Speed Density

All engines are equipped with a Manifold Absolute Pressure (MAP) sensor (gasoline), or Barometric Pressure (BARO) sensor (diesel), and use the speed density method to compute airflow rate. PCM uses manifold pressure to calculate the airflow rate. The MAP/BARO sensor responds to manifold vacuum changes due to engine load and speed changes. The PCM sends a voltage signal to the MAP/BARO sensor. Manifold pressure changes result in resistance changes in the MAP/BARO sensor.

By monitoring MAP/BARO sensor signal voltage, the PCM determines manifold pressure. On gasoline engines, if MAP sensor fails, the PCM supplies a fixed MAP sensor value, and uses the TP sensor to control fuel.

Some models also use an Intake Air Temperature (IAT) sensor. The IAT sensor allows PCM to determine intake air temperature. PCM uses signal to delay EGR until intake air temperature reaches about 40°F (5°C). If intake air temperature becomes excessively high, PCM compensates by slightly retarding timing.