In Back to the Future Part 3 Marty follows Doc back to 1885 to rescue him from being shot at the hands of Buford 'Mad Dog' Tannen. He arrives in 1885 some time after Doc has written the letter that Marty receives in 1955. Unfortunately for them the DeLorean springs a leak and looses fuel which leaves them short of a way to get the DeLorean up to 88mph, the Mr Fusion only powers the time circuits, but the engine runs on standard gasoline and always has.
Why don't they just use the other DeLorean?
There are now TWO DeLoreans in 1885. The one Marty arrived in and the one Doc has hidden in a mine with repair instructions for his 1955 counterpart. Why don't they just syphon the fuel from that DeLorean.
This may be a plot hole, but there may be perfectly logical explanation for it.