Back to the Future III: 2 Time Machines in 1885
When Marty went back to 1885 to rescue Doc, they had to find a way to get the time machine up to 88mph without any gas which Doc said wouldn't be available to them for years, but at the time, there were 2 time machines; there was the one that Marty used to go from 1955 to 1885 and the same time machine hidden in the cave that Doc ensured would be available to Marty in 1955 so he could go home.
They had to leave one of the time machines in the cave so that the Marty stuck in 1955 could use it to get out and had to use the other to get out of 1885 before Bufford killed one of them, but if gas was the main problem, couldn't they have siphoned whatever gas was left in the time machine Doc hid in the cave or used parts from one to repair the other while leaving the other in good enough condition that the 1955 Doc could repair for Marty?