Engine Controls - Basic Diagnostic Procedures - 6.0L W/bifuel & Cng Engines — Chevrolet Chevy Express H2500 (2003-2003)

INTRODUCTION

Note. Unless specified otherwise, Savana information also applies to Savana Special.

The following diagnostic steps help prevent overlooking simple problems. This is also where to begin diagnosis for a no start condition.

Before proceeding with basic diagnostic procedures, ensure testing procedure is followed, as some systems which may result in a no-start condition may cause a Diagnostic Trouble Codes (DTC) to be stored in the Powertrain Control Module (PCM). See DIAGNOSTIC STARTING POINT - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or DIAGNOSTIC STARTING POINT - ENGINE CONTROLS under SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.

If no-start condition still exists after checking and repairing all DTCs, perform a careful and complete visual inspection. Many problems result from mechanical breakdowns, poor electrical connections or damaged/misrouted vacuum hoses. See VISUAL INSPECTION under PRELIMINARY INSPECTION & ADJUSTMENTS. Before condemning the computerized system, perform each test listed in this article.

Note. Perform all voltage tests with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure.

VISUAL INSPECTION

Visually inspect all electrical wiring. Look for chafed, stretched, cut or pinched wiring. Ensure electrical connectors fit tightly and are not corroded. Ensure vacuum hoses are properly routed and not pinched or cut. If necessary, see VACUUM DIAGRAMS - EXPRESS & SAVANA article or VACUUM DIAGRAMS - SIERRA & SILVERADO article to verify routing and connections. Inspect air induction system for possible vacuum leaks.

Cylinder Compression Pressure

Check engine mechanical condition with compression gauge, vacuum gauge, or engine analyzer. See engine analyzer manual for specific instructions. For compression specifications, see ENGINE COMPRESSION SPECIFICATIONS table. Install and tighten spark plugs to 11 ft. lbs. (15 N.m).

Application	Specification - psi (kPa)
Normal Compression Pressure	(1)
Minimum Compression Pressure	(1)
Compression Variance	(2)
(1) Lowest compression reading should not be less than 70 percent of highest compression reading. No cylinder compression reading should be less than 100 psi (689.5 kPa). (2) Information is not available from manufacturer.

(1)	Lowest compression reading should not be less than 70 percent of highest compression reading. No cylinder compression reading should be less than 100 psi (689.5 kPa).

(2)	Information is not available from manufacturer.

ENGINE COMPRESSION SPECIFICATIONS

Cylinder Combustion Pressure Leakage

Disconnect negative battery cable. Remove spark plugs. Install Cylinder Head Leakdown Tester (J-35667 A). Measure each cylinder on compression stroke, with both valves closed.
Apply air pressure to cylinder leakdown tester. Record cylinder leakage readings for each cylinder. Any cylinder with 25 percent leakage or more requires service.
Inspect following primary areas in order to properly diagnose a leaking cylinder. If air is heard from intake or exhaust system, remove valve rocker arm cover of suspect cylinder head. Ensure that both valves are closed. Inspect cylinder head for a broken valve spring or remove and inspect suspect cylinder head. If air is heard from crankcase system at crankcase oil filler tube, remove piston from suspect cylinder. Inspect piston and connecting rod assembly. Inspect engine block. If bubbles are observed in radiator, remove and inspect both cylinder heads. Inspect engine block.
After performing test, remove leakdown tester. Install spark plugs and reconnect negative battery cable.

Exhaust System Backpressure

Before replacing any components, check exhaust system for restrictions. Use vacuum gauge or low pressure (0-5 psi) gauge to check exhaust system.

If vacuum gauge is used, connect vacuum gauge hose to intake manifold vacuum port and start engine. Observe vacuum gauge. Partially open throttle and hold steady. If vacuum gauge reading slowly drops after stabilizing, exhaust system should be checked for restriction. If using low pressure gauge, connect gauge in following manner

Check At Oxygen Sensor - Remove oxygen sensor. Install backpressure tester in place of oxygen sensor. After test is completed, coat oxygen sensor threads with anti-seize compound before installation.

Diagnosis

Start engine and allow it to reach normal operating temperature. Increase engine speed to 2000-2500 RPM and note gauge. If reading exceeds 1.25 psi (.09 kg/cm 2 ), exhaust system is restricted.
Check exhaust system for collapsed pipe, heat distress and possible internal muffler failure. If none of these conditions exist, check for restricted catalytic converter. Replace as necessary.

