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human
[Back To The Future III] why didnt Doc just use the fuel from the Delorean that he brought back to put into the delorean that Marty bought back.

I cannot remember if this was mentioned in the movie, but the reason they couldnt use the delorean to go back to 1985 from 1885 is because the fuel line broke and all the fuel spilt out, so why couldnt marty and doc dig up the delorean that brought doc back and drain the fuel from that one and take its fuel line and put it back in Martys delorean?
gpt
Doc was happy living his life in 1885 and wanted to stay there.  He says as much in his letter.  He left the DeLorean with the intent that Marty (and 1955 Doc) would repair the DeLorean in 1955 and send Marty back home to 1985, leaving Doc back in 1885.   He never intended (nor expected) Marty to come back to 1885 and get him.

1885 Doc wouldnt have been planning the need to refuel another motor vehicle, let alone another DeLorean.  That said, instead of leaving any amount of fuel to sit and waste away for 70 years (and it would have gone bad), he probably drained it himself and put it to some better use on his own, being the scientist that he is.