Rough, Unstable, or Incorrect Idle and Stalling. GMC Truck Safari

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 2002 🢒 Safari - 2WD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Inspections

Action

DEFINITION: The engine runs unevenly at idle. If severe enough, the engine or vehicle may shake. The engine idle speed may vary. The condition may be severe enough to stall the engine.

Preliminary

Refer to Symptoms - Engine Controls .

Sensor

•	Inspect the heated oxygen sensor (HO2S).

Inspect for silicon contamination from fuel or improperly used sealant. The sensor will have a white powdery coating. The condition will result in a low or a high but false signal voltage, or rich exhaust indication. The control module will reduce the amount of fuel delivered to the engine causing a severe driveability problem.

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Inspect the throttle position (TP) sensor. If a sticking throttle shaft or binding linkage causes a high TP sensor open throttle indication, the control module will not control the idle. Monitor the TP sensor voltage. A scan tool or voltmeter should read less than 0.9 volts with the throttle closed.

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Inspect the engine coolant temperature (ECT) sensor using the scan tool in order to compare the engine coolant temperature with the ambient air temperature on a cold engine. If the coolant temperature reading is more than 5 degrees more than or less than the ambient air temperature on a cold engine, Inspect for a high resistance or for a low resistance in the coolant sensor circuit or the sensor itself.

Fuel System

•	Test the fuel pressure. Refer to Fuel System Diagnosis .

•	Perform the fuel injector balance test. Refer to Fuel Injector Balance Test with Tech 2 .

•	Perform the fuel injector coil test. Refer to Fuel Injector Solenoid Coil Test .

•	Inspect the evaporative emission (EVAP) control system. Refer to Service Bay Test .

Ignition System

•	Inspect the ignition output using the J 26792 spark tester or the equivalent. Refer to Distributor Ignition (DI) System Diagnosis .

•	Inspect the spark plugs. Refer to Spark Plug Inspection .

•	Inspect the spark plug wires. Refer to Ignition System Specifications .

Additional

•	Inspect for vacuum leaks. Vacuum leaks can cause a higher than normal idle and low idle air control (IAC) counts.

•	Inspect the IAC operation. Refer to Idle Air Control (IAC) System Diagnosis .

•	Inspect the control module grounds for being clean, tight, and in their proper locations. Refer to Power and Grounding Component Views .

•	Inspect the scan tool to determine if the control module is receiving an A/C signal.

•	Inspect the battery cables and ground straps. They should be clean and secure.

•	Inspect the crankcase ventilation valve for proper operation by placing a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve.

Engine Mechanical

Inspect for the following:

•	Broken motor mounts--Refer to Engine Mount Inspection in Engine Mechanical.

•	Improper valve timing

•	Low compression--Refer to Engine Compression Test in Engine Mechanical.

•	Bent pushrods--Refer to Valve Rocker Arm and Push Rod Cleaning and Inspection in Unit Repair.

•	Worn rocker arms--Refer to Valve Rocker Arm and Push Rod Cleaning and Inspection in Unit Repair.

•	Broken or weak valve springs

•	Worn camshaft lobes--Refer to Camshaft and Bearings Cleaning and Inspection in Unit Repair.