PURPOSE: To prevent an engine from stall due to overlean air fuel ration in low speed vehicle by controlling a secondary air amount in an intake system according to the change speed of running parameter showing the speed reduction condition in the speed reduction and low vehicle speed so long as an automatic transmission is not in gear.
CONSTITUTION: To a control circuit 20 are input values detected by a negative pressure switch 7, absolute intake pressure sensor 10, crank angle sensor 11, water temperature sensor 12, intake temperature sensor 13, O2 sensor 14, vehicle speed sensor 16, neutral switch 19, etc. When the control circuit 20 judges speed reduction on the bass of the change amount of absolute intake pressure in the cooling water temperature and vehicle speed higher than predetermined ones, it controls the opening of a proportional electromagnetic valve 9 provided in a secondary intake air supply path 8 on the basis of the change amount of absolute intake pressure. When an automatic transmission is however in gear within a low vehicle speed region, the control of proportional electromagnetic valve 9 in the speed reduction is stopped to close said valve.
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