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:

NOTE: A higher secondary ignition voltage is required to operate vehicle on CNG than on gasoline.

Inspect for proper secondary ignition system voltage output using Spark Tester (J-26792).
Remove and inspect spark plugs for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits.
Test for proper ignition system operation.
Test for crossfire between spark plugs. Test for spark plug wire failure. Wetting down the secondary ignition system with a water spray bottle may aid in locating damaged or deteriorated components.
Inspect engine for intake manifold leaks, improper valve timing, proper engine cylinder compression, sticking or leaking intake or exhaust valves or exhaust system leakage.
Inspect intake and exhaust system for casting flash.
Check fuel system performance. See FUEL SYSTEM PRESSURE TEST under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
Review service bulletins for AF ECM or PCM programming updates.
Verify that correct CNG software and calibration has been programmed into PCM.