Rough, Unstable, or Incorrect Idle, Stalling

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
^ Perform the visual and physical checks as described in Preliminary Checks. Testing and Inspection

Check: For vacuum leaks.

Check: Powertrain Control Module (PCM) grounds for being clean, tight and proper routing. Refer to Diagrams for PCM wiring. Diagrams

FUEL SYSTEM
Monitoring the long term fuel trim will help identify an excessively lean or rich condition.

Lean Condition
A lean condition is present when the long term fuel trim value reads near 160. Refer to "Diagnostic Aids" located in DTCs 44 and 64.

Rich Condition
A rich condition is present when the long term fuel trim value reads near 108. Refer to "Diagnostic Aids" located in DTCs 45 and 65. Powertrain Controls - ECM/PCM

Check: Incorrect minimum idle speed may be caused by foreign material accumulation in the throttle bore, coking on the throttle valve or on the throttle shaft, or throttle body tampering.

Check: Injector harness locations. Make sure each injector is connected to the correct injector/cylinder according to the firing order (firing order: 1-8-4-3-6-5-7-2).

Check: For injectors that are shorted internally. Compare injector resistance to one another. Injector resistance should be within 1.0 ohm of each other.

Check: Injector balance. Refer to Chart C-2A Injector Tests. Chart C-2A Injector Coil Test Procedure

Check: For fuel in pressure regulator hose. If fuel is present, replace regulator assembly.

Check: Evaporative Emission (EVAP) control system. Refer to Chart C-3 EVAP Purge Solenoid Valve Check. Chart C-3 EVAP Canister Purge Solenoid Check

Check: The Heated Oxygen Sensors (HO2S) should respond quickly to different throttle positions. If they don't, check the HO2S for silicon contamination from fuel, or use of improper RTV sealant. The sensors will have a white powder coating, and will result in a high but false signal voltage (rich exhaust indication). The PCM will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem.

IGNITION SYSTEM
Check: For proper ignition voltage output with spark tester J26792 (ST 125).

Check: Perform misfire check. Refer to Chart C-4B Misfire Check. Chart C-4B Misfire Check

Check: Distributor vent system. Refer to Chart C-4C Distributor Vent System Check. Chart C-4C Distributor Vent System Check

Check: Spark plugs. Remove spark plugs, check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits. Repair or replace as necessary.

ENGINE MECHANICAL
Check: Perform a cylinder compression check.

Check: For correct camshaft or weak valve springs.

ADDITIONAL CHECKS
Check: Manifold Absolute Pressure (MAP) sensor. Refer to Chart C-1D MAP Output Check. Chart C-1D Manifold Absolute Pressure (MAP) Output Check

Check: Throttle linkage for sticking or binding.

Check: Park/Neutral Position (PNP) switch circuit. Refer to Chart C-1A P/N Position Switch Diagnosis or use Tech 1 scanner. Verify that scan tool indicates vehicle is in drive with gear selector in drive or overdrive. Chart C-1A Park/Neutral Switch Diagnosis

Check: Idle Air Control (IAC) operation. Refer to Chart C-2C Idle Air Control Valve Check. Chart C-2C Idle Air Control (IAC) System Check

Check: Exhaust Gas Recirculation (EGR) valve. There should be no EGR at idle. Refer to Chart C-7 EGR System Check. Chart C-7 EGR System Check

Check: A/C signal to PCM. Tech 1 scanner should indicate A/C is being requested whenever A/C is selected. If problem exists with A/C "ON," check A/C system operation. Refer to Chart C-10 A/C Clutch Circuit Diagnosis. Chart C-10 A/C Clutch Control Circuit Diagnosis

Check: Check AIR Injection Reaction (AIR) system. There should be no air to exhaust ports while in "Closed Loop." Refer to Chart C-6 Secondary Air Injection System Check. Chart C-6 Secondary Air Injection (AIR) System Check

Check: Crankcase ventilation valve for proper operation by placing finger over inlet hole in valve end several times. Valve should snap back. If not, replace valve. Refer to Emissions Control Systems/Positive Crankcase Ventilation. Positive Crankcase Ventilation

Check: PROM calibration I.D. for most current and/or incorrect software. Refer to PROM - Programmable Read Only Memory for calibration information.

Check: For broken motor mounts.

Check: Generator output voltage. Repair if less than 9 volts or more than 16 volts.