Fuel Control (Carburetted Models)
All models (except Skyhawk, Cimmaron, Cavalier & Firenza) are equipped with "feedback" carburetors which contain an electrically operated Mixture Control (M/C) solenoid. The M/C solenoid operates single or dual metering rods in float bowl. The rods meter fuel supplied too idle and main systems and vary air/fuel ratio within a pre-calibrated range. An idle air bleed circuit also operates in conjunction with metering rods.
The ECM, responds to inputs from data sensors and constantly adjusts air/fuel mixture. The 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 is cycled (turned on and off) at a rate of 10 times per second.
When ECM responds to signals received from oxygen sensor, the CCC system is in closed loop operation. Under certain operating conditions, ECM may ignore sensor and use pre-programmed instructions. Operating conditions which cause ECM to ignore oxygen sensor signals result in open loop mode. For example, during cold engine starts (engine speed below 200 RPM), M/C solenoid is turned off by ECM to provide a rich mixture.