Rough, Unstable, or Incorrect Idle, Stalling

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

Preliminary Checks
^ Perform the visual/physical checks as described under Diagnostic Notes in this section.
^ CHECK: For vacuum leaks.
^ CHECK: ECM/PCM grounds for being clean, tight and proper routing. See ECM,PCM wiring diagrams.

Fuel System

Notice: Monitoring O2S voltage will help identify the cause of the problem.

If the system is running Lean, refer to "Diagnostic Aids" found in Diagnostic Trouble Code 25 under Computers and Control Systems/Procedures/Diagnostic Charts.

If the system is running Rich, refer to "Diagnostic Aids" found in Diagnostic Trouble Code 26 under Computers and Control Systems/Procedures/Diagnostic Charts.

^ CHECK: Incorrect minimum idle speed may be caused by foreign material accumulation in the throttle bore coking. on the throttle valve or on the throttle shaft.
^ CHECK: Clean injectors.
^ CHECK: Fuel injector spray balance.
^ CHECK: For fuel in pressure regulator vacuum hose. if fuel is present, replace regulator assembly.
^ CHECK: EVAP TVV operation.
^ CHECK: The 02S should respond quickly to different throttle positions. If it does not, check the 02S 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/PCM will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem.

Ignition System
^ CHECK: Ignition timing. See "Vehicle Emission Control Information" label.
^ CHECK: Ignition system; wires, plugs. rotor, etc.

Engine Mechanical
^ CHECK: Perform a cylinder compression check.
^ CHECK: For camshaft wear or weak valve springs.

Additional Checks
^ CHECK Manifold absolute pressure (MAP) sensor.
^ CHECK Throttle linkage for sticking or binding.
^ CHECK Idle air control (IAC) valve operation.
^ CHECK A/C signal to ECM/PCM. If problem exists with A/C ON check A/C idle system operation.
^ CHECK Service Bulletins for ECM/PCM calibration updates.
^ CHECK For broken motor mounts.
^ CHECK Generator output voltage. Repair if less than 12 or more than 16 volts.
^ CHECK Electrical load idle-up circuit.
^ CHECK PCV valve for proper operation.
^ CHECK EGR valve. There should be no EGR at idle (California Emissions only).