For 1990-2009 cars

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

Checks | Action
--- | ---
Definition: Engine runs unevenly at idle. If severe, the engine or vehicle may shake. Engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine.
Preliminary Checks |

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

• | Refer to Symptoms .

• | Search for service bulletins.

Fuel System |

• | Check system fuel pressure. Refer to Fuel System Diagnosis .

• | Check the fuel injectors. Refer to Fuel Injector Balance Test .

• | Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis Testing.

• | Check the fuel filter for being plugged or restricted.

• | Check items which can cause an engine to run rich (long term fuel trim significantly in the negative range). Refer to Diagnostic Aids for DTC P0172 Fuel Trim System Rich .

• | Check items that can cause an engine to run lean (long term fuel trim significantly in the positive range). Refer to Diagnostic Aids for DTC P0171 Fuel Trim System Lean .

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

• | Check for fuel in the Fuel Pressure Regulator vacuum hose.

Sensor/System |

• | Check the EGR valve for a stuck open pintle.

• | Check for proper operation of the MAP Sensor. Refer to Manifold Absolute Pressure Sensor Output Diagnosis .

• | Check for proper operation of the TP sensor.

• | Check for proper operation of the ECT sensor.

• | Check the Oxygen sensor (O2S 1). The O2S 1 should respond quickly to different throttle positions. If not, check the O2S 1 for silicon or other contaminants from fuel or the use of improper RTV sealant. The sensor may have a white powdery coating. Silicon contamination causes a high but false O2S 1 signal voltage (rich exhaust indication). The PCM will reduce the amount of fuel delivered to the engine, causing a severe driveability problem. Refer to Oxygen sensor in Information Sensors/Switches Description .

Ignition System |

• | Check for proper ignition voltage output with the J 36012-A spark plug jumper wires and J 26792 spark tester or equivalent.

• | Check the spark plugs for the following conditions:

- | Check for wet or fuel fouled spark plugs.

- | Check for cracks on the spark plugs.

- | Check for wear.

- | Check for improper spark plug gap.

- | Check for burned electrodes or heavy deposits.

• | If the spark plugs are fuel or oil fouled, the cause of the fouling must be determined before replacing the spark plugs. Refer to Spark Plug Visual Diagnosis in Engine Electrical.

• | Check the Ignition Control Module (ICM) for proper ground connection.

• | Check the ignition coils for cracks or carbon tracking. Refer to Electronic Ignition (EI) System Diagnosis .

Engine Mechanical |

• | Check the following:

- | Compression. Refer to Engine Compression Test in Engine Mechanical.

- | Sticking or leaking valves.

- | Worn camshaft(s) lobe(s).

- | Valve timing.

- | Broken or weak valve springs.

- | Balance shaft broken, damaged or incorrectly installed.

- | Excessive oil in combustion chamber-Leaking valve seals.

• | For incorrect basic engine parts. Inspect the following:

- | Camshafts

- | Cylinder head

- | Pistons, etc.

• | Refer to Engine Mechanical.

Additional Checks |

• | Check exhaust system for possible restrictions. Check for the following:

- | Inspect exhaust system for damaged or collapsed pipes.

- | Inspect mufflers for heat distress or possible internal failure.

- | For possible plugged Three-Way catalytic converter. Refer to Restricted Exhaust System Check .

• | Check for proper operation of the crankcase ventilation system.

• | Check for malfunctioning engine mounts. Refer to Engine Mechanical.

• | An engine miss condition can be caused by Electromagnetic Interference (EMI) on the reference circuit. EMI can usually be detected by monitoring engine RPM with a scan tool. A sudden increase in RPM with little change in actual engine RPM change, indicates EMI is present. If a problem exists, check routing of secondary ignition wires, high voltage components (Near Ignition Control circuits).

• | Check for intake and exhaust manifold passages for casting flash. Refer to Engine Mechanical .