What damage could be done to a engine if E85 was put in a 2003 Chevy Astro van - about 15 gallons?

Answer 1

In all honestly very very likely none.

To ensure it, avoid heavy throttle application, the sooner you can get a significant amount of gas in the tank the safer. E85 or pure ethanol is NOT near as corrosive as people are led to believe. Methanol on the other hand is pretty rough stuff to some materials. But those materials are often very rare in most fuel injection systems.

Stoichiometric A/F ratio for pure gasoline is 14.67:1, for E85 it is 9.67:1. So the fuel injectors need to be able to deliver approx 50% MORE fuel at WOT (Wide Open Throttle) than the standard gas injectors.

The stock lambda O2 sensor will properly control the fuel mixture to maintain "stoich"

keeping the computer making proper fuel mixture.

Most cars were already safe to use ethanol fuel since the early 80's. But being the carbs or fuel injection cannot supply ENOUGH fuel at WOT or heavy throttle, it can cause damage at WOT due to running lean. Being 15 gallons is not full tank, you are probably running around 65-70% ethanol and the rest gas.

You very likely will get a P0171 and/or P1074 fault (lean bank 1 and lean bank 2). Once you get more gasoline in the tank, after 2 diving cycles it will clear and after 40 or so driving cycles without the fault it will be cleared from the history.