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

Testing & Diagnostics 1 illustration ~1846 words

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.

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»(ref-152801-S36073921212003022600000) under FUEL SYSTEMS. Test for an accurate fuel indication from the Fuel Tank Pressure (FTP) sensor. Refer to «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159156-S06111585432003111300000) 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»(ref-158498-S20484766492003100100000) 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»(ref-158498-S20484766492003100100000) 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»(ref-152801-S42213517772003022600000).
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»(ref-152801-S01659124862003022600000). 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.

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.