A method cleaning up internal components of a fuel injector for a diesel engine. The method includes operating a fuel injected diesel engine on a fuel composition that includes a major amount of diesel fuel and from about 5 to about 500 ppm by weight of a reaction product derived from (a) a hydrocarbyl substituted dicarboxylic acid, anhydride, or ester and (b) an amine compound or salt thereof of the formula wherein R is selected from the group consisting of hydrogen and a hydrocarbyl group containing from about 1 to about 15 carbon atoms, and R1 is selected from the group consisting of hydrogen and a hydrocarbyl group containing from about 1 to about 20 carbon atoms. The reaction product is characterized by a particular FTIR spectrum.