GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 GMC C Sierra - 2WD 🢒 Engine 🢒 Engine Controls - 6.6L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Inspection

Action

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 the engine.

Preliminary Inspection
    • Refer to Important Preliminary Inspection Before Starting in Symptoms - Engine Controls .
    • Search for bulletins.
    • Inspect for RPO KPJ. If equipped, inspect for low coolant, low oil pressure, or an engine over-temperature conditions.
    • Inspect the vehicle for factory or aftermarket accessories that may contact the body and chassis. A grounding out condition may simulate a misfire or rough run concern.
    • Compare the scan tool data at idle with the scan tool data list. Refer to Scan Tool Data List .
    • Inspect the engine control module (ECM) grounds for being clean, tight, and in the proper locations.
    • Remove and inspect the air filter element for dirt or for being restricted. Refer to Air Cleaner Element Replacement . Replace as necessary.

Sensor Inspection
    • Turn ON the ignition with the engine OFF. Observe the actual fuel rail pressure with a scan tool. The actual fuel rail pressure should be between 1.0-1.8 MPa. If it is not, inspect for high resistance in the fuel rail pressure (FRP) sensor circuits or the FRP sensor. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
    • Test for an intermittent FRP sensor signal by wiggling the harness between the sensor and the ECM with the ignition ON and the engine OFF, while monitoring the parameter with a scan tool.
    • Observe the FRP sensor and the fuel pressure regulator commanded percent at idle. FRP should be near 40 MPa, and commanded percent should be near 40 percent. If the command is high, a fuel pressure or sensor exists.
    • Inspect for an intermittent crankshaft position (CKP) sensor signal. Refer to Intermittent Conditions .
    • Inspect the CKP sensor reluctor wheel. Remove the CKP sensor and attempt to move the reluctor wheel front to back or side to side with a probe. If there is any movement the reluctor wheel retaining bolts are loose.
    • Inspect for proper crankcase ventilation.

Fuel System Inspection
    • Perform the fuel injector balance test. Refer to Fuel Injector Balance Test with Tech 2 .
    • Inspect for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Inspect for a restriction in the fuel return system. Refer to Fuel Return System Diagnosis .
    • Inspect for a contaminated fuel condition. Refer to Contaminants-in-Fuel Diagnosis .
    • Inspect for a sticking Fuel Pressure Regulator. The symptom for this condition will be an idle surge of at least 100 RPM. This surge will be from 50 RPM above Desired Idle Speed to 50 RPM below Desired Idle Speed. Refer to Fuel System Diagnosis - High Pressure Side .

Engine Mechanical Inspection
    • Inspect engine mechanical for the following:
       - Inspect the cylinder compression. Refer to Engine Compression Test in Engine Mechanical - 6.6L.
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in the combustion chamber--Leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical - 6.6L.
    • Inspect the following components for incorrect basic engine parts:
       - Camshaft--Refer to Camshaft and Bearings Cleaning and Inspection in Engine Mechanical - 6.6L.
       - Cylinder heads--Refer to Cylinder Head Cleaning and Inspection in Engine Mechanical - 6.6L.
       - Pistons, connecting rods, or bearings--Refer to Piston, Connecting Rod, and Bearing Cleaning and Inspection in Engine Mechanical - 6.6L.

Additional Inspection
    • Inspect the following components of the exhaust system for possible restrictions:
       - The exhaust system for damaged or collapsed pipes
       - The exhaust manifold for a collapsed inner wall
       - The mufflers for heat distress or possible internal failure
    • Electromagnetic interference (EMI) on the reference circuit can cause an engine miss condition. A scan tool can usually detect EMI by monitoring the engine RPM. A sudden increase in RPM with little change in actual engine RPM change indicates that EMI is present. If a problem exists, inspect routing of high voltage components, such as fuel injector wiring, near the sensor circuits.
    • Inspect the park neutral position (PNP) switch circuit.
    • Inspect for a short to 5 volts on one of the fuel injector control circuit between the ECM and the FICM.
    • Inspect for faulty motor mounts. Refer to Engine Mount Inspection in Engine Mechanical - 6.6L.
    • Inspect the intake manifold and the exhaust manifold passages for casting flash.