The optimum idle operating point for an internal combustion engine, in terms of fuel economy, emission levels, and idle quality, is a function of both the engine operating and environmental conditions. This paper reports on the development of an idle mode control system which adaptively adjusts the idle operating point in response to changes in these conditions. The control system described continually identifies the optimum idle operating point, defined in terms of engine crankshaft speed (fuel economy) and roughness level (idle quality), and maintains operation at this point under any set of operating conditions.The control system objective is to maintain idle operation at the minimum speed possible without adversely effecting idle quality or emission levels. This is accomplished by operating at an idle speed which results in the maximum acceptable level of engine roughness. The roughness level is determined by an on line measurement technique based on crankshaft speed fluctuations. The roughness set-point is modified adaptively to reduce the frequency of poor combustion events that give rise to increased levels of unburned hydrocarbons (HC). Thus, the idle mode control system strives to reach the best compromise among fuel economy, idle quality, and emission levels under any operating conditions.