Idle Control of Internal Combustion Engines (ICEs) is an important mode of Engine Management Systems (EMS) due to its effects on Fuel Consumption (FC) and pollutions produced in urban traffics. In this paper an optimal controller is used to control the idle speed with respect to minimization of FC in transient situations. In order to design an optimal controller a state space representation of engine in demanded. Subspace identification method is used to derive a proper state space model of engine around idle speed state. The required input/output data is generated using a mean value model. A finite time optimal controller is designed with the aim of minimizing both fuel consumption and speed fluctuations. The influence of finite time value on gross fuel consumption is then studied under various weighting factors. It is shown that the fuel consumption is sensitive to the controlling time duration for finite time LQR.