Abstract: There are occasions when an intermittent abnormality occurs in a water temperature sensor that is of a type that does not cause a sudden change in a detection value of the water temperature sensor. The movement of the detection value of the water temperature sensor when this type of abnormality occurs may be, for example, a movement in which the detection value of the water temperature sensor becomes lower even through warm up has been proceeding after engine start up has begun. When this type of movement of the detection value occurs, if the detection value of the water temperature sensor reduces, after engine start up has begun, to a value lower than that when start up began, it is immediately determined based on this fact that an abnormality of the water temperature sensor is occurring.

Abstract: An engine ECU executes a program including the steps of: detecting an air intake; subtracting an air fuel ratio detected by an upstream air fuel ratio sensor from that detected by a downstream air fuel ratio sensor; using the calculated air fuel ratio difference to determine a range for a lower GA range; determining a range for a higher GA range; and determining from the combination of the determined ranges of the lower and higher GA ranges whether a section A's exhaust system operates normally or has abnormality.

Abstract: There are occasions when an intermittent abnormality occurs in a water temperature sensor that is of a type that does not cause a sudden change in a detection value of the water temperature sensor. The movement of the detection value of the water temperature sensor when this type of abnormality occurs may be, for example, a movement in which the detection value of the water temperature sensor becomes lower even through warm up has been proceeding after engine start up has begun. When this type of movement of the detection value occurs, if the detection value of the water temperature sensor reduces, after engine start up has begun, to a value lower than that when start up began, it is immediately determined based on this fact that an abnormality of the water temperature sensor is occurring.

Abstract: An engine ECU executes a program including the steps of: detecting an air intake; subtracting an air fuel ratio detected by an upstream air fuel ratio sensor from that detected by a downstream air fuel ratio sensor; using the calculated air fuel ratio difference to determine a range for a lower GA range; determining a range for a higher GA range; and determining from the combination of the determined ranges of the lower and higher GA ranges whether a section A's exhaust system operates normally or has abnormality.

Abstract: A boost pressure control system for a vehicle engine equipped with a supercharger and a torque converter having a lock-up clutch, in which the boost pressure is controlled by adjusting the degree of the opening of an air bypass valve and the ON/OFF position of the supercharger in such manner that the different control characteristics of the boost pressure are selected in accordance with whether the lock-up clutch of the torque converter is turned ON or OFF, i.e., when the lock-up clutch is turned ON, the supercharger is started at a lower engine load, and the degree of the opening of the air bypass valve is set smaller, thereby enabling the lock-up operation of the torque converter under running conditions in which a higher driving torque is required and, thereby widening the operating range of the lock-up operation of the torque converter and improving the fuel economy of the vehicle.

Abstract: A boost pressure control system for an engine, by which a boost pressure of the engine is controlled in accordance with a load of the engine. The system controls the boost pressure in two different control modes, i.e., an acceleration mode and a normal mode, and when a degree of acceleration required becomes larger than a predetermined value, the acceleration mode is selected in which the boost pressure is made higher than in the normal mode. To avoid a sudden drop of the boost pressure, the system maintains the acceleration mode even after the completion of the acceleration, until the engine load becomes low enough that the boost pressure drops to the level at which the switching to the normal mode does not cause a sudden pressure drop.

Abstract: A boost pressure control system for an engine, by which the degree of opening of the air bypass valve is adjusted and on/off of a mechanical supercharger operation is controlled to obtain an appropriate boost pressure. In the system, the degree of opening of the air bypass valve is adjusted in accordance with the load of the engine, based on different characteristics selected in accordance with whether the operation of the supercharger is started or stopped. If the engine is operated in a non-boost region, when the supercharger is operating the degree of opening of the air bypass valve is made larger to reduce the compression work of the supercharger, and when the operation of the supercharger is stopped, the degree of opening of the air bypass valve is made smaller to ensure that the air flow through the supercharger maintains the rotation of the rotors of the supercharger.

Abstract: A diagnostic system of an air-fuel ratio control device including an O.sub.2 sensor and an air bleed control valve actuated in response to a signal output by the O.sub.2 sensor. When the electric control current fed into the air bleed control valve reaches an upper limit or lower limit and remains at the upper limit or lower limit for a fixed time, the diagnostic system determines that the air-fuel ratio control device has malfunctioned. When the temperature of the engine cooling water is high, a diagnosis is prohibited, to prevent an incorrect diagnosis.