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 Diagnostic System Check - Engine Controls .

• | Refer to Symptoms - Engine Controls .

Sensor Checks |

• | Inspect the operation of the heated oxygen sensor (HO2S) 1. Refer to:

- | DTC P0131

- | DTC P0132

- | DTC P0133

- | DTC P0134

• | Check the throttle position (TP) sensor. A sticking throttle shaft or a binding throttle linkage will cause a high TP sensor voltage (open throttle indication). Under these conditions the powertrain control module (PCM) may not control the idle. Monitor the TP sensor voltage with the scan tool or a voltmeter. The indicated voltage should be less than 1.25 volts with throttle closed.

• | Inspect for a shifted or fixed engine coolant temperature (ECT) sensor. Test the ECT sensor using the scan tool. Compare the engine coolant temperature with the ambient air temperature on a cold engine. If the engine coolant temperature is more than 5°C (9°F) more or less than the ambient air temperature, check for a high resistance in the coolant sensor circuit or the sensor itself. Refer to Temperature Versus Resistance .

• | Check the performance of the mass air flow (MAF) sensor. Refer to Diagnostic Aids in DTC P0101 .

Fuel System Checks |

• | Check if a Rich or Lean system can cause the condition. Drive the vehicle at the speed of the complaint. Monitoring the Fuel Trim may help identify the problem.

- | Lean--The Long Term Fuel Trim will be approaching 20 percent. Refer to DTC P0171 , Diagnostic Aids.

- | Rich--The Long Term Fuel Trim will be approaching -20 percent. Refer to DTC P0172 , Diagnostic Aids.

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

• | An evaporative emission (EVAP) canister purge solenoid that allows excess purge at idle, can cause a rich fueling condition that affects engine performance. Refer to Evaporative Emission Control System Diagnosis .

• | Check the fuel injectors for leaking. Refer to Fuel Injector Balance Test with Special Tool .

Ignition System Checks |

• | Check for the proper secondary voltage output with a J 26792 spark tester or equivalent.

• | Check the condition of the spark plugs. Refer to Spark Plug Inspection .

• | Check the spark plug wires for the correct electrical resistance. Refer to Spark Plug Wire Inspection .

Engine Mechanical Check |

• | Check for any of the following engine mechanical concerns:

- | Faulty hydraulic lifter assemblies

- | Broken or weak valve springs

- | Low compression

- | Incorrect valve timing

- | Incorrect or worn camshaft

• | Refer to Symptoms - Engine Mechanical in Engine Mechanical.

Additional Checks |

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

• | Check the air inlet system (ducts, air cleaner, MAF sensor) for any leaks or restrictions.

• | Check the throttle body for blockage and a damaged or binding throttle valve.

• | Check for binding of the accelerator cable or pedal. Inspect the accelerator cable for the proper adjustment. Refer to Accelerator Cable Adjustment .

• | Check for clean and tight battery cables and ground straps.

• | Check the PCM grounds for clean, tight, and proper connections. Refer to

#### Power and Ground

Engine Controls Components
MIL and DLC
Handling ESD Sensitive Parts Notice
H/L, HTR, IG and LAMP Fuses
HAZ, D/L, TAIL, STOP and DOME Fuses
BATT, ABS, FI and ACC Fuses
BATT, ABS, FI and ACC Fuses
Handling ESD Sensitive Parts Notice
G100, G101 and G102
.

• | Check the exhaust gas recirculation (EGR) valve for leaks and proper operation. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis .

• | Check for a DTC C1215 in the antilock brake system (ABS) idle-up circuit. Refer to DTC C1215 in Antilock Brake System.

• | Check the idle air control (IAC) system for proper operation. Use a scan tool in order to command the IAC valve to increase/decrease engine idle speed. Refer to Idle Air Control (IAC) System Diagnosis .

• | The PCM is designed to respond to changes in engine load. Inspect the electrical load idle-up system for proper operation. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .

• | Check the scan tool in order to determine if the PCM is receiving an A/C signal. Idle speed should be increased with the A/C ON. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .

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

• | Check the positive crankcase ventilation (PCV) valve for proper operation by placing a finger over inlet hole in valve end several times. The valve should snap back. Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical.

• | Inspect the engine cooling system for proper operation. An engine overheating condition or the failure of the engine to reach normal operating temperature may affect engine idle speeds. Refer to Engine Overheating and Engine Fails To Reach Normal Operating Temperature in Engine Cooling.

• | A malfunctioning air limiting valve in the throttle body could cause the following conditions:

- | A slightly higher than normal idle speed on a hot engine when the air limiting valve is stuck open

- | A lower than normal idle speed on a cold engine when the air limiting valve is stuck closed

• | If either condition is present and the IAC system is OK, replace the air limiting valve. Refer to Throttle Body Assembly Replacement .