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.
- Perform careful visual check as described in "Before Starting". - Important Preliminary Checks
- Perform "Diagnostic Circuit Check". Testing and Inspection
- CHECK:
- Motor Mounts for damage, grounding out on the frame or mispositioned.
- Throttle linkage for sticking or binding.
- Ignition timing. See "Vehicle Emission Control Information" label.
- ECM ground circuits.
- IAC system. See Code 35. FOR THE 2.5L MOTOR ONLY.
- Generator output voltage. Repair if less than 9, or more than 16 volts.
- PIN switch circuit.
- PCV valve for proper operation, by placing finger over inlet hole in valve end several times. Valve should snap back. If not, replace valve.
- Evaporative emission control system.
- A/C signal to ECM terminal "B8". "Scan" tool should indicate A/C is being requested whenever A/C is selected and the pressure cycling switch is closed.
- Controlled idle speed.
- MAP sensor - ignition "ON," engine stopped. Compare MAP voltage with known good vehicle.
Voltage should be the same ±400 mV (0.4 volt).
or
Start and idle engine. Disconnect sensor electrical connector. If idle improves, substitute a known good sensor and recheck.
- A/C refrigerant pressure too high. Check for overcharge or faulty pressure switch.
- For injector(s) leaking. Check fuel pressure CHART A-6. Chart A-6 Fuel System Pressure Test
- If a sticking throttle shaft or binding linkage causes a high TPS voltage (open throttle indication), the ECM will not control idle. Monitor TPS voltage. "Scan" and/or Voltmeter should read less than 0.95 volt with throttle closed.
- Vacuum leaks can cause higher than normal idle.
- Battery cables and ground straps should be clean and secure. Erratic voltage will cause IAC to change its position, resulting in poor idle quality.
- Minimum idle air rate.
- Check AIR system.
- EGR valve: There should be no EGR at idle.
- Run a cylinder compression check.
- Inspect oxygen sensor 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 ECM will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem.
- Check ignition system: wires, plugs, rotor, etc.
- Monitoring block learn will help identify the cause of the problem.
If the system is running lean(block learn greater than 138), refer to "Diagnostic Aids" on text of Code 44. DTC 44
If the system is running rich (block learn less than 118), refer to "Diagnostic Aids" on text of Code 45. DTC 45
- Stalling may be due to an incorrect idle air rate. Refer to Code 35 (FOR THE 2.5L MOTOR ONLY) or "Idle Speed Error".