Rough, Unstable, or Incorrect Idle, Stalling
DefinitionThe engine runs unevenly at idle. If severe enough, the engine or vehicle may shake. The engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine. The engine idles at the incorrect speed.
Preliminary Check
Refer to "Important Preliminary Checks." Testing and Inspection
Sensor Checks
^ Check the Heated Oxygen Sensor 1 (HO2S 1). Refer to "DTC P0131, P0132, P0133, P0134." Testing and Inspection
^ Check the Throttle Position (TP) sensor - If a sticking throttle shaft or binding linkage causes a high TP sensor voltage open throttle indication, the PCM will not control idle. Monitor the TP sensor voltage. A scan tool and/or voltmeter should read less than 1.25 (25%) volts with throttle closed.
^ Check the Engine Coolant Temperature (ECT) sensor - Using the scan tool, compare the engine coolant temperature with the ambient air temperature on a cold engine. If the engine coolant temperature is more than 5°C greater or less than the ambient air temperature, check for a high resistance in coolant sensor circuit or the sensor itself. Refer to DTC P0117 or P0118 "Diagnostic Aids." P0117
P0118
^ Check the Mass Air Flow (MAF) sensor response and accuracy.
Fuel System Checks
^ Check to determine if a Rich or Lean system causes the condition. Drive the vehicle at the speed of the complaint. Monitoring the Fuel Trim will help identify the problem.
- Lean - The Long Term Fuel Trim will be greater than 20%. Refer to DTC P0171 "Diagnostic Aids." P0171
- Rich - The Long Term Fuel Trim will be less than -20%. Refer to DTC P0172 "Diagnostic Aids." P0172
^ Check the Evaporative Emission (EVAP) control system.
^ Check the fuel injector(s) for leakage.
^ Check the fuel pressure. Refer to "Table A-7B." Related Tests, Information and Procedures
Ignition System Checks
^ Check the ignition voltage output using the spark tester J 26792 or equivalent.
^ Check the spark plugs for the following:
- Terminal screw - bent, loose (rotate and pull)
- Ceramic Insulator - cracks, tracking, deposits (red or black dust)
- tip Insulator - cracks, tracking, deposits (oil, carbon, glazing, fuel additive), loose (rattles)
- Center electrode - loose (wiggles side to side)
- Platinum pads - missing, wear, gap size (too large or too small)
^ Check the spark plug cables by connecting an ohmmeter to the ends of each cable in question. If the meter reads over 30,000 ohms, replace the cables.
^ Check the ignition timing.
Additional Checks
^ Check for vacuum leaks. Vacuum leaks can cause higher than normal idle and low Idle Air Control (IAC) counts.
^ Check the PCM grounds for clean, tight, and proper routing. Refer to PCM Wiring Diagram.
^ Check the scan tool to determine if the PCM is receiving an A/C signal, (with A/C "ON" an increase in idle speed should be indicated). If a problem exists with the A/C "ON", check the A/C system operation.
^ Check for the Exhaust Gas Recirculation (EGR) being "ON" while idling, which will cause roughness, stalling, and hard starting.
^ Check the battery cables and ground straps. They should be clean and secure. Erratic voltage will cause IAC to change its position, resulting in poor idle quality.
^ Check A/C refrigerant pressure for being too high or for a faulty high pressure switch.
^ Check the Crankcase Ventilation Valve for proper operation by placing a finger over inlet hole in valve end several times. The valve should snap back. If not, replace the valve.
Engine Mechanical Check
^ Check the engine for the following:
- Broken motor mounts
- Improper valve timing
- Low compression
- Bent push rods
- Worn rocker arms
- Broken or weak valve springs
- Worn camshaft lobes