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 in Preliminary Checks. Preliminary Checks

CHECK: For Vacuum leaks.

CHECK: Engine Control Module (ECM) grounds for being clean, tight and proper routing.

FUEL SYSTEM

NOTE: To determine if the condition is caused by a rich or lean system, the car should be driven at the speed of the complaint. Monitoring Block Learn will help identify a problem.

Lean: Block Learn above 142. Refer to Diagnostic Aids of Diagnostic Trouble Code 44.
DTC 44

Rich: Block Learn above 118. Refer to Diagnostic Aids of Diagnostic Trouble Code 45.
DTC 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 Diagnostic Charts / Chart C-3.
Chart C-3: Evaporative Emission Canister Purge Valve

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 Diagnostic Charts / Chart C-1D.
Chart C-1D: Manifold Absolute Pressure (MAP) Sensor

CHECK: Throttle Linkage for sticking or binding.

CHECK: Park/Neutral Position Switch Circuit. See Diagnostic Charts / Chart C-1A, or use "Scan Tool" and be sure tool indicates Vehicle is in drive with Gear Selector in Drive or Overdrive.
Chart C-1A: Park/Neutral Position (PNP) Switch

CHECK: Idle Air Control operation, see Diagnostic Charts / Chart C-2C.
Chart C-2C: Idle Air Control (IAC) System Check

CHECK: Exhaust Gas Recirculation Valve. There should be no Exhaust Gas Recirculation at idle. Refer to Diagnostic Charts / Chart C-7A.
Chart C-7A: EGR System Check

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, refer to Diagnostic Charts / Chart C-10.
Chart C-10: Air Conditioning System Function Check

CHECK: Positive Crankcase Ventilation Valve for proper operation.

CHECK: For broken Motor Mounts.

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