A device for affecting operational magnitudes of an I.C. engine having exhaust gas return includes a control device for determining the amounts of fuel to be supplied and of exhaust gas to be recirculated. A control valve in the exhaust gas return conduit is activated to switch in rapid succession the exhaust gas return on and off. Corresponding pressure changes in the intake pipe are detected by a single pressure sensor whose output signals are applied to the control device which uses the difference of the two pressure signals for modifying the amount of fuel and recirculated exhaust gas.