Engine Cranks But Does Not Run (LC8FHZ)

Engine Cranks But Does Not Run (LC8\FHZ)

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.

* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Circuit/System Description

This Engine Cranks but Does Not Run diagnostic is an organized approach to identify a condition which causes the engine to crank but does not continue to run. This diagnostic directs the service technician to the appropriate system diagnosis. This diagnostic assumes the vehicle system voltage levels are adequate for starter motor operation. Refer to Battery Inspection/Test Battery Inspection/Test and Engine Cranks Slowly Testing and Inspection. The fuel level supply must be adequate, and the fuel quality must be able to sustain the combustion process.

Diagnostic Aids

Inspect for any of the following conditions:

* An open Ignition/Injector fuse may prevent the engine from starting and/or continuing to run. This engine application uses 2 fuses, one for each cylinder bank, to supply ignition 1 voltage to the ignition module/coil assemblies and also to the fuel injectors. If either fuse is open, ignition spark and fuel injection spray will be lost for the entire cylinder bank affected.

* Insufficient fuel can cause a no start condition. Verify that the 1/4 turn isolation valve and all 4 CNG fuel tank manual shut-off valves are ON. Thoroughly inspect the fuel supply lines and components for partial blockage or restrictions.

* Fuel injectors with partially blocked and restricted nozzles, or a malfunctioning solenoid, can cause a no start condition.

* There may be fuel spray at the fuel injectors and the indicated fuel pressure may be correct, yet there may not be enough fuel to start the engine. If the fuel injectors and the injector circuit are OK and fuel spray is detected, the fuel injector ON time may be inadequate. If the engine control module (ECM) receives incorrect inputs from the various information sensors, the fuel delivered by the fuel injectors may be inadequate to start the engine. Observe all the engine data parameters with a scan tool and compare the values indicated with the expected values or the values from a known good vehicle.

* Observe the Engine Speed parameter while cranking the engine. The scan tool should indicate a steady 200-300 RPM while cranking. If erratic values such as sudden spikes in the engine speed are displayed, the engine reference signal is not stable enough for the engine to start and run properly.

* Inspect the engine for good secure electrical grounds.

* If the engine almost starts and then stalls, inspect for an open in the ground circuits of the crankshaft position sensor and the camshaft position sensor.

* A vehicle that starts and runs after being brought to the repair shop for a no start condition may have an ignition system that is susceptible to moisture. Spray water on the ignition system components and the wiring in order to verify for an engine starting or will not stay running concern.

* When disconnecting electrical connectors or removing fuses and relays, always inspect electrical terminals for corrosion and for adequate terminal tension.

* Use the J 35616 for any test that requires probing the underhood electrical center terminals, component wire harness terminals, or the controller wire harness terminals.

Reference Information

Schematic Reference

Engine Controls Schematics [1][2]Electrical Diagrams

Connector End View Reference

* Component Connector End Views 1-2 Shift Solenoid (SS) Valve (M30)

* Electrical Center Identification Views Fuse Block - Underhood Label

Electrical Information Reference

* Circuit Testing Circuit Testing

* Connector Repairs Connector Repairs

* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections

* Wiring Repairs Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References Control Module References for scan tool information

Special Tools

* CH-48027 Digital Pressure Gauge

* J 35616 GM-Approved Terminal Test Kit

* J 35616-A/BT-8637 Connector Test Adapter Kit

* J 26792 Spark Tester

Circuit/System Verification

1. Engine cranking for 15 seconds, observe the DTC information with a scan tool. Verify that DTC P0117, P0118, P0122, P0123, P0201-P0208, P0222, P0223, P0300-P0308, P0335, P0336, P0351-P0358, P0562, P0563, P0601-P0604, P0606, P0607, P062F, P0633, P0685, P0690, P1631, P1682, or P2610 is not set.

• If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle Diagnostic Trouble Code (DTC) List - Vehicle.

2. Ignition ON, view the security indicator. The security indicator should not remain illuminated after the vehicle bulb check has completed.

• If the security indicator remains illuminated after the bulb check, refer to Diagnostic Trouble Code (DTC) List - Vehicle Diagnostic Trouble Code (DTC) List - Vehicle , and diagnose any theft deterrent DTCs set as current.

3. Engine cranking, observe the scan tool Engine Speed parameter. The scan tool should indicate an engine speed greater than 0 RPM.

• If the engine speed is 0 RPM, refer to DTC P0335 P0335 or DTC P0336 P0336 for further diagnosis.

4. Ignition OFF, connect a J 26792 onto the boot of a spark plug wire and ground for each cylinder bank of the engine.

Note:
An erratic or weak spark is considered a no spark condition.

5. While cranking the engine, verify that both spark testers spark.

• If the spark tester does not spark for any cylinder tested, refer to Electronic Ignition System Diagnosis Electronic Ignition (EI) System Diagnosis.

Note:
Additional DTCs may set when using the fuel pump relay output control.

6. Ignition ON, command the fuel pump relay ON with a scan tool. You should hear the high pressure lock-out solenoids energize.

• If the high pressure lock-out solenoids do not energize, refer to Fuel System Diagnosis (Without FPCM) Fuel System Diagnosis (Without FPCM)Fuel System Diagnosis (LC8\K07\UFP) Fuel System Diagnosis (LC8K07UFP)Fuel System Diagnosis (LC8\FHZ) Fuel System Diagnosis (LC8FHZ)Fuel System Diagnosis (FPCM) Fuel System Diagnosis (FPCM).

7. Ignition OFF, install the CH-48027. Refer to Fuel Pressure Gauge Installation and Removal Fuel Pressure Gauge Installation and Removal.

Note:

* Before verifying fuel pressure, allow the engine to cool until the engine coolant temperature is below 60°C (150°F). Coolant temperatures higher than 60°C (150°F) will result in higher than normal fuel pressure, due to hot soak fuel boiling. If the fuel pressure reading is taken while hot soak fuel boiling is present, pressure readings may exceed 930 kPa(135 psi) and will not be accurate.

* Do NOT start the engine.

* The fuel pump relay may need to be commanded ON several times in order to obtain the highest possible fuel pressure.

8. Ignition ON, command the fuel pump ON with a scan tool. The fuel pressure should be between 620-758 kPa (90-110 psi).

• If not within the specified range, refer to Fuel System Diagnosis (Without FPCM) Fuel System Diagnosis (Without FPCM)Fuel System Diagnosis (LC8\K07\UFP) Fuel System Diagnosis (LC8K07UFP)Fuel System Diagnosis (LC8\FHZ) Fuel System Diagnosis (LC8FHZ)Fuel System Diagnosis (FPCM) Fuel System Diagnosis (FPCM).

9. Verify the following conditions do not exist:

* Collapsed air intake duct to the throttle body

* Restricted air filter element

* Gas or oil fouled spark plugs-Refer to Spark Plug Inspection Testing and Inspection

* A skewed manifold absolute pressure (MAP) sensor-Refer to DTC P0106 P0106.

* A skewed engine coolant temperature (ECT) sensor-Refer to DTC P0116 P0116.

* Restricted exhaust system-Refer to Restricted Exhaust Restricted Exhaust.

* An engine mechanical condition, for example, worn timing chain or low compression-Refer to Symptoms - Engine Mechanical Symptoms - Engine Mechanical and Camshaft Timing Chain and Sprocket Cleaning and Inspection 39. Camshaft Timing Chain and Sprocket Cleaning and Inspection.

Repair Instructions

Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.