Putting sugar in a diesel fuel may not cause any damage because it does not dissolve in the fuel. Instead, the sugar will sink and settle to the bottom of a container, according to the I-CAR website. Sugar may affect fuel filters.
Fuel filters are designed to prevent contaminants from reaching a car engine. If the vehicle is started up, any sugar in the fuel tank may clog the filters and perhaps the fuel pump and injectors, preventing the engine from operating.
Vandals sometimes put sugar into a car's fuel tank believing it will turn into sludge inside the engine. This does not happen, but to check for contamination in the fuel, a mechanic typically inspects the filters or disconnects the fuel line, pumps a sample of fuel into a clear glass container and examines it after it settles.
Cleaning the Tank
The fuel tank and fuel line must be cleaned out and plugged filters replaced if they are clogged with sugar. Fuel injector cleaners can be used for engines with an injection system.