The speed density system is only needed when there is a mass air flow (MAF) sensor malfunction. If the powertrain control module (PCM) detects a malfunction with the MAF sensor circuit, the PCM defaults to speed density fuel management.

Three sensors provide the PCM with the basic information for the fuel management portion of its operation. The 3 specific signals to the PCM establish the engine speed and air density factors. The engine speed signal comes from the ignition system. 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 in order 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 PCM to control injector pulse width.