Rough, Unstable, or Incorrect Idle, Stalling

DEFINITION:


The engine runs unevenly at idle. If bad enough, the car 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.

CHECK:
- Perform the usual checks for obvious problems (vacuum leaks, vacuum and electrical circuits correctly connected arcing at distributor or plug wires etc.)
- Ignition timing. See Emission Control Information label.
- Injectors for leaking or restriction. Visually check fuel spray from injectors. If there is spray from only one injector, then there is a malfunction in the injector assembly, or in the signal to the injector assembly. The malfunction can be isolated, by switching the injector connectors. If the problem remains with the original injector, after switching the connectors, then the injector is defective. Replace the injector. If the problem moves with the injector connector, then, the problem is an improper signal in the injector circuits.
- Fuel pressure.
- IAC valve for sticking or improper operation.
- 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 1.2 volts with throttle closed.
- EGR "ON" while idling will cause roughness, stalling, and hard starting.
- Check PCV valve for proper operation.
- Battery cables and ground straps should be clean and secure. Erratic voltage will cause IAC valve to change its position resulting in poor idle quality. IAC valve will not move if system voltage is below 9 or greater than 17.8 volts.
- Power Steering - ECM should compensate for Power Steering loads. Loss of this signal would be most noticeable when parking and steering loads are high.
- MAP Sensor - Ignition "ON", engine stopped. Compare MAP voltage with known good vehicle. Voltage should be within 400 mv (.4 volts).
- Perform compression test.
- Inspect Oxygen sensor for silicone 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.