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: Do not compare power output of the vehicle while operating on CNG to a vehicle operating on gasoline. Both fuels have different drive feel characteristics.

Compare customer vehicle with a similar unit. Verify the customer's concern.
Remove air filter and check for dirt or a restriction.
Inspect transmission shift pattern and downshift operation.
Inspect PCM for correct CNG calibration. Program PCM, if necessary.
Inspect for proper fuel pressure. See FUEL SYSTEM PRESSURE TEST under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
Inspect the High Pressure Regulator (HPR) filter for restriction.

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

Test for proper secondary ignition system voltage output using Spark Tester (J-26792).

NOTE: More timing advance is required to operate vehicle on CNG than on gasoline.

Test for proper operation of the Knock Sensor (KS) system. 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 exhaust system for a possible restriction. Restrictions can include a failed catalytic converter, damaged or collapsed pipes, or an internally failed muffler.
Correct engine compression.
Correct engine valve timing.
A worn or incorrect camshaft.
Test for proper A/C compressor clutch operation. An A/C clutch that is engaged all the time may cause this symptom.
Test for proper transmission Torque Converter Clutch (TCC) operation, if equipped. See appropriate DIAGNOSIS article in AUTOMATIC TRANSMISSIONS.
Test for proper generator operation.
Inspect for proper operation of accelerator pedal. A stretched accelerator cable may cause this symptom.