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 all of the High Pressure Lock-Off (HPL) solenoids for proper operation. An inoperative HPL will cause a decrease in amount of fuel available. This will be perceived as poor fuel economy.
Inspect air filter for proper type and for dirt or restriction.
Inspect vacuum hoses for leaks, restrictions and proper connections as shown on the Vehicle Emission Control Information (VECI) label.
Review driver's driving habits. Check for A/C compressor clutch being engaged all the time, excessive idling or stop and go driving, proper tire inflation, low CNG refill pressure (check fill station pressure), excessively heavy loads being carried, or too much acceleration, too often.
Advise vehicle operator to fill the fuel tank and recheck fuel economy.
Test 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 for fuel system leakage.
Test the ECT sensor for proper operation by performing the following:
1. Allow engine temperature to reach ambient air temperature by leaving the engine off for several hours or overnight for most accurate results.
2. Turn ignition on, with engine off.
3. Using scan tool, compare 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.
Remove and inspect spark plugs for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits.
Test for an open Ignition Control (IC) circuit.
Inspect spark plug wires for cracking, hardness or proper connections and routing.

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

Test for proper operation of 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.