Engine Controls - Basic Diagnostic Procedures - 6.0L W/bifuel & Cng Engines: Overview Chevrolet Silverado 1500 HD

System Description

The PCM controls the fuel system while the vehicle is operating on CNG. The PCM receives signals from various engine sensors and determines the correct amount of fuel required by the engine. Fuel is stored in cylindrical fuel tanks. A full tank can contain fuel at 3600 psi (24,820 kPa). A High Pressure Lock-Off (HPL) solenoid in each tank and a Low Pressure Lock-Off (LPL) solenoid in the fuel line prevent fuel flow. The PCM commands only the HPL's open for one second at every ignition ON for a CNG prime pulse. The CNG prime pulse is performed in order to charge the fuel line and also allows Fuel Tank Pressure (FTP) sensor to monitor the amount of fuel pressure in the tanks. The PCM commands BOTH the HPL's and the LPL open when engine RPM indicates the engine is cranking or running on CNG.

This fuel delivery system utilizes a High Pressure Regulator (HPR) in order to lower the fuel pressure from tank pressure down to an injector delivery pressure. When the PCM commands the HPL's open, fuel begins to flow through the HPR. The HPR reduces pressure to between 130-200 psi (896-1379 kPa). The fuel exiting the HPR flows through the intermediate fuel line to the fuel rail mounted LPL solenoid. Fuel flows out of the LPL and through the fuel rails to the injectors.

Test Description

The numbers below refer to step numbers on diagnostic procedure.

1. 2 - This step verifies that the HPL solenoid and the FTP sensor are functioning. A fuel tank pressure value that decreases while the engine is operating indicates the HPL is not opening or the manual tank valve is closed.
2. 3 - This step verifies the FRP sensor is displaying the correct fuel pressure.
3. 4 - This step tests for a leak in the fuel system between the LPL and fuel injectors. The fuel pressure should remain constant during the test.
4. 5 - This step tests the HPR output pressure. If the fuel pressure is within the specified values, then the HPR, fuel lines, and filters are okay.
5. 6 - This step tests the operation of the AF lock-off relay controlling the HPL solenoid.
6. 9 - This step tests the operation of the AF lock-off relay controlling the LPL solenoid.
7. 13 - This step verifies that a fuel filter is not the cause of the condition. If fuel pressure is within the specified range after the fuel filter replacement, then a plugged filter was the cause of the condition.

