Speed Density System

DESCRIPTION
The Sequential Multiport Fuel Injection (SFI) system is a speed and air density system. The system is based on "speed density" fuel management.
Three specific data sensors provide the Engine Control Module (ECM) with the basic information for fuel management portion of its operation. That is three specific signals to the ECM establish the engine speed and air density factors.

The engine speed signal comes from the distributor low resolution circuit. The ECM uses this information to determine engine speed (RPM).
Air density is derived from Intake Air Temperature (IAT) and Manifold Absolute Pressure (MAP) sensor inputs. The IAT sensor measures the air temperature that is entering the engine. The IAT signal works in conjunction with the MAP sensor to determine air density. As the intake manifold pressure increases, the air density in the intake manifold also increases and additional fuel is required. This information from the IAT and MAP sensors is used by the ECM to control injector pulse width.

The speed density system is only needed when there is a Mass Air Flow (MAF) sensor malfunction. If the ECM detects a malfunction with the MAF sensor circuit the ECM will default to speed density fuel management.