Rough, Unstable, or Incorrect Idle, Stalling

Engine Control Module (ECM)

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.
^ Injector balance.
^ If a sticking throttle shaft or binding linkage causes a high Throttle Position Sensor (TPS) voltage (open throttle indication) the Engine Control Module (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.
^ Battery cables and ground straps should be clean and secure. Erratic voltage will cause Idle Air Control (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 - Engine Control Module (ECM) should compensate for Power Steering loads. Loss of this signal would be most noticeable when parking and steering loads are high.
^ Manifold Absolute Pressure (MAP) Sensor - Ignition "ON", engine stopped. Compare MAP voltage with known good vehicle. Voltage should be within 400 mv (.4 volts).
^ Air flow sensor (sticking, improper CO adjustment or poor connection causing erratic voltage signal).
^ A/C compressor or relay.
^ A/C refrigerant pressure too high. Check for overcharge or faulty pressure switch.
^ Cooling fan inoperative.
^ Canister Purge System.
^ PCV valve for proper operation by placing finger over inlet hole in valve end several times. Valve should snap back. If not replace valve.
^ Run a cylinder compression check.
^ Inspect Oxygen sensor for silicone contamination from fuel, or use of improper Room Temperature Vulcanizing (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 for fuel in pressure regulator vacuum hose. If present, replace pressure regulator assembly.