Circuit Description

WARNING: This page is about a different car, the 2003 GMC Savana and 2003 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.

The Powertrain Control Module (PCM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. CNG Fuel delivery is controlled differently during Open and Closed Loop. During Open Loop, the PCM determines fuel delivery based on sensor signals without Oxygen Sensor (O2S) input. During Closed Loop, the O2S inputs are added and used by the PCM to calculate short and long term fuel trim fuel delivery adjustments. If the O2S indicate a lean condition, fuel trim values will be above zero percent. If the O2S indicate a rich condition, fuel trim values will be below zero percent. Short term fuel trim values change rapidly in response to the Heated Oxygen Sensor (HO2S) voltage signals. Long term fuel trim makes coarse adjustments in order to maintain an air/fuel ratio of 16.5:1 while operating on CNG. If the PCM detects an excessively lean condition, DTC P0171 or P0174 sets.