The use of an olefin-polymerizable carboxylic acid copolymer, wherein the copolymer comprises at least one free carboxylic acid side group in protonated form or in salt form, i) wherein the copolymer is obtainable by copolymerizing a) at least one ethylenically unsaturated, polymerizable polycarboxylic anhydride with b) at least one polymerizable olefin; and then derivatizing the copolymer from step by partial or complete reaction of the anhydride radicals of the copolymer from step with water, or at least one hydroxyl compound, or at least one primary or secondary amine, or mixtures thereof; to form a copolymer derivative containing carboxyl groups; or ii) wherein the copolymer is obtainable by copolymerizing a) at least one ethylenically unsaturated, polymerizable mono- or polycarboxylic acid with b) at least one polymerizable olefin; and then derivatizing the copolymer from step by partial reaction of the carboxyl radicals of the copolymer with at least one hydroxyl compound, at least one primary or secondary amine; or mixtures thereof, to form a copolymer derivative having a reduced content of free carboxyl groups; or iii) wherein the copolymer is obtainable by copolymerizing a) at least one ethylenically unsaturated, polymerizable mono- or polycarboxylic acid with b) at least one polymerizable olefin; as a diesel fuel additive for reducing and/or preventing internal diesel injector deposits (IDIDs) in direct injection diesel engines. i) wherein the copolymer is obtainable by copolymerizing a) at least one ethylenically unsaturated, polymerizable polycarboxylic anhydride with b) at least one polymerizable olefin; and then derivatizing the copolymer from step by partial or complete reaction of the anhydride radicals of the copolymer from step with water, or at least one hydroxyl compound, or at least one primary or secondary amine, or mixtures thereof; to form a copolymer derivative containing carboxyl groups; or copolymerizing a) at least one ethylenically unsaturated, polymerizable polycarboxylic anhydride with b) at least one polymerizable olefin; and a) at least one ethylenically unsaturated, polymerizable polycarboxylic anhydride with b) at least one polymerizable olefin; and then derivatizing the copolymer from step by partial or complete reaction of the anhydride radicals of the copolymer from step with water, or at least one hydroxyl compound, or at least one primary or secondary amine, or mixtures thereof; to form a copolymer derivative containing carboxyl groups; or ii) wherein the copolymer is obtainable by copolymerizing a) at least one ethylenically unsaturated, polymerizable mono- or polycarboxylic acid with b) at least one polymerizable olefin; and then derivatizing the copolymer from step by partial reaction of the carboxyl radicals of the copolymer with at least one hydroxyl compound, at least one primary or secondary amine; or mixtures thereof, to form a copolymer derivative having a reduced content of free carboxyl groups; or copolymerizing a) at least one ethylenically unsaturated, polymerizable mono- or polycarboxylic acid with b) at least one polymerizable olefin; and a) at least one ethylenically unsaturated, polymerizable mono- or polycarboxylic acid with b) at least one polymerizable olefin; and then derivatizing the copolymer from step by partial reaction of the carboxyl radicals of the copolymer with at least one hydroxyl compound, at least one primary or secondary amine; or mixtures thereof, to form a copolymer derivative having a reduced content of free carboxyl groups; or iii) wherein the copolymer is obtainable by copolymerizing a) at least one ethylenically unsaturated, polymerizable mono- or polycarboxylic acid with b) at least one polymerizable olefin; as a diesel fuel additive for reducing and/or preventing internal diesel injector deposits (IDIDs) in direct injection diesel engines. a) at least one ethylenically unsaturated, polymerizable mono- or polycarboxylic acid with b) at least one polymerizable olefin; as a diesel fuel additive for reducing and/or preventing internal diesel injector deposits (IDIDs) in direct injection diesel engines.