That's it!

Marty McFly:

Bartender says that's the strongest stuff they got. [Doc begins to pour alcohol into the DeLorean's gas cap] Doc:
Try it, Marty. [Marty is trying to start the engine in the DeLorean. The engine begins to sorely start as he keeps turning the keys to work the engine] Doc:
Give it more gas... [the DeLorean sounds as if its about to start until the fuel-injection manifold blows apart from the car with a lot smog as the car dies; Doc picks it up] Doc:
Damn! It blew the fuel injection manifold. Strong stuff alright, it'll take me a month to rebuild it. Marty McFly:
A month? Doc, you're gonna get shot on Monday! Doc:
I know, I know...! Wait, I've got it! we can simply roll it down a steep hill... no, no, we'd never find a smooth enough surface. Unless... of course, ice! We'll wait until winter, when the lake freezes over we'll... Marty McFly:
Winter? Doc! Monday, it's three days away! Doc:
Okay, okay, let's think this through logically... we know that it won't run under its own power and we know we can't pull it, but if we could find a way to push it up to 88 miles per hour... [a whistle blows in the distance and Doc looks out the window at the arriving train] Doc:
That's it!