Rough, Unstable, or Incorrect Idle, Stalling

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

Preliminary Check
Refer to Important Preliminary Checks. Testing and Inspection

Sensor Checks

^ Check the Heated Oxygen Sensor (HO2S).

Check 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 sever driveability problem. Refer to the following items:

- DTC P0131 HO2S Circuit Low Voltage Sensor 1. P0131
- DTC P0132 HO2S Circuit High Voltage Sensor 1. P0132
- DTC P0133 HO2S Circuit Slow Response Sensor 1. P0133
- DTC P0134 HO2S CKT Insufficient Activity Sensor 1. P0134
- DTC P0137 HO2S Circuit Low Voltage Sensor 2. P0137
- DTC P0138 HO2S Circuit High Voltage Sensor 2. P0138
- DTC P0140 HO2S CKT Insufficient Activity Sensor 2. P0140
- DTC P0141 HO2S Heater Circuit Sensor 2. P0141

^ Check the Throttle Position (TP) sensor. If a sticking throttle shaft or binding linkage causes a high TP sensor open throttle indication, the Control Module will not control the idle. Monitor the TP sensor voltage. A scan tool and/or voltmeter should read less than 0.85 volts with the throttle closed.
^ Check 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. If the coolant temperature reading is more than 5 degrees greater than or less than the ambient air temperature on a cold engine, check for a high resistance in the coolant sensor circuit or the sensor itself. Refer to the following items:

- DTC P0118 ECT Sensor Circuit High Voltage Diagnostic Aids. P0118
- DTC P0125 ECT Excessive Time to Closed Loop Diagnostic Aids. P0125

Fuel System Checks

1. Check to determine if a Rich or Lean system causes the condition. Drive the vehicle at the speed of the complaint. Monitoring the Fuel Trim will help identify the problem.

^ Lean - The Long Term Fuel Trim is greater than 150. Refer to DTC P0132 HO2S Circuit High Voltage Sensor 1 for the 4.3L Diagnostic Aids. P0132
^ Refer to DTC P0138 HO2S Circuit High Voltage Sensor 2 for Diagnostic Aids. P0138
^ Rich - The Long Term Fuel Trim is less than 115. Refer to DTC P0131 HO2S Circuit Low Voltage Sensor 1 Diagnostic Aids. P0131
Refer to DTC P0137 HO2S Circuit Low Voltage Sensor 2 Diagnostics Aids. P0137

2. Check the fuel injector driver circuit.

^ Disconnect the injector harness connector at the injectors.
^ Connect an injector test light between the terminals of each injector connector and note the light while cranking.
^ If the test light fails to blink at any connector, it is a faulty injector drive circuit harness, connector, or terminal.

3. Perform the fuel injector coil test. Refer to Fuel Injector Coil Test. Fuel Injector Coil Test
4. Perform the fuel injector balance test. Refer to Fuel Injector Balance Test. Fuel Injector Balance Test
5. Check the Evaporative Emission (EVAP) control system. Refer to EVAP Control System Diagnosis. System Diagnosis - EVAP Control
6. Perform a cylinder compression test. Refer to Engine.
7. Check for leaking fuel injectors.
8. Check the fuel pressure. Refer to Fuel System Pressure Test. System Diagnosis - Fuel System Pressure Test

Ignition System Checks

^ Check the ignition output voltage using the spark tester J 26792 or the equivalent. Refer to Enhanced Ignition System Diagnosis. System Diagnosis - Enhanced Ignition
^ Check the spark plugs.

1. Remove the spark plugs.
2. Check for the following conditions:

^ Wet plugs
^ Cracks
^ Wear
^ Improper gap
^ Burned electrodes
^ Blistered insulators
^ Heavy deposits
^ Check the spark plug wires by connecting an ohmmeter to the ends of each wire in question. If the meter reads over 30,000 ohms, replace the wires.

Additional Checks

^ Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle and low Idle Air Control (IAC) counts.
^ Check the IAC operation. Refer to the following items:

- DTC P0506 Idle Control System Low RPM. P0506
- DTC P0507 Idle Control System High RPM. P0507
- DTC P15O9 IAC System High RPM. P1509
- Fuel System Diagnosis.

^ Check the Control Module grounds for being clean, tight, and in their proper locations. Refer to Component Locations.
^ Check the Transmission Range (TR) pressure switch assembly operation. Refer to Transmission Fluid Pressure Switch Assembly. Testing and Inspection
^ Check the scan tool to determine if the Control Module is receiving an A/C signal. Refer to A/C Compressor Clutch Control Diagnosis. A/C Compressor Clutch Control Diagnosis If a problem exists with the A/C ON, check the A/C system operation. Refer to Heating and Air Conditioning.
^ Check for the Exhaust Gas Recirculation (EGR) being ON while idling which will cause roughness, stalling, and hard starting. Refer to EGR System Diagnosis. System Diagnosis - EGR
^ Check the battery cables and ground straps. They should be clean and secure. Erratic voltage will cause the AC to change its position resulting in poor idle quality.
^ Check the IAC Valve. The IAC will not move if the system voltage is not within 9 to 16 volts.
^ Check the A/C refrigerant pressure for being too high or for a faulty high pressure switch.
^ Check the Crankcase Ventilation Valve for proper operation by placing a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve. Refer to Crankcase Ventilation System.

Engine Mechanical Check

Check for the following:

^ Broken motor mounts.
^ Improper valve timing.
^ Low compression.
^ Bent push rods.
^ Worn rocker arms.
^ Broken or weak valve springs.
^ Worn camshaft lobes.
^ Refer to Engine.