Rough, Unstable, or Incorrect Idle, Stalling

Definition: Engine runs unevenly at idle. If severe, the engine or car may shake. Engine idle speed may vary in rpm. Either condition may be severe enough to stall engine.


PRELIMINARY CHECKS

- Perform the careful visual/physical checks as described at start of "Symptoms".

- CHECK: For vacuum leaks

- CHECK: Engine control module (ECM) grounds for being clean, tight and proper routing. See ECM wiring diagrams.


FUEL SYSTEM

- NOTICE: Monitoring L.T. Fuel Trim will help identify the cause of the problem. If the system is running lean (L.T. Fuel Trim greater than 142), refer to "Diagnostic Aids" of diagnostic trouble code 44.

If the system is running rich (L.T. Fuel Trim less than 118), refer to "Diagnostic Aids" of diagnostic trouble code 45.

- CHECK: Incorrect minimum idle speed may be caused by foreign material accumulation in the throttle bore coking, on the throttle body or on the throttle shaft.

- CHECK: Clean injectors.

- CHECK: For fuel in fuel pressure control valve hose. If fuel is present, replace regulator assembly.

- CHECK: Evaporative Emission Control System, refer to "Evaporative Emission Control System".

- CHECK: The Heated oxygen sensor should respond quickly to different throttle position, if it does not, check the heated oxygen sensor for silicon contaminations 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 heated oxygen sensor signal indication). The ECM will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem.

IGNITION SYSTEM

- CHECK: Ignition timing, using a "Scan tool".

- CHECK: Ignition system; wires, plugs, electronic ignition, etc.


ENGINE MECHANICAL

- CHECK: Perform a cylinder compression check.

- CHECK: For correct camshaft or weak valve springs.


ADDITIONAL CHECKS

- CHECK: Manifold absolute pressure sensor. Refer to CHART C-10.

- CHECK: Throttle linkage for sticking or binding.

- CHECK: Park/neutral position switch circuit. See CHART C-1A, or use "Scan tool" and be sure tool indicates vehicle is in drive with gear selector in drive or overdrive.

- CHECK: Idle air control operation, use CHART C-2C.

- CHECK: Exhaust gas recirculation valve. There should be no exhaust gas recirculation at idle. Refer to CHART C-7A.

- CHECK: Air conditioning (A/C) signal to engine control module, "Scan tool" should indicate A/C is being requested when ever A/C is selected. If problem exists with A/C "ON," check A/C system operation CHART C-10.

- CHECK: Positive crankcase ventilation valve for proper operation. Refer to "Positive Crankcase Ventilation (PCV)".

- CHECK: For broken motor mounts.

- CHECK: Generator output voltage. Repair if less than 9 or more than 16 volts.