A control system for a mixture-compressing internal combustion engine with a throttle flap valve disposed in an intake manifold and an idle channel bypassing the throttle flap valve. An additional channel, terminating downstream of the idle channel in the intake manifold, with the additional channel merely forming an air channel. A valve is provided for controlling a communication between the additional channel and the intake manifold. The valve is connected with a diaphragm a back surface of which is adapted to be exposed to the intake manifold vacuum through a line connected to the intake manifold. A three/two-way on-off valve, influenced by a control device, is disposed in the line connected to the intake manifold. The on-off valve is adapted to vent the backside of the diaphragm when the intake manifold vacuum is above an idle value during a coasting of the engine so as to enable air to be introduced into the intake manifold through the additional channel.