What does "burning the midnight oil" mean?
Over the next nine months they tuned just about every moving part, replacing the suspension, radiator, and alternator, fitting it with front struts, a hatch spoiler and a seamless roll cage, and picking a proper array of rally tyres. It was quite an undertaking, but well worth the effort.
After burning the midnight oil for nearly a full year, making sure not a single loose screw could be found, the big day came.

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It means they worked late into the night. In this case it means they worked many late nights throughout the year.

Answer

"To burn the midnight oil" means to stay up late at night in order to work/study. The "oil" refers to the oils used in oil lamps.

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