Toyota Corolla 1988-1997 Repair Guide

Computerized Engine Systems



Engines equipped with a feedback carburetor use a simple system designed to keep the air/fuel ratio at an optimum of 14.7:1; excluding warm-up and acceleration. The carburetor is designed to run richer than it normally should. This sets up a rich limit of system operation. When a leaner operation is desired, the computer (ECM) energizes the electronic air bleed control valve (EBCV) to introduce additional air into the carburetor's main metering system and into the carburetor's primary bore. Once the air/fuel ratio is detected as being too lean by the oxygen sensor, the ECM will de-energize the EBCV and close both bleed ports. By shutting off the air, the mixture begins moving back towards the rich limit. The system is operating in the "closed loop'' mode, during which it will adjust itself and react to these adjustments. On these engines, the ECM receives information from the oxygen sensor, vacuum switches and the distributor.

The Electronic Fuel Injection (EFI) system precisely controls fuel injection to match engine requirements. This in turn reduces emissions and increases driveability. The ECM receives input from various sensors to determine engine operating conditions. These sensors provide the input to the control unit which determines the amount of fuel to be injected as well as other variables such as idle speed. These inputs and their corresponding sensors include:

Intake manifold absolute pressure-MAP or Vacuum Sensor
Intake air temperature-Intake Air Temperature Sensor
Coolant temperature-Water Temperature Sensor
Engine speed-Pulse signal from the distributor
Throttle valve opening-Throttle Position Sensor
Exhaust oxygen content-Oxygen Sensor