M System

After the X system has controlled air fuel mixtures to a nearly ideal ratio, the M system provides a finer control based on engine load. The M system consists of air control valve A, frequency solenoid valve A, control unit, and a check valve on 1986 Accord models.
The diaphragm chamber of air control valve A receives the same carburetor vacuum as the EGR valve, and air is supplied to the intake manifold in proportion to the amount of air entering the carburetor. Frequency solenoid valve A is controlled by the electronic control unit based on signals from the oxygen sensor. When the oxygen sensor indicates that air/fuel mixtures are too rich the control unit operates the frequency valve, which in turn applies additional vacuum to air control valve A in order to increase the valve opening.