Abstract: A diagnosis device for an exhaust gas recycling device diagnoses whether or not an exhaust gas recycling device is operating normally; the temperature of an exhaust gas recycling passage is detected by a temperature detection means, a decision is made by an exhaust gas recycling operating region detection means whether or not the operating region is such as to carry out exhaust gas recycling, when a decision is made that the operating region is such as to carry out exhaust gas recycling a counter is incremented and when otherwise is decremented, and a decision that the exhaust gas recycling is not being carried out normally, that is that the exhaust gas recycling device has failed, is made when the count value of this counter is at least a certain value and the temperature of the exhaust gas recycling passage is not more than a certain value.

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.

Abstract: A diagnosing apparatus for diagnosing first and second vacuum switches and a throttle switch of an exhaust gas purification apparatus. The diagnosing apparatus includes first and second vacuum switches, a throttle switch, an engine speed sensor, and a computer. The second vacuum switch outputs a signal when the value of vacuum is greater than 80 mm Hg vacuum. The second vacuum switch outputs a signal when the value of vacuum is less than 350 mm Hg vacuum. The throttle switch outputs a signal when the throttle valve is open more than 9 degrees. The engine speed sensor detects an engine revolution number. A predetermined relationship is stored in the computer of the diagnosing apparatus and the relationship includes at least three ranges or areas.

Abstract: A method for controlling intake flow of an internal combustion engine having an intake port including a helical passage and a straight passage both extending towards a common port end opening which opens to the combustion chamber of the engine, in such a manner that a minimum proportion of intake flow is directed through the straight passage during idling operation of the engine regardless of the temperature condition of the engine, a maximum proportion of intake flow is directed through the straight passage during loaded operation at any non idling load of the engine when the engine is not warmed up, and a proportion of intake flow increasing according to the level of engine load is directed through the straight passage during loaded operation of the engine when the engine has been warmed up. This method is performed by an intake passage switchover control valve system operated conveniently by utilizing intake vacuum under a proper switchover thereof which itself incorporates the engine load performance.

Abstract: An ignition timing control system for an engine using a vacuum advance unit connected to a distributor and communicated with an intake passage of the engine, which comprises: a throttle valve plate received within the intake passage; first and second ports formed in the wall of the intake passage; conduits connecting the first and second ports with the vacuum advance unit; and heat-sensitive valve means disposed in the conduit connecting the second port and the advance unit. The first port is located upstream of the valve plate and exposed to the substantially atmospheric pressure while the valve plate remains in its idle position, but exposed to the vacuum when the valve plate is opened from the idle position. The second port is kept located downstream of the valve plate and exposed to the vacuum even when the valve plate is opened to its fast-idle position. The second port is exposed to the substantially atmospheric pressure when the valve plate is further opened from the fast-idle position.