As automotive emissions standards tightened, and fuel economy became more important, the widespread use of Electronic Fuel Injection increased dramatically. As a result engine performance also improved. The Sentra and Pulsar models use two different types of electronic fuel injection. The first type uses a single fuel injector mounted in the throttle body housing. This system is known as Throttle Body Fuel Injection (TBI). The TBI system is used on E16i and GA16i engines.
The E15ET, CA16DE, CA18DE, GA16DE and SR20DE engines use Multi-port Fuel Injection (MFI). The MFI system uses one fuel injector for each cylinder. The injectors are mounted in the intake manifold and are controlled by the engine control module. The injectors are pulsed together or in pairs depending on the specific application.
On all fuel injected engines a mixture ratio feedback system is used for optimum three-way catalyst action. The engine control module (ECM) consists of a microcomputer, inspection lamps, a diagnostic mode selector and connectors for signal input, output, and power supply. The ECM controls the amount of fuel injected, ignition timing, mixture ratio feedback, idle speed, fuel pump operation, air injection valve (AIV) operation, exhaust gas recirculation (EGR) and evaporative emission canister (EVAP canister) purge operation.
The air/fuel ratio is fine tuned using a heated oxygen sensor (HO 2 S) or oxygen sensor (O 2 S) and the ECM varies the injection pulse width to maintain the mixture inside the stoichiometric window. When the ECM uses the HO 2 S or O 2 S as the primary source of feedback information, this is called "closed loop'' control. Under certain conditions the ECM may ignore the HO 2 S or O 2 S. Some of these conditions are:
Without constant adjustment, the ideal mixture (called the stoichiometric window) is rarely obtained from any car, because certain factors such as: manufacturing variations, worn parts and the operating environment may cause the mixture ratio to change. For this reason, the actual fuel setting and theoretical fuel setting is measured by the ECM and recorded. The ECM uses this data to return the fuel mixture to the "learned" setting rather than the factory setting. This ability to "learn" the ideal setting is called "fuel trim." The data for fuel trim is stored in the volatile memory of the ECM. The contents of the fuel trim data may change under certain conditions: if ECM power is lost for approximately 24 hours, the operating adjustments change for an extended period of time, or there is a component malfunction.