He says he put in $20 worth, which as our local E85 prices of $2.59, leads to about 8 gallons.

Now, he did this at a gas station about 3 miles from my house, and claims it ran fine on the way home. I also found it to drive just fine, other than the fuel rates on the DIC being very bad. I was very careful with it and ran it down to about 3/8 tank of fuel and then topped it off with BP 93 gasoline.

Mileage numbers stayed bad over the next 100 miles or so, but it continued to run fine.

After parking it for about 6 hours, I started it to head home and it ran quite badly if pushed past about a quarter throttle and 2300 RPM. I topped it off again with BP 93 fuel (about 8 gallons) and the problem persists. I have since disconnected the fuel line and extended it into a gas can, jumpered the fuel pump relay, drained the tank to E on the gauge, and added 20 gallons of new BP 93 fuel. The problem remains. Hoping for a service engine light soon so that I can chase this further.

Anyone have any ideas?

I have never seen an accidental misfueling do this.