Chrysler Full-Size Vans 1967-1988 Repair Guide

General Information


The Chrysler Electronic Feedback Carburetor (EFC) system incorporates an oxygen sensor, a three-way catalytic converter, an oxidizing catalytic converter, a feedback carburetor, a solenoid-operated vacuum regulator valve, and a Combustion Computer. Also incorporated into the system are Chrysler's Electronic Spark Control.

In Chrysler's system, "Combustion Computer" is a collective term for the Feedback Carburetor Controller and the Electronic Spark Control computer, which are housed together in a case located on the air cleaner. The feedback carburetor controller is the information processing component of the system, monitoring oxygen sensor voltage (low voltage/lean mixture, high voltage/rich mixture), engine coolant temperature, manifold vacuum, engine speed, and engine operating mode (starting or running). The controller examines the incoming information and then sends a signal to the solenoid-operated vacuum regulator valve (also located in the Combustion Computer housing), which then sends the proper rich or lean signal to the carburetor.


Electronic Feedback Control (EFC) System

The EFC system is essentially an emissions control system which utilizes an electronic signal, generated by an exhaust gas oxygen sensor to precisely control the air/fuel mixture ratio in the carburetor. This in turn allows the engine to produce exhaust gases of the proper composition to permit the use of a three-way catalyst. The three-way catalyst is designed to convert the three pollutant (1) hydrocarbons (HC), (2) carbon monoxide (CO), and (3) oxides of Nitrogen (NOx) into harmless substances.

There are two operating modes in the EFC system:

  1. Open Loop-air/fuel ratio is controlled by information programmed into the computer at manufacture.
  3. Closed Loop-air/fuel ratio is varied by the computer based on information supplied by the oxygen sensor.

When the engine is cold, the system will be operating in the open loop mode. During that time, the air/fuel ratio will be fixed at a richer level. This will allow proper engine warm up. Also, during this period, air injection (from the air injection pump) will be injected upstream in the exhaust manifold.

Both closed loop and open loop operation are possible in the EFC system. Open loop operation occurs under any one of the following conditions: coolant temperature under 150°F; oxygen sensor temperature under 660°F; low manifold vacuum (less than 4.5 in. Hg. engine cold, or less than 3.0 in. Hg. engine hot); oxygen sensor failure; or hot engine starting. Closed loop operation begins when engine temperature reaches 150°F.

Oxygen Sensor

The oxygen sensor is a device which produces electrical voltage. The sensor is mounted in the exhaust manifold and must be heated by the exhaust gas before producing a voltage. When there is a large amount of oxygen present (lean mixture) the sensor produces a low voltage. When there is a lesser amount present, it produces a higher voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich/lean switch. The voltage is transmitted to the Spark Control Computer. The computer sends a signal to the Oxygen Feedback Solenoid mounted on the carburetor to change the air/fuel ratio back to stoichiometric.

Electric Choke Assembly

An electric heater and switch assembly is sealed within the choke housing. Electrical current is supplied through the oil pressure switch. A minimum of 4 psi (2.7 kPa) oil pressure is necessary to close the contacts in the oil pressure switch and feed current to the automatic choke system. Electricity must be present when the engine is running to open the choke and keep it open.

The heater can be tested with a direct B+ connection. The choke valve should reach the open position within five minutes.

Operation of any type, including idling should be avoided if there is any loss of choke power. Under this condition, any loss of power to the choke will cause the choke to remain fully on during the operation of the vehicle. This will cause a very rich mixture to burn and result in abnormally high exhaust system temperatures, which may cause damage to the catalyst or to the underbody parts of the car. It is advised that the electric choke power not be disconnected to troubleshoot cold start problems.