Abstract: A diesel engine idle speed control system and method wherein a preset running characteristic map is corrected by a learning value updated on the basis of the output of a PID under predetermined conditions for each of different engine load modes, thereby determining a desired injection quantity. The map indicates a relationship between an engine speed and a fuel injection quantity for each of accelerator openings, including an accelerator opening of zero percent. A judgment is made on the present load mode of the engine, and a basic injection quantity corresponding to the present accelerator opening and engine speed is calculated from the running characteristic map. When the vehicle operating condition is in an idle mode, learning is executed so as to optimize a feedback correction quantity used for feedback control for each load mode, and a corrected basic injection quantity is calculated from the basic injection quantity and the learning value. Relearning is executed according to a predetermined condition.