Fuel Control Operation

California vehicles are equipped with "feedback" carburetors. These carburetors contain an electrically operated mixture control (M/C) solenoid. The M/C solenoid operates single or dual metering rods in the float bowl of the carburetor.

A metering rod supplements fuel supply to idle and main systems, varying air/fuel ratio within a pre-calibrated range. The M/C solenoid also controls air/fuel ratio through use of an idle air bleed circuit that operates in conjunction with the metering rod(s).

The ECM reacts to input from data sensors by constantly adjusting air/fuel mixture to maintain efficient engine performance. ECM controls M/C solenoid by providing a ground for solenoid.

When solenoid is energized, fuel flow through carburetor is reduced, providing a leaner mixture. When solenoid is de-energized, fuel flow through carburetor is increased, providing a richer mixture. The solenoid cycles (turns on and off) 10 times per second.

During closed loop operation, the ECM adjusts fuel mixture in response to signals received from the oxygen sensor. Under certain operating conditions, the ECM may ignore inputs from some sensors and substitute pre-programmed data to operate the engine.

During cold engine starts (engine speed below 200 RPM), M/C solenoid is turned off by the ECM. This provides a rich mixture to the engine. Under certain operating conditions, the ECM ignores oxygen sensor signals. When these conditions exist, the CCC system then operates in open loop mode.