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In the third Back to the future movie, Marty digs up a 70-year-old Delorean in a mine, and uses it to go back to 1885. Then, upon arrival, he gets his fuel tank pierced and it leaks all the fuel, trapping him in that time period.

Other questions have mentioned that they might have been able to siphon the fuel from the other Delorean waiting in the mine, but many reasons seem to prevent this course of actions.

But what about preventing the leak itself? They can see the hole the Arrow left, therefore know the exact location the Arrow would hit the car. They would only need to add a very small piece of metal on that exact spot to prevent the Arrow puncturing the car, and neither marty nor 1955-doc knows the new-model delorean enough to risk messing with it by removing that extra piece. They probably wouldn't even notice it. There is no paradox, as the 1955 team wouldn't do anything differently, the metal can be small enough to have no effect on anything (weight, aerodynamism, etc), so everything should play out the same, except for their own delorean to suddenly be full again.

BTTF already established its time travel rules with the possiblity to change in real time objects and events (Marty losing his hand in the first movie), therefore it would not be surprising to see fuel suddenly appearing in their own car.

Other options would also exist, like modifying the car to have a hidden backup fuel tank that automatically fills up with the main one, but isn't used, thus making them "find" this backup tank on their own car right away.

I'm mostly wondering if, and why, doc didn't mention any timey-wimey options, and went straight to the unlikely method of moving that fast with 100 years old tech.

He even invented long-burning chemical logs for the train, he should have at least tried something easier first in my opinion?

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    At a guess, since the whole thing is rather slapdash Mad Science and not something they really understand, they were still under the assumption that they shouldn't twist timelines around like a pretzel. – Radhil Feb 21 '17 at 17:17
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    I'd say that the challenge was not in repairing the tank, but finding and drilling for oil, and refining it into gasoline. They didn't have the time to do that much. – VBartilucci Feb 21 '17 at 19:55
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    It's not about repairing the tank. What I said was about preventing the tank being damaged in the first place, therefore having the fuel reappear like Marty's hand disappeared in the first movie. – Kaito Kid Feb 21 '17 at 20:30
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    Just in case anyone else is confused like I was, the question is asking why didn't 1885 Doc put extra plating on the gas tank of the DeLorean that he (Doc) arrived in due to the lightning strike. That way, when Marty and 1955 Doc pulled that car from the mine, it would be protected from the arrow before Marty ever traveled to 1885. The arrow would not pierce the gas tank and would remain full. – Alarion Feb 21 '17 at 20:48
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    He's not thinking 4th-dimensionally. – Bishop Feb 21 '17 at 22:20
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Why they didn't

Doc was at that point still very much against disturbing the timelines. He was stuck in 1885, but instead of asking Marty to come fetch him, he urged him to return to 1985 and destroy the time machine.
He was also adamant he would not fall in love in 1885.

He has hidden the DeLorean, which has worked, since Marty is now in 1885. Going back to the hidden DeLorean to add a piece of metal plating is exactly the sort of thing that could disturb a great deal, causing Marty to disappear from the timeline that landed him in 1885.

Whether it would have worked

It may not have worked. Remember that they didn't have that much time before Doc's date of death. As we've seen, the timeline does not change instantly. In the first film, it took a while for the McFly-children in the photograph to fade. Had they added a reinforcement to the DeLorean, the ripples in the timeline may not have been fast enough to reach them in time to prevent Doc's death.

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    I didn't think about the time-delay (that kind of delay makes no sense at all yet many time-travel stories have it and it makes me mad haha), but you make a very good point. Indeed, that seems like the most important factor when considering ANY plan that would involve using time travel indirectly (even without touching the delorean, like sending a second letter to 1955), and in that case it really is a huge risk since Doc is dying very soon. Good catch! – Kaito Kid Feb 21 '17 at 21:46
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    Also, BTTF 2's "alternate timeline" system contradicts BTTF 1's "overwritten timeline" system, so I think Doc probably realized he didn't really know how all of that worked and decided not to mess with it, – Liesmith Feb 24 '17 at 1:21
  • It may not necessarily contradict if we imagine that time is moving forward and going into the future allows us to see the changes there immediately. If we are in the past our relics from the future need time to propagate backwards. There's probably holes in my thinking but it could be "reasonable" that items from future timelines that no longer exist take time to disappear if they're brought to the past, but jumping to the future the timeline changes update quicker (but not instantaneously since Biff was able to return the Delorean to Marty in BTTF 2). – Phil Patterson Feb 24 '17 at 15:45
  • @Liesmith: How does it contradict? I assumed that the "restored" timeline in BTTF2 is not the original one, but a newly generated one (caused by Doc and Marty's actions) that simply ends up with the same sequence of events as the original one (but technically it is different). – Flater Jun 21 '17 at 14:41
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    @Liesmith: If you take that as inescapable truth, then Doc and Marty in the future would have ceased to exist immediately when old man Biff arrived in the past. There would be no attempts to change the past (which is the entire plot of the movie) as no one remembers what things used to be like. The movies showed a slight lag between a change, and the timeline actually changing. E.g; the slowly fading photograph. Assuming Biff returned while that delay was still ongoing, the new timeline would not have activated immediately. – Flater Jun 22 '17 at 7:33

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