GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Computer Controlled Catalytic Converter (C-4) System


The C-4 System, installed on certain 1979 and all 1980 cars sold in California, is an electronically controlled exhaust emission system. The purpose of the system is to maintain the ideal air/fuel ratio at which the catalytic converter is most effective. As the system is designed around feedback control of the air/fuel mixture, it was a predecessor of engine control systems that were soon to follow.

Major components of the system include an Electronic Control Module (ECM), an oxygen sensor, an electronically controlled carburetor, and a three-way oxidation reduction catalytic converter. The system also includes a maintenance reminder flag connected to the odometer which becomes visible in the instrument cluster at regular intervals, signaling the need for oxygen sensor replacement.

The oxygen sensor, installed in the exhaust manifold, generates a voltage which varies with exhaust gas oxygen content. Lean mixtures (more oxygen) reduce voltage; rich mixtures (less oxygen) increase voltage. The voltage output is monitored by the ECM which uses the information in engine control calculations.

An engine temperature sensor installed in the engine coolant outlet monitors engine coolant temperatures. Vacuum control switches and throttle position sensors also monitor engine conditions and supply signals to the ECM.

The Electronic Control Module receives input signals from all sensors. It processes these signals and generates a control signal sent to the carburetor. The control signal cycles between on (lean command) and off (rich command). The amount of on and off time is a function of the input voltage sent to the ECM by the oxygen sensor.

Rochester Dualjet (2-barrel) E2ME and E4ME (4-barrel) carburetors are used with the C-4 system. Basically, an electrically operated mixture control solenoid is installed in the carburetor float bowl. The solenoid controls the air/fuel mixture which is metered to the idle and main metering systems. Air metering to the idle system is controlled by an idle air bleed valve. It follows the movement of the mixture solenoid to control the amount of air bled into the idle system, enriching or leaning out the mixture as appropriate. Air/fuel mixture enrichment occurs when the fuel valve is open and the air bleed valve is closed. All cycling of this system, which occurs ten times per second, is controlled by the ECM. A throttle position switch informs the ECM of the open or closed throttle operation. A number of different switches are used, varying with application. When the ECM receives a signal from the throttle switch, indicating a change of position, it immediately searches its memory for the set of operating conditions that result in an ideal air/fuel ratio, and shifts to that set of conditions. The memory is continually updated during normal operation.

A Check-Engine light is included in the C-4 System installation. When a fault develops, the light comes on, and a trouble code is set into the ECM memory. However, if the fault is intermittent, the light will go out, but the trouble code will remain in the ECM memory as long as the engine is running. The trouble codes are used as a diagnostic aid, and are pre-programmed.

Unless the required tools are available, troubleshooting the C-4 System should be confined to mechanical checks of electrical connectors, vacuum hoses and the like. Should you access to the necessary diagnostic equipment, a diagnostic trouble code chart may be found later in this section. Otherwise, all diagnosis and repair should be performed by a qualified mechanic.

For more information concerning C-4 System diagnosis and repair, refer to the Basic Troublshooting procedures under the CCC system, later in this section.