For 1990-2009 cars

#### Rough, Unstable, or Incorrect Idle, Stalling

Checks | Action
--- | ---
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. The engine idles at an incorrect speed.
Preliminary Check |

• | Refer to Powertrain On Board Diagnostic (OBD) System Check .

• | Refer to Symptoms .

Sensor Checks |

• | Inspect the heated oxygen sensors (HO2S 1 and HO2S 2). Refer to DTC P0130 through DTC P0141.

• | Inspect the throttle position (TP) sensor. A sticking throttle shaft or a binding throttle linkage will causes a high TP sensor voltage. Under these conditions the PCM may not control the idle. Monitor the TP sensor angle with the scan tool. The indicated angle should be approximately 11 percent with the throttle closed.

• | Inspect the engine coolant temperature (ECT) sensor. A fixed or inaccurate ECT sensor reading can cause the engine idle to surge or race. Using the scan tool, compare the engine coolant temperature with the ambient air temperature on a cold engine. If the engine coolant temperature is 5°C (41°F) more than or 5°C (41°F) less than the ambient air temperature, inspect the resistance of the coolant sensor. Refer to Temperature Versus Resistance . If the engine coolant temperature is significantly less than the ambient air temperature, inspect the coolant sensor electrical circuit for high resistance.

• | Inspect the mass air flow (MAF) sensor operation. Run the engine to normal operating temperature. Turn OFF all accessories and observe the MAF sensor at idle with a scan tool. If the MAF sensor reading is more than 3.5 g/s refer to DTC P0101 Mass Air Flow (MAF) Sensor Performance .

Fuel System Checks |

• | Inspect the operation of the fuel system for rich or lean condition. Operate the vehicle under the conditions that caused the concern. Monitor the Fuel Trim parameter on a scan tool in order to identify the problem.

- | Lean -- The Long Term Fuel Trim will be more than 20 percent. Refer to Diagnostic Aids.

- | Rich -- The Long Term Fuel Trim will be less than -20 percent. Refer to Diagnostic Aids.

• | Test the fuel pressure while the condition exists. Refer to Fuel System Diagnosis .

• | Inspect the evaporative emission (EVAP) control system. Refer to Evaporative Emission Control System Diagnosis .

• | Test the fuel injectors for leaking. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) .

Ignition System Checks |

• | Test for the proper secondary voltage output with a J 26792 .

• | Inspect the spark plugs for any of the following conditions:

- | Any wet or fuel fouled plugs

- | A terminal screw that is bent or loose

- | A ceramic insulator that has cracks, carbon tracking, or deposits

- | A tip insulator that has cracks, carbon tracking, deposits, or is loose

- | A center electrode that is loose

- | An incorrect spark plug gap

- | Missing platinum pads

• | Inspect the ignition coil boots for damage.

• | Observe the IAC Duty Cycle parameter on the scan tool at idle, while operating the rear defogger, the A/C system, or the headlights. The IAC Duty Cycle parameter should increase 2-7 percent when operating the engine loads. If the IAC Duty Cycle parameter remains fixed, check for a short to ground in the Diagnostic Request circuit from the PCM to the TE1 terminal of the underhood diagnostic link connector.

Engine Mechanical Check |

• | Inspect for any of the following engine mechanical problems:

- | Any faulty hydraulic lifter assemblies

- | Any broken or weak valve springs

- | A low compression

- | An incorrect valve timing

- | Any sticking or leaking valves

- | An incorrect or worn camshaft

• | Refer to the appropriate procedures in Engine Mechanical.

Additional Checks |

• | Inspect for vacuum leaks. Vacuum leaks can cause a higher than normal idle speed.

• | Verify the PCM grounds are clean, tight, and in the proper locations. Refer to the PCM Wiring Diagram.

• | Check the scan tool to determine if the PCM is receiving an A/C signal. The idle speed should be increased with the A/C ON. Refer to Air Conditioning (A/C) Idle Circuit Diagnosis .

• | Inspect the battery cables and the ground straps. They should be clean and secure.

• | Inspect the A/C refrigerant pressure for being too high or for a faulty high pressure switch.

• | Test for the proper operation of the IAC system. Use a scan tool to command the IAC valve to increase and decrease engine speed.

• | The engine will run rough and the engine can stall if the CMP actuator solenoid is advancing camshaft timing at idle. Observe the Cam Phase Angle parameter on a scan tool at engine idle. If the Cam Phase Angle is not 0 degrees refer to DTC P1349 Intake Camshaft Position (CMP) System .

• | Inspect the PCV valve for proper operation by placing a finger over the inlet hole in the valve end several times. The valve should snap back. Refer to Positive Crankcase Ventilation Valve Replacement .

• | Inspect that the idle speed is increasing for changes in electrical loads on the engine. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .

• | Verify that the PCM is increasing the idle speed when the P/S system pressure is increased on turns. Refer to PSP Switch Diagnosis .