The engine usually takes a few fills to clear the fog. When there's a lot of players, it's possible people have already been filling it, so it can take you only one fill or two because people have been helping you. When there aren't a lot of people logged on, you need to fill it about 4-5 times, I think?
Just for reference:
The Town Square's Engine opens the Junkyard.
The Engine Bay's Engine opens the Greenshade Caverns.
The Northern Slurries' Engine opens the Abandoned Labs.