Engine Control Module (ECM)

The Electronic Control Unit (ECU) uses all of the engine and vehicle operations sensors to determine the proper amount of fuel to be delivered to maintain the correct air-fuel ratio to provide the minimum exhaust emissions. The ECU also controls (part or all of) the operation of the secondary air, electric choke, idle speed and distributor advance control systems to enhance driveablity.
The air-fuel ratio is maintained by the ECU in one of two operating modes.

OPEN LOOP CONTROL (NO FEEDBACK CONTROL):
During engine start, warm-up, high load operation and deceleration, the air-fuel ratio is controlled in open-loop operation. The ECU operates based on pre-programmed map values, for engine speed, throttle valve opening angle and engine coolant temperature. During deceleration, the Slow Cut Solenoid Valve (SCSV) limits fuel flow for better fuel economy and for prevention of overheating of the catalysts. Map value is a value previously established and stored in the Read Only Memory (ROM) of the ECU.

CLOSED LOOP CONTROL (FEEDBACK CONTROL):
After engine warm-up, the air-fuel ratio is controlled by computations based on the feedback from the oxygen sensor signal. The oxygen sensor output voltage changes sharply at the stoichiometric ratio. The control unit senses this oxygen sensor signal and provides feedback control to the Feedback Solenoid Valve (FBSV), maintaining the stoichiometric ratio that will produce the best purification rate in the 3-catalyst converter. In this state, the Slow Cut Solenoid Valve (SCSV) is kept wide open (100% duty). When the FBSV is energized, the main fuel jet in the FBSV is closed, leaving the primary main jet passage as the only fuel metering passage. This reduces the amount of fuel, resulting in leaner air-fuel mixture. When the FBSV is de-energized, the main fuel jet in the FBSV is opened, providing two fuel metering passages (including the primary main jet passage). Since this will increase the amount of fuel, a richer air-fuel mixture is obtained. With the ON-OFF operation of the SCSV, the slow (idle) fuel passage is opened and closed similarly. The air-fuel ratio at deceleration is controlled in this manner.