Rough, Unstable or Incorrect Idle, Stalling

DEFINITION
The engine runs unevenly at idle. If bad enough, the vehicle may shake. Also, the idle may vary in RPM (called "hunting"). Either condition may be severe enough to cause stalling. Engine idles at incorrect speed.

PRELIMINARY CHECKS
Refer to Preliminary Check. - Important Preliminary Checks

SENSORS
Oxygen Sensor (O2S)
Check the O2S for silicon contamination from fuel, or use of improper RTV sealant. The sensor will have a white, powdery coating, and will result in a high but false signal voltage (rich exhaust indication). The control module will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem.

Throttle Position (TP) sensor
If a sticking throttle shaft or binding linkage causes a high TP sensor voltage open throttle indication, the control module will not control idle. Monitor TP sensor voltage. A scan tool and/or voltmeter should read less than 0.85 volt with throttle closed.

Engine Coolant Temperature (ECT) sensor
Using a scan tool, compare engine coolant temperature with ambient temperature on a cold engine.
If engine coolant temperature reads 5 degrees greater than or less than ambient air temperature. Check for high resistance in coolant sensor circuit or sensor itself. Refer to Diagnostic Charts / DTC 15 Coolant Temperature Circuit. Testing and Inspection

Manifold Absolute Pressure (MAP) sensor
Check the MAP sensor response and accuracy. Refer to Diagnostic Charts / C-Charts / Manifold Absolute Pressure Output Test. Testing and Inspection

FUEL SYSTEM
To determine if the condition is caused by a rich or lean system, the vehicle should be driven at the speed of the complaint. Monitoring fuel trim will help identify problem.

Lean - Long term fuel trim greater than 150.
Refer to Diagnostic Aids in Diagnostic Charts / DTC 44 Oxygen Sensor Circuit (Lean). Testing and Inspection

Rich - Long term fuel trim less than 115.
Refer to Diagnostic Aids in Diagnostic Charts / DTC 45 Oxygen Sensor Circuit (Rich). Testing and Inspection

CHECK:

- Idle speed control actuator operation (7.4L only).
- Evaporative emission control system.
- Perform a cylinder compression check.
- For injector(s) leaking. Check fuel pressure. Refer to Diagnostic Charts / Chart A-6 Fuel System Diagnosis. Testing and Inspection

IGNITION SYSTEM
Check ignition system and ignition timing.

ADDITIONAL CHECKS
CHECK:

- Vacuum leaks can cause higher than normal idle and low Idle Air Control (IAC) counts. Refer to Diagnostic Charts/ C-Charts/ Idle Air Control System Check. Testing and Inspection
- Control module grounds for clean, tight, and proper routing.
- Transmission Range (TR) pressure switch assembly.
- Use scan tool to determine if control module is receiving A/C request signal. If problem exists with A/C "ON," check A/C system operation. Refer to Diagnostic Charts / C-Charts / A/C Request Signal Diagnosis. Testing and Inspection
- EGR "ON" while idling, will cause roughness, stalling and hard starting. Refer to Diagnostic Charts / C-Charts / EGR System. Testing and Inspection
- Battery cables and ground straps should be clean and secure. Erratic voltage will cause IAC to change its position, resulting in poor idle quality.
- IAC valve will not move if system voltage is below 9 volts or greater than 16 volts.
- A/C refrigerant pressure too high or faulty high pressure switch.
- 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.

ENGINE MECHANICAL
Check for broken motor mounts, proper valve timing, low compression, bent pushrods, worn rocker arms, broken or weak valve springs, worn camshaft lobes.