NO-START DIAGNOSIS

Note. For terminal and circuit identification, see WIRING DIAGRAMS .

Definition - No-start is defined as engine cranks properly, but does not start. Engine may fire a few times.

Preliminary Checks

Before performing following tests, check battery condition, engine cranking speed and for adequate fuel in tank. Perform diagnostic system check following any repairs. See DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG or DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.

Perform fuel system checks. Test for proper fuel pump relay operation. Test for proper fuel system pressures. Go to «FUEL SYSTEM PRESSURE TEST»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) under FUEL SYSTEMS. Test for an accurate fuel indication from the Fuel Tank Pressure (FTP) sensor. Refer to «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.
Perform sensor checks: Test ECT sensor for proper operation by performing the following: Allow engine temperature to reach ambient air temperature by leaving engine off for several hours or overnight for most accurate results. Turn ignition on, with engine off. Using a scan tool, compare ECT and IAT sensors displayed temperature. The sensors should be within 5°F (3°C) of each other. Replace ECT sensor or repair ECT sensor circuit, if ECT is not about the expected value. Test TP sensor for proper operation. With throttle blade at full rest position, the monitored TP sensor voltage on a scan tool should be less than 0.85 volt. If TP sensor is not about the expected voltage, check for a faulty TP sensor, faulty TP sensor circuit, or sticking throttle shaft or linkage/cable assembly. Test for proper operation of the Crankshaft Position (CKP) sensor. For testing procedures and specifications, perform appropriate diagnostic test. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-157345-S13357277872003081100000) in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article or «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-48l-53l-60l-gasoline__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L EXPRESS & SAVANA - GASOLINE article. Test for proper operation of the MAP sensor (if equipped). For testing procedures and specifications, perform appropriate diagnostic test. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-157345-S13357277872003081100000) in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article or «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-48l-53l-60l-gasoline__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L EXPRESS & SAVANA - GASOLINE article.

CNG TANK FUEL PRESSURE RELEASE

CAUTION

Fuel system is under pressure. Pressure must be released prior to servicing fuel system.

Start and operate engine at idle. Ensure engine is operating on CNG by observing that the Fuel Indicator Lamp (FIL) is off and the AFO Enable on scan tool display indicates engine is operating on alternative fuel.
Using scan tool, select the High Pressure Lock-Off (HPL) solenoid special function.
While engine is operating on CNG, command the HPL solenoid off and observe fuel tank pressure sensor data parameter. Fuel tank pressure should slowly decrease. If fuel pressure fails to decrease, go to step 5. When engine stalls or a switchover to gasoline is observed, go to next step.
Turn ignition off. Remove the ALT FUELS fuse to disable the CNG fuel system. A small amount of CNG will remain in fuel lines and components. Slowly open the fittings in order to release any remaining pressure when disconnecting a fuel line. Fuel pressure relief for the CNG system is now complete. If service requires that the CNG fuel tanks to be relieved of all internal pressure, refer to «CNG FUEL TANK VENTING PROCEDURE»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines__cng-fuel-tank-venting-procedure).
Close the manual lock-down screw located on the end of each High Pressure Lock-Off (HPL) solenoid. The manual shut-off fitting is closed by rotating the Allen screw clockwise until fully seated. (Scheme 8)
Turn ignition off. Disconnect negative battery cable. Slowly loosen the fuel line fitting at the lock-off solenoid elbow several turns. Fuel pressure relief for the CNG system is complete when fuel line can be pulled away from the "O" ring face seal and fuel does not vent.
When reinstalling, ALWAYS replace "O" ring face seals at all fittings that were disconnected or loosened. After repairs are complete, tighten any loosened fittings to correct torque. Leave manual lock-down screws closed on any vehicle that will require CNG fuel tank venting. Open the manual lock-down screws located on the HPL manual shut-off fittings by rotating the Allen screw counterclockwise until fully seated on vehicles that does not require venting of CNG fuel tanks. If service requires that the ignition be turned ON with CNG components or lines disconnected, keep the ALT FUELS fuse removed to prevent CNG fuel venting during an ignition ON prime pulse.

