re: Back to the Future: Part 2-3 Timeline issue...

This is from the BTTF FAQ.

LINK


1.20: In I885, when Marry tells Doc they're out of gas; why don't they just go to the Delgado Mine, dig up the DeLorean where Doc hid it and get the gas out of it?

A: There are two logical answers to this one…

The car mechanic's answer: As anyone who has stored an automobile for a long period of time can tell you, you always drain all of the fluids out of the car before putting it into storage. Doc most certainly would have drained the gas out of the DeLorean if he was going to leave it hidden for 70 years. At the 1955 drive-in, Doc specifically says "I put gas in the tank" indicating that the DMC must have had an empty tank when they found it in the mine.

The time travel theory answer: Even if Doc had not drained the tank, he still would not have gone back into the mine for fear of creating a time paradox by accidentally damaging the DeLorean, the mine, or who knows what. After all, since Marty is now back in 1885, Doc's plan obviously worked, and worked perfectly. But what if Doc were to go back into the mine and accidentally cause a cave-in that causes even more damage to the DeLorean? What happens to the future of that DeLorean, when it's unearthed in 1955? And what might that do to Marty and the undamaged future DeLorean now in 1885? As an analogy, imagine a time traveler going back in time, finding himself as a child, and cutting off that child's hand with a meat cleaver. What happens to the adult time-traveler's hand? That would definitely risk a time paradox, and we know that Doc would never go out of his way to risk such a thing for fear of (in the worst case scenario) unraveling the fabric of the space-time continuum and destroying the entire universe.