For 1990-2009 cars

Checks | Action
--- | ---
DEFINITION: The engine runs unevenly at idle. The engine or vehicle may shake if the condition is severe enough. The engine idle speed may vary in RPM. The 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 performance of the heated oxygen sensor (HO2S) sensor 1 and the HO2S 2. Refer to Diagnostic Aids in DTC P0131 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 voltage with the scan tool or a voltmeter. The indicated voltage should be less than 1.25 volts with the throttle closed.

• | Check the engine coolant temperature (ECT) sensor. Use a scan tool in order to compare the engine coolant temperature reading on a cold engine with the ambient air temperature reading of the same engine. A coolant temperature reading that is more than 5°C (41°F) different than the ambient air temperature reading (on a cold engine) may indicate a high resistance in the coolant sensor circuit or the sensor itself. Refer to Temperature Versus Resistance .

• | Check the performance of the MAP sensor. Refer to DTC P0106 Manifold Absolute Pressure (MAP) System Performance in Diagnostic Aids.

Fuel System Checks |

• | Determine whether a rich or a lean fueling condition can cause the concern. Drive the vehicle at the speed of the concern. Monitor the Fuel Trim parameters on a scan tool in order to identify the fuel system status.

- | Lean -- The Long Term Fuel Trim value will be greater than 20%. Refer to DTC P0171 Fuel Trim System Lean in Diagnostic Aids.

- | Rich -- The Long Term Fuel Trim value will be greater than -20%. Refer to DTC P0172 Fuel Trim System Rich in Diagnostic Aids.

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

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

• | Check the fuel injector for leaking.

Ignition System Checks |

• | Check for adequate secondary ignition voltage with a J 26792 spark tester 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 cable that reads over 30,000 ohms.

• | Check the ignition timing. Refer to Ignition Timing Adjustment .

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

• | 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 being clean, tight, and in the proper location. Refer to

#### PCM Power and Ground

.

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

• | Check the EGR Valve for excessive flow or sticking open. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis .

• | Check that the battery cables and ground straps are clean and secure.

• | An idle concern that occurs when the A/C is ON may be caused by A/C refrigerant pressure that is too high or a faulty high pressure switch.

• | Check the idle speed control (ISC) system for proper operation. Use a scan tool in order to command the ISC motor to increase/decrease idle speed.

• | Check the PCV Valve for proper operation. Refer to Crankcase Ventilation System Inspection .

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