Diagnostic Procedure

Perform Alternative Fuels Diagnostic System Check - Engine Controls. See ALTERNATIVE FUELS DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article. After performing Alternative Fuels Diagnostic System Check - Engine Controls, go to next step.
Did you perform Fuel Injector Coil Test? If yes, go to next step. If no, go to FUEL INJECTOR COIL TEST (BI-FUEL & CNG) .
Start and idle engine. Observe Fuel Rail Pressure (FRP) sensor parameter with scan tool. Is fuel pressure within 40-50 psi (275-345 kPa)? If yes, go to next step. If no, go to FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG) .
Turn OFF the ignition. Turn ON the ignition, with the engine OFF. Monitor Fuel Rail Pressure (FRP) sensor parameter with scan tool. Does fuel rail pressure remain constant? If yes, go to next step. If no, go to FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG) .

CAUTION: DO NOT repeat any portion of this test before running engine in order to prevent engine from flooding.

NOTE: Record fuel pressure value immediately after CNG fuel injector stops pulsing. Fuel pressure may rise after fuel injector stops pulsing. DO NOT record higher fuel pressure value.

With scan tool, select Fuel Injector Balance Test function, within Special Functions menu. Select an injector to be tested. Press Enter. This will prime fuel system. Record fuel pressure indicated by fuel pressure sensor after fuel rail pressure stabilizes. This is first pressure reading.
Energize fuel injector by depressing Pulse Injector button on scan tool. This will energize injector and decrease fuel pressure. Record fuel pressure indicated by FRP sensor after fuel injector has stopped pulsing. This is second fuel pressure reading. Press Enter again to bring you back to Select Injector screen. Repeat for each fuel injector. Subtract second pressure reading from first pressure reading for one fuel injector. The result is pressure drop value. Obtain a pressure drop value for each fuel injector. Add all individual pressure drop values. This is total pressure drop. Divide total pressure drop by number of fuel injectors. This is average pressure drop. Does any fuel injector have a pressure drop value that is more than average pressure drop or less than average pressure drop by 2.0 psi (14 kPa)? If yes, go to next step. If no, go to SYMPTOM DIAGNOSIS under SYMPTOMS (BI-FUEL & CNG) in TROUBLE SHOOTING - NO CODES - TRUCKS article.
Replace affected CNG fuel injector. After repairs, go to next step.
Operate vehicle in order to verify repair. Does a driveability condition still exist? If yes, go to SYMPTOM DIAGNOSIS under SYMPTOMS (BI-FUEL & CNG) in TROUBLE SHOOTING - NO CODES - TRUCKS article. If no, system is okay.