1. Perform diagnostic system check. See «DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS»(ref-159156-S07074324102003111300000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG or «DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS»(ref-159297-S00366181602003112400000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article. After performing diagnostic system check, go to next step.
2. Connect vehicle to a CNG dispensing station and verify that the CNG fuel tank is at least 1/3 full. Fill CNG fuel tank, if necessary. Observe the Fuel Tank Pressure (FTP) sensor data display with a scan tool. Attempt to start and idle the engine. Does the FTP sensor display a steady pressure at or greater than 1200 psi (8274 kPa)? If yes, go to step next. If no, go to step 6.
3. Attempt to start and idle engine. Operate engine at idle until normal operating temperature is obtained. If engine will not start on CNG, use the pressure values observed during cranking. Observe and record the FRP sensor display with a scan tool. Compare the recorded scan tool display pressure values to the value range of 130-200 psi (896-1379 kPa)? Is FRP sensor value within the specified range? If yes, go to next step. If no, go to step 8.
4. Turn ignition off. Turn ignition on, with engine off. Command the LPL solenoid ON and OFF with a scan tool in order to charge the fuel rail with pressure. Monitor the fuel rail pressure for 5 minutes after commanding the LPL solenoid closed. Does the fuel rail pressure drop more than 3 psi (21 kPa)? If yes, go to step 10. If no, go to next step.
5. If engine will not start on CNG, use the pressure values observed during cranking. Observe FRP sensor display with the scan tool while cranking the engine. Operate engine at idle until normal operating temperature is obtained. Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive. With engine idling, observe and record the FRP sensor display on scan tool. Observe and record the pressure indicated by the FRP sensor display during engine load by performing the following steps: Apply parking brake. Firmly apply and hold brake pedal. Place vehicle in forward gear. Observe pressure while depressing throttle to 75 percent or greater for a maximum of 10 seconds. Allow engine to idle for a minimum of 2 minutes to cool transmission. Compare the recorded pressure values to the value range of 130-200 psi (896-1379 kPa). Are all of the fuel pressure values within the specified range? If yes, problem is intermittent. If no, go to step 12.
6. Turn ignition on, with engine off. Command the HPL on and off with scan tool. Listen for operation of the HPLs. HPL is located on CNG tank. Can operation of the HPLs be heard? If yes, go to next step. If no, diagnose fuel lock-off relay. See «ALTERNATIVE FUEL LOCK-OFF RELAY»(ref-153237-S00750211782003041600000) under MODULES, MOTORS, RELAYS & SOLENOIDS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
7. Ensure manual tank valves are fully open by turning the screws fully counterclockwise. (Схема №1) Did you have to open the manual tank valves? If yes, go to step 2. If no, go to step 14.
8. Is the fuel pressure greater than 200 psi (1379 kPa)? If yes, go to step 15. If no, go to next step.
9. Turn ignition on, with engine off. Command the LPL on and off with scan tool. Listen for operation of the LPL. LPL is located on left engine valve cover. Can operation of the LPL be heard? If yes, go to step 12. If no, diagnose AF fuel lock-off relay. Relay is attached to underhood fuse/relay block. See «ALTERNATIVE FUEL LOCK-OFF RELAY»(ref-153237-S00750211782003041600000) under MODULES, MOTORS, RELAYS & SOLENOIDS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
10. Inspect for a fuel system leak between the LPL solenoid and CNG fuel injectors. Replace any leaking components as necessary. Did you find and correct the condition? If yes, go to step 16. If no, go to next step.
11. Raise CNG fuel rails, lines and injectors as an assembly. Command the LPL on with scan tool. Locate and replace the leaking CNG fuel injector. After repairs, go to step 16.
12. Inspect the fuel line between the HPL and HPR and the fuel line between the HPR and LPL for kinks, bends, damage or obstruction. An obstructed fuel line may be cool or frosted near the obstruction when fuel flows through the line. Repair or replace components as necessary. Did you find and correct the condition? If yes, go to step 16. If no, go to next step.
13. Release CNG fuel system pressure. Replace CNG in-line filter. Replace the HPR filter. Observe the FRP sensor display with a scan tool while cranking engine. Operate engine until normal operating temperature is obtained. Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive. Observe and record the FRP sensor display at engine idle. Observe and record the FRP sensor display during engine load by performing the following steps: Apply parking brake. Firmly depress and hold brake pedal. Place vehicle in forward gear. Observe the pressure while depressing the throttle to 75 percent or greater for a maximum of 10 seconds. Allow the engine to idle for a minimum of 2 minutes to cool the transmission. Compare the recorded pressure values to the value range of 130-200 psi (896-1379 kPa). Are all fuel pressure values within the specified range? If yes, go to step 16. If no, go to step 15.
14. Replace FTP sensor. FTP sensor is threaded into HPL, and threaded into CNG tank end. After replacing sensor, go to step 16.
15. Replace the HPR. After replacing HPR, go to next step.
16. Release CNG fuel system pressure. Turn ignition off. Remove all remaining test equipment. Start and operate engine at idle. Inspect for CNG fuel system leaks using an Ultrasonic Leak Detector (J-41416) or soapy water. Repair any leaks as necessary. Clear DTC information from PCM with a scan tool. Turn ignition off and wait 30 seconds. Start engine and operate vehicle. Observe MIL. Observe vehicle performance and driveability. Does the vehicle operate on CNG with normal driveability and no MIL illumination? If yes, system is okay. If no, see «DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS»(ref-159156-S07074324102003111300000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or «DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS»(ref-159297-S00366181602003112400000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.