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 for the following:

Test for proper EVAP Purge system operation (RPO KL6 engine only). 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.
Inspect fuel injectors for leakage or diagnose fuel system. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
If engine continues to run smoothly when ignition is turned off, inspect for proper ignition switch adjustment or a short to voltage in the AF ECM or PCM ignition feed circuit.