Abstract: The present invention aims at preventing deterioration of driveability due to lean combustion, in a direct injection spark ignition type internal combustion engine. To this end, there is calculated a target engine-torque tTe which is to be generated by the engine, based on an engine driving condition, while detecting an actual engine-torque Te which is being actually generated by the engine. There is further calculated a deviation state quantity .DELTA.TQ=Te-tTe, which represents a deviation state between the target engine-torque and the actual engine-torque. Then, the lean combustion is inhibited when the deviation state quantity .DELTA.TQ is equal to or larger than a predetermined value. Namely, homogeneous lean combustion and stratified lean combustion are inhibited, and the combustion mode is switched to homogeneous stoichiometric combustion.

Abstract: A first passage provided with a first valve for guiding vaporized fuel from a fuel tank into a canister, and a second passage provided with a second valve connecting this canister with an intake pipe downstream or a throttle, are provided. A pressure detecting mechanism is provided between this first valve and second valve, and a third valve is provided for leading fresh air into the canister. When the engine running condition satisfies a predetermined positive pressure test condition, the second and third valves are closed and the first valve is opened. In this state, the presence or absence of a leak is determined from the pressure detected by the pressure detecting mechanism. In other words, it is determined for example that the fuel tank has no leak if the pressure rises above a predetermined value.

Abstract: A diagnostic system for an idle speed control valve is constructed to determine abnormality of the idle speed control valve by detecting a variation in an intake air amount produced upon forced opening and closing operation of the idle speed control valve.