All engines are equipped with a MAP sensor, and use the speed density method to compute the airflow rate. PCM uses manifold pressure to calculate the airflow rate. The MAP sensor responds to manifold vacuum changes due to engine load and speed changes. The PCM sends a voltage signal to the MAP sensor. Manifold pressure changes result in resistance changes in the MAP sensor.
By monitoring MAP sensor signal voltage, the PCM determines manifold pressure. If MAP sensor fails, the PCM supplies a fixed MAP value, and uses the TP sensor to control fuel.
Some models also use a Intake Air Temperature (IAT) sensor. Sensor allows PCM to determine intake air temperature. Control module 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.