Mitsubishi Pick-ups and Montero 1983-1995 Repair Guide

Feedback Carburetor System (FBC)


All Mitsubishi feedback carburetors employ an oxygen sensor as part of the feedback system controlling the air/fuel mixture within the carburetor. Although this technology is now rather dated in view of the sophisticated fuel injection controls available today, the FBC system represents the first real attempt to control fuel delivery by means other than gravity and air pressure.

Reduced to its simplest form, the oxygen sensor reads the content of the exhaust and communicates electrically with the Engine Control Module (ECM). This micro computer evaluates the signal and adjusts the amount of fuel entering the engine to achieve the best combustion and therefore the lowest emissions. Although the oxygen sensor plays a critical role in this control process, the ECM also interprets signals from other sensors reading throttle position, engine temperature and other variables.

In ordinary operating conditions after engine warm up, the air/fuel ratio is within the usable range of the oxygen sensor. By attempting to keep the air and fuel mixed at the proportionally perfect (stoichiometric) ratio, the exhaust will be almost completely processed by the catalytic converter. If the exhaust changes drastically, pollutants may exceed the capacity of the converter. This constant process of checking the exhaust and adjusting the mixture is called closed loop operation.

There are times during normal operation, however, when the exhaust is outside these usable limits. Conditions of engine start-up, partially warmed driving, high load operation or sudden deceleration can each cause sufficient exhaust conditions to exceed the range of the oxygen sensor. For this reason, when the ECM is advised by the sensors of one of these conditions, the system enters open loop operation. The ECM then controls the carburetor based on pre-programmed values (often called default values) and disregards the signal from the oxygen sensor. These pre-programmed values are in the ROM (Read Only Memory) of the ECM and, since they are installed at manufacture, cannot be altered by the oxygen sensor. They control the carburetor to allow the quickest warm up of a cold engine or the best compromise of performance and emissions under the existing condition.

The feedback carburetor is covered in Fuel System .