In this research, we developed control solutions for the problem of precisely adjusting the idle speed of a spark ignition internal combustion engine, using a recently developed Adaptive Posicast Controller (APC) for time-delay systems. The objective was to regulate the engine speed to a prescribed set-point in the presence of accessory load torque disturbances such as those due to air conditioning and power steering. The adaptive controller, integrated with the existing proportional spark controller, was used to drive the electronic throttle actuator. Following successful simulation studies, we conducted experiments using vehicles provided by Ford Motor Company. Experimental results showed dramatic performance improvements with the proposed control solution over the development controller.