Fuel Injection System - PFI: Description: Notes

All General Motors vehicles with Port Fuel Injection (PFI) systems have fuel delivery controlled by an on-board computer, or Electronic Control Module (ECM). The ECM is the heart of the system and reads output signals from sensors to determine proper air/fuel ratio, ignition timing and idle speed. The ECM also has the ability to perform some diagnostic work on itself and the system.

Standard PFI systems feature simultaneous double-fire injection. On these systems, all injectors pulse once each engine revolution. Thus 2 injections of fuel are mixed with incoming air to produce a charge for each combustion cycle. On Century, Ciera, Delta 88, Electra, Ninety-Eight, Regal, Regency and Toronado models, the injectors are pulsed sequentially (one-by-one) in spark plug firing order. There is one injection of fuel for every combustion cycle. This system is referred to as Sequential Fuel Injection (SFI). Camaro, Corvette and Firebird PFI systems are referred to as Tuned Port Injection (TPI).

All 3 systems maintain constant fuel pressure to the injectors. Therefore, the air/fuel ratio is adjusted by varying injector pulse width, or injector on time. The ECM processes information from various sensors to compute the pulse width. There are 2 major sub-systems; the fuel system and the electronic control system. The mechanical portion of the port fuel injection system consists of fuel injectors, throttle body, fuel rail, fuel pressure regulator, idle air control (IAC) valve, fuel pump and fuel pump relay.