GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Checks

Action

DEFINITION: The engine runs unevenly at idle. The engine or vehicle may shake if the condition is severe. The engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine.

Preliminary Checks

Refer to Symptoms .

Sensor Checks
    • Inspect the Heated Oxygen Sensor (HO2S) performance.
    • Inspect for silicon contamination from fuel or improperly used sealant. The sensor will have a white powdery coating. The sensor will result in a high, but false, signal voltage (rich exhaust indication). The control module will reduce the amount of fuel delivered to the engine causing a severe driveability problem. Refer to the following DTC tables in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual:
       - DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1
       - DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1
       - DTC P0133 HO2S Slow Response Bank 1 Sensor 1
       - DTC P0134 HO2S Insufficient Activity Bank 1 Sensor 1
       - DTC P0135 HO2S Heater Circuit Bank 1 Sensor 1
       - DTC P0137 HO2S Circuit Low Voltage Bank 1 Sensor 2
       - DTC P0138 HO2S Circuit High Voltage Bank 1 Sensor 2
       - DTC P0140 HO2S Insufficient Activity Bank 1 Sensor 2
       - DTC P0141 HO2S Heater Circuit Bank 1 Sensor 2
    • Inspect the Engine Coolant Temperature (ECT) sensor using the scan tool to compare the engine coolant temperature with the ambient air temperature on a cold engine. Inspect for a high resistance in the coolant sensor circuit or the sensor itself if the coolant temperature reading is more than 5 degrees greater than or less than the ambient air temperature on a cold engine. Refer to DTC P1115 ECT Sensor CKT Intermittent High Voltage Diagnostic Aids or DTC P0118 ECT Sensor Circuit High Voltage Diagnostic Aids in Engine Controls in the C/K Truck Service Manual.
    • Inspect the Manifold Absolute Pressure (MAP) sensor response and accuracy. Refer to DTC P0106 MAP System Performance in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.

Fuel System Checks
    • Inspect for a rich or lean symptom that causes the condition. Drive the vehicle at the speed of the complaint. Monitoring the oxygen sensors will help identify the problem.
    • Inspect for fuel in the pressure regulator vacuum hose. Replace the regulator assembly if fuel is present.
    • Inspect the Evaporative Emission (EVAP) control system. Refer to EVAP Control System Diagnosis in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.
    • Perform a cylinder compression test.
    • Inspect for leaking fuel injectors. Refer to Fuel Injector Circuit Diagnosis in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.
    • Inspect the fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect the Gas Distribution Adapter (GDA) for proper installation and connection.