GM Firebird 1982-1992 Repair Guide

System Descriptions

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The Multi-Port Fuel Injection (MPFI or MFI) systems were first introduced on the 1985 models. They may be called different names (Port Fuel Injection-PFI, Tuned Port Injection-TPI, Sequential Fuel Injection-SFI), however, all of the systems are similar in operation.

The systems are controlled by an Electronic Control Module (ECM) which monitors the engine operations and generates output signals to provide the correct air/fuel mixture, ignition timing and idle speed. Input information to the ECM is provided by the oxygen sensor, temperature sensors, detonation sensor, mass air flow sensor and throttle position sensor. A system may use all or some of these sensors, depending on the year and engine application. The ECM also receives information concerning engine rpm, road speed, transmission gear position, power steering and air conditioning status.

All of the systems use multiple injectors, aimed at the intake valve at each intake port, rather than the centrally located injector(s) found on the earlier Throttle Body Injection (TBI) and Crossfire Injection systems. The injectors are mounted on a fuel rail and are activated by a signal from the electronic control module. The injector is a solenoid-operated valve which remains open depending on the width of the electronic pulses (length of the signal) from the ECM; the longer the open time, the more fuel is injected. In this manner, the air/fuel mixture can be precisely controlled for maximum performance with minimum emissions.

There are two different types of fuel management systems used on the multi-port fuel injection engines. The mass air flow system and the speed density system.

The mass air flow systems measure the mass of air that is drawn into the engine cylinders, rather than just the volume. The sensor contains a hot-wire sensing unit, which is made up of an electronic balanced bridge network. Whenever current is supplied to the sensor, the bridge is energized and the sensing hot-wire is heated. As the air enters the mass air flow sensor, it passes over and cools the hot wire. When the hot wire is cooled, it's resistance changes and additional current is needed to keep the bridge network balanced. This increase in current is sent to the computer as a voltage signal and is used to calculate the mass of the incoming air. The ECM uses this information to determine the duration of fuel injection pulse, ignition timing and EGR operation.

The speed density systems calculate the volume of air moving through the intake. The ECM establishes the speed factor through a signal from the ignition module. The Manifold or Intake Air Temperature (MAT/IAT) and the Engine Coolant Temperature (ECT) sensors work together to assure that proper temperature information gets to the ECM while the Manifold Absolute Pressure (MAP) sensor monitors the changes in manifold pressure which results from changes in engine loading. These three sensors contribute to the density factor. Together, these inputs (engine speed, coolant temperature sensor, etc...) are the major determinants of the air/fuel mixture delivered by the fuel injection system.

The following engines and fuel management systems are covered in this section:



1985-1989 2.8L MPFI-Mass Air Flow
 
1990-1992 3.1L MPFI-Speed Density
 
1989 3.8L Turbo SFI-Mass Air Flow
 
1985-1989 5.0L and 5.7L TPI-Mass Air Flow
 
1990-1992 5.0L and 5.7L TPI-Speed Density
 

 
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