A method of substantially removing, or reducing the occurrence of, injector deposits in a diesel engine operated using a diesel fuel containing a minor amount of a metal-containing species is disclosed. The method includes adding to a diesel fuel the reaction product between a hydrocarbyl-substituted succinic acid or anhydride and hydrazine. The diesel engine is equipped with fuel injectors having a plurality of spray-holes, each spray-hole having an inlet and an outlet, and the fuel injectors have one or more of the following characteristics: (i) spray-holes which are tapered such that the inlet diameter of the spray-holes is greater than the outlet diameter; (ii) spray-holes having an outlet diameter of 0.10 mm or less; (iii) spray-holes where an inner edge of the inlet is rounded; (iv) 6 or more spray-holes; (v) an operating tip temperature in excess of 250° C.