I think for more modern vehicles that can run on E85 or traditional gasoline, they make the same miles per gallon? Something about able to advance timing to take advantage of the high octane fuel making more power per stroke and increase MPG's. Obviously our antiquating 4Runners don't have anything like that.
I have heard of complete E85 swaps like someone else is doing that did not replace the fuel pump and the fuel lines. There's very few rubber parts along that path so it wouldn't be terribly hard to convert them either to stainless steel or some other flexible line. Those would be the only two components that would not be able to handle the increased fuel needs of having to compensate for E15.
The ECU will read it like this: because E15 has less energy density it'll start to run a little lean. The oxygen sensor (or 2001 later vehicles, Air Fuel Ratio sensor) will immediately increase fuel trims to compensate. I don't see a scenario where it would ever increase the long term fuel trims over 30% and trip a code, I don't think even E85 would do that. Over time, the ethanol
could start to cause the rubber to deteriorate to the point of causing fuel delivery options. The pump should continue to operate as it's always on at the same rate, it doesn't scale with throttle. I do not think that fuel pressure will drop as the 4Runner doesn't max out the pump if naturally aspirated.
Overall, I believe based on my work on the 5VZ platform we should be fine other than the obvious MPG drop as it's less energy dense fuel. California has been E10 only for about 5-6 years now and I have not had any problems except for watching my MPG drop. Below is a chart of the drop, you can see the baseline go down in 2017. It'll be less severe with a 5% add instead of 10% add but you get the idea.
