The performance of a closed-loop electronic fuel injection timing control system for diesel engines has been investigated, both experimentally and analytically. The Electronic Control System (ECS) studied is a version of the "Optimizer," a peak seeking control which can find the maximum of one variable with respect to another. In this case, it was used to find the timing for maximum brake torque (MBT). The ECS can also be operated in a "biased" mode in which it will hold the timing either advanced or retarded of MBT, but in a fixed relationship to it. Performance and emissions of a medium duty engine equipped with the ECS were measured on an engine dynamometer. The results clearly demonstrate that, for a variety of operating conditions and for two fuels, the ECS can find and hold the timing at MBT or in fixed relationship to it. BSFC, smoke, and NOx values obtained under ECS control were, in most cases, the same as those obtained with the timing set manually to the same mean value as that selected by the ECS. A computer model of the engine/ECS combination which successfully simulates the system under the conditions of the experimental tests is described. This model has been used to predict the influence of several ECS modifications on its performance.