Circuit/System Description

The powertrain control module (PCM) uses input from several engine sensors in order to determine the correct engine speed at idle. With the desired idle speed calculated, the PCM determines the correct throttle opening and the corresponding throttle position (TP) needed to obtain the desired engine speed. The PCM sends the desired throttle position or target value to the TAC module. The TAC module achieves the desired throttle position by commanding the throttle control motor to position the throttle valve at the target value. If necessary, the PCM adjusts the target throttle position in order to achieve the desired engine idle speed.

The PCM also determines the typical volume of intake air used by a properly operating engine during the first moments of cold start operation. The PCM monitors the mass air flow (MAF) sensor readings during the first 13 seconds of cold engine operation and compares those to the calculated values. Ignition system timing is preset during cold engine startup. If the initial ignition system timing retard value is not detected for the expected length of time, the DTC can set.