The valve and spring/coil assemblies not shown. A valve pulses and admits vacuum to the mechanical EGR valve according to engine speed and load. It is controlled by a 555 timer circuit and a throttle position sensor. When no EGR is needed, for example, during acceleration, the valve closes and vacuum is bled from the EGR valve via a bypass. This system is far more accurate than normally, especially considering manifold vacuum behaves differently when using a high-efficiency fuel system.
