Possible Causes & Corrections

WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Savana, 2003 Chevrolet Silverado, and 2003 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.

Check the following items:

Test for proper operation and performance of oxygen sensor. Inspect oxygen sensor for silicone contamination caused by fuel or use of improper engine sealants. The oxygen sensor will have a White powdery coating. The sensor will output a high, but false signal voltage. This false rich condition may cause the AF ECM or PCM to reduce amount of fuel to engine.
Test the ECT sensor for proper operation by performing the following:
1. Allow the engine temperature to reach ambient air temperature by leaving engine off for several hours or overnight for most accurate results.
2. Turn ignition on, with engine off.
3. Using scan tool, compare the ECT and IAT sensors displayed temperature. The sensors should be within 5°F (3°C) of each other.
4. Replace ECT sensor or repair ECT sensor circuit, if ECT is not near the expected value.
Test for proper MAP sensor operation. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article. A MAP sensor that does not respond quickly and accurately to manifold pressure changes may cause this symptom.
Determine if condition is rich or lean by operating vehicle under the conditions of the concern. Monitor oxygen sensor output.
Test for leaking fuel injectors.
Inspect for gasoline in the fuel pressure regulator vacuum hose, if equipped. This indicates a faulty gasoline fuel pressure regulator.
Inspect for proper operation of CNG fuel injectors. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
Test for proper EVAP Purge system operation. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.
Test for proper alternative fuel system pressures. See FUEL SYSTEM PRESSURE TEST under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
Inspect for manifold or air induction system leaks.
Test PCV system for proper operation and valve type. See EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
Test for proper engine cylinder compression. See CYLINDER COMPRESSION PRESSURE underMECHANICAL INSPECTION in PRELIMINARY INSPECTION & ADJUSTMENTS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.