To prevent or inhibit the formation of a deposit in an engine, esp. in an intake valve, by adding a novel fuel-sol. polylactone arom. ester to a fuel.

A polylactone represented by formula I (wherein R1 is 1-100C hydrocarbyl; R2 is 2-5C alkylene; and x is 1-25) is reacted with an acyl halide represented by formula II (wherein R5 and R6 are each H, hydroxyl, nitro, amino, or 1-6C N,N-dialkylamino provided they are not simultaneously H; and Z is halogen) in an inert solvent at about 25-150°C to give a fuel-sol. polylactone arom. ester represented by formula III (wherein R3 and R4 are each the same as R5 and R6) having a mol.wt. of about 250-5,000. The obtd. polylactone arom. ester is added in an amt. of 50-2,500ppm by wt. to a hydrocarbon fuel to prevent or inhibit the formation of a deposit in an engine.