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 engine.

PRELIMINARY CHECKS
- Perform the careful visual / physical checks as described in Preliminary Checks.
- Check for vacuum leaks.
- Check Powertrain Control Module (PCM) grounds for being clean, tight and proper routing.
- Service Bulletins for EPROM updates.

IGNITION SYSTEM CHECKS
- Ignition system. Spark plugs and ignition components. Refer to Ignition System. Ignition System

ENGINE MECHANICAL CHECKS
- Perform a cylinder compression test.
- For correct camshaft.

FUEL SYSTEM CHECKS
- Monitoring Long Term (L.T.) fuel trim will help identify a problem.
Lean - L.T. Fuel Trim near 198. Refer to "Diagnostic Aids" in the text of Diagnostic Chart 44. Testing and Inspection
Rich - L.T. Fuel Trim near 58. Refer to "Diagnostic Aids" in the text of Diagnostic Chart 45. Testing and Inspection
- Injector balance. Refer to Chart C-2A. Testing and Inspection
- For fuel in pressure regulator hose. If fuel is present, replace regulator assembly.
- Evaporative Emission Control (EVAP) system, Use Chart C-3. Testing and Inspection
- The Oxygen Sensor (O2S). The O2S should respond rapidly to the varying quantities of oxygen in the exhaust system. After the O2S has reached operating temperature, the voltage should change quickly, and can go as high as 1 volt and as low as 0.1 volt in less than a second. If this response time is slow or voltage is at a steady level, the O2S should be checked for silicon, glycol or other contamination The O2S may have a white powdery coating (silicon contamination) or a green coating (glycol contamination) that would result in a false signal voltage to the Powertrain Control Module, and may lead to a driveability complaint.

ADDITIONAL CHECKS
- Manifold Absolute Pressure (MAP) sensor. Refer to Chart C-1D. Testing and Inspection
- Throttle linkage for sticking or binding.
- Park / Neutral Position (PNP) switch or trans range switch circuit. Use Tech 1 scan tool and be sure tool indicates vehicle is in drive with gear selector in drive or overdrive (if equipped).
- Idle Air Control (IAC) valve operation, Use Chart C-2C. Testing and Inspection
- A/C operation. If problem exists with A/C "ON," check A/C system operation CHART C-10. Testing and Inspection
- For broken motor mounts.
- Generator output voltage. Repair if less than 9 or more than 16 volts.
- For intake valve deposits.