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 |

• | Check the Heated Oxygen Sensor (HO2S) sensor 1 and 2. Refer to DTC P0130 through DTC P0141.

• | Check the Throttle Position (TP) sensor. A sticking throttle shaft or a binding throttle linkage will causes a high TP sensor voltage (open throttle indication). Under these conditions the PCM may not control the idle. Monitor the TP sensor angle with the scan tool. The indicated angle should be aproximately 11% with throttle closed.

• | Check the Engine Coolant Temperature (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 (41°F) greater or less than the ambient air temperature, check for a high resistance in coolant sensor circuit or the sensor itself. Refer to Temperature Versus Resistance .

• | Check the MAP sensor for a quick response and accuracy.

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 greater than 20%. Refer to Diagnostic Aids.

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

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

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

• | Check 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 |

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

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

- | Wet (fuel fouled) plugs.

- | A terminal screw that is bent or loose (rotate and pull).

- | A ceramic insulator that has cracks, carbon tracking or deposits (red or black dust)

- | A tip insulator that has cracks, carbon tracking, deposits (oil, carbon, glazing, fuel additive), or is loose (rattles).

- | A center electrode that is loose (wiggles side to side)

- | An incorrect spark plug gap that is too large or too small.

- | Platinum pads that are missing.

• | Check the spark plug cables by connecting an ohmmeter to the ends of each cable. Replace any spark plug cables with a resistance greater than 25,000 ohms.

• | Observe the Spark parameter and the IAC Duty Cycle parameter on the scan tool at idle, while operating the rear defogger, the A/C system, and the headlights. The Spark parameter should increase 2-3 degrees and the IAC Duty Cycle parameter should increase 4-7% when operating the engine loads. If the Spark and IAC Duty Cycle parameters remain fixed, check for a short to ground in the Diagnostic Request circuit from PCM terminal C2-15 to the underhood diagnostic link connector.

Engine Mechanical Check |

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

- | Faulty hydraulic lifter assemblies

- | Broken or weak valve springs

- | Low compression

- | Incorrect valve timing

- | Sticking or leaking valves

- | Incorrect or worn camshaft

• | Refer to Engine Mechanical.

Additional Checks |

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

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

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

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

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

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

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

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

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