For 1990-2009 cars

Checks | Action
--- | ---
DEFINITION: The engine runs unevenly at idle. The engine or the vehicle may shake if the condition is severe enough. The engine idle speed may vary in the 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 sensors 1 and 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 cause a high TP sensor voltage. Under these conditions the PCM may not control the idle. Monitor the TP sensor voltage with the scan tool or with 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 which is more than 5°C (41°F) above or below the ambient air temperature reading may indicate a high resistance in the coolant sensor circuit or in the ECT sensor. Refer to Temperature Versus Resistance .

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

• | Check the engine reference signal in the CKP sensor with a scan tool. Observe the Engine Speed parameter while running the engine at idle. The scan tool should indicate steady RPM readings and linear RPM readings while running. If sudden increases or decreases are displayed in the engine speed, the engine reference signal is not stable enough for the engine to run correctly. Check for a short to ground in the ignition reference low circuit of the CKP sensor. A short to ground in the ignition reference low circuit of the CKP sensor will cause a faulty engine reference signal to be sent to the PCM. However, this will not set a CKP sensor DTC.

Fuel System Checks |

• | Determine if a rich fueling condition 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 value of the Long Term Fuel Trim will approach 20%. Refer to DTC P0171 Fuel Trim System Lean Diagnostic Aids.

- | Rich - The value of the Long Term Fuel Trim will approach -20%. Refer to DTC P0172 Fuel Trim System Rich Diagnostic Aids.

• | Check for a fuel injector that is faulty, is leaking, or is operating incorrectly. Perform the Fuel Injector Coil Test and the Fuel Injector Balance Test. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) and to Sequential Multiport Fuel Injector Balance Test .

• | 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 .

Ignition System Checks |

• | Check for adequate secondary ignition voltage with a J 26792 spark tester or with an equivalent.

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

- | Wet plugs or fuel fouled plugs.

- | A bent or loose terminal screw.

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

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

- | A center electrode that is loose

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

- | Refer to Spark Plug Visual Diagnosis in Engine Electrical.

• | Check the spark plug cables by connecting an ohmmeter to the ends of each cable. Replace any spark plug cable that has a reading over 30,000 ohms.

Engine Mechanical Check | Check for any of the following engine mechanical concerns:

• | An incorrect valve lash

• | Broken valve springs or weak valve springs

• | Low compression

• | Incorrect valve timing

• | Sticking valves or leaking valves

• | An incorrect camshaft or a worn camshaft

Refer to Engine Compression Test in Engine Mechanical.

Additional Checks |

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

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

• | Check the PCM grounds for being clean, tight, and in the correct location. Refer to

#### Cell 21: Power and Ground

.

• | An idle concern that occurs when you turn ON the A/C may indicate a fault in the A/C idle-up system. Idle speed should increase when you turn ON the A/C. Check the scan tool in order to determine if the PCM is receiving an A/C signal. Refer to Air Conditioning (A/C) Idle Circuit Diagnosis .

• | Inspect the engine for the correct idle speed. Refer to Idle Speed Inspection .

• | A/C refrigerant pressure that is too high, or a faulty high pressure switch, may cause a rough running condition or a stalling condition when you turn ON the A/C.

• | Check the idle air control (IAC) system for correct operation. Using a scan tool, command the IAC valve to increase or to decrease the engine idle speed.

• | Check the PCV valve for the correct part and for the correct 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 .

• | Check the PSP switch for correct operation. Refer to PSP Switch Diagnosis .