Scheme 8

CNG FUEL TANK VENTING PROCEDURE

Note. Venting of natural gas to the atmosphere may be prohibited in some areas. Before venting the CNG fuel tank, contact the local gas company to ensure that all local, state and federal regulations are observed. Venting cannot be stopped once the High Pressure Lock-Off (HPL) has been opened with the manual vent tool. Venting CNG fuel tank will damage the HPL. Venting is only necessary for HPL replacement, internal tank cleaning, tank replacement or tank shipping.

Release fuel system pressure. See «CNG TANK FUEL PRESSURE RELEASE»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines__cng-tank-fuel-pressure-release). Disconnect negative battery cable. Remove fuel tank cover. In order to vent rear tanks on cargo/passenger vans, tanks must be removed from vehicle.
Disconnect fuel outlet line from the tank. Install a cap supplied from the CNG Tank Vent Kit (J-42435) to fuel line fitting of fuel outlet port. Tighten cap to 27 ft. lbs. (37 N.m).
Leave Fuel Tank Pressure (FTP) sensor installed, if equipped. Disconnect fuel inlet line from tank.
Connect female end of vent hose to fuel line fitting. Connect male end of vent hose to a vent stack. Connect the ground strap to one of the fittings and to vent stack ground.
Ensure that manual lock-down screw of the manual shut-off fitting is open by rotating the Allen screw counterclockwise until fully seated. (Scheme 8) If tank begins venting when the manual lock-down screw is opened, do not proceed to the next step until tank stops venting.
Remove manual shut-off fitting from the HPL. Obtain manual vent tool supplied from the CNG Tank Vent Kit (J-42435). Rotate screw on manual vent tool counterclockwise in order to fully retract plunger. Install manual vent tool into HPL port with plunger fully retracted. Tighten manual vent tool to 108 INCH lbs. (12 N.m).
Slowly turn the manual vent tool screw clockwise until the plunger is fully extended. Open vent kit in-line valve. The tank should always contain some pressure and begin to vent.
Do not leave tank unattended during venting. Allow CNG fuel tank to vent until the pressure gauge reads zero psi. When tank stops venting, inspect vent line or HPL for a frozen fitting. Fitting will eventually thaw and fuel may continue to vent.
When tank is completely vented, remove vent line from fuel fitting. Remove fuel fitting from fuel outlet port. Remove manual vent tool. If tank is to be reused, replace the HPL.

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.

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.
3 - This step verifies the FRP sensor is displaying the correct fuel pressure.
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.
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.
6 - This step tests the operation of the AF lock-off relay controlling the HPL solenoid.
9 - This step tests the operation of the AF lock-off relay controlling the LPL solenoid.
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.

Perform diagnostic system check. See «DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-system-check-alternative-fuels) 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.
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.
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.
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.
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.
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»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-system-component-testing-60l-wbifuel-cng) under MODULES, MOTORS, RELAYS & SOLENOIDS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
Ensure manual tank valves are fully open by turning the screws fully counterclockwise. (Scheme 8) Did you have to open the manual tank valves? If yes, go to step 2. If no, go to step 14.
Is the fuel pressure greater than 200 psi (1379 kPa)? If yes, go to step 15. If no, go to next step.
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»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-system-component-testing-60l-wbifuel-cng) under MODULES, MOTORS, RELAYS & SOLENOIDS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
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.
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.
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.
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.
Replace FTP sensor. FTP sensor is threaded into HPL, and threaded into CNG tank end. After replacing sensor, go to step 16.
Replace the HPR. After replacing HPR, go to next step.
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»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-system-check-alternative-fuels) 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.

IGNITION SYSTEMS

Note. For additional ignition system and component diagnosis, see IGNITION SYSTEMS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.

CAMSHAFT POSITION SENSOR

For testing procedures and specifications, perform appropriate diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article or DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L EXPRESS & SAVANA - GASOLINE article.

CRANKSHAFT POSITION SENSOR

ELECTRONIC IGNITION SYSTEM

Diagnose using gasoline engine test procedures. See ELECTRONIC IGNITION SYSTEM DIAGNOSIS under IGNITION SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4.8L, 5.3L & 6.0L "C", "G", "H", "K", "N", "S" & "T" SERIES - FLEX FUEL & GASOLINE article.

IDLE SPEED & IGNITION TIMING

Idle speed and ignition timing are controlled by PCM. Incorrect idle speeds are normally caused by dirty throttle plate or vacuum leaks. Ensure all vacuum components are functioning properly.