GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 TopKick C-Series (Conventional) C6/C7/C8 🢒 Engine 🢒 Engine Controls and Fuel - 7.8L (LG4) 🢒 Engine Cranks But Does Not Run

Description

The Engine Cranks but Does Not Run diagnostic table is an organized approach to identifying a condition that causes an engine to not start. The diagnostic table directs the service technician to the appropriate system diagnosis. The diagnostic table assumes the following conditions are met:

Important: The engine will start even if exhaust brake is fully engaged.

    • The battery is completely charged and terminals are cleaned tight.
    • The engine cranking speed is normal.
    • There is adequate fuel in the fuel tank.
    • There is no air in the fuel line.
    • Filters, air and fuel, are clean.
    • Fuses and slow blow fuses are normal.

Diagnostic Aids

If an intermittent condition is suspected, the following may cause an intermittent:

    • Poor connections
    • Misrouted harness
    • Rubbed through wire insulation
    • Broken wire inside the insulation

Important: The Actual Fuel Rail Pressure on scan tool will only update with the engine running or one time after the key is cycled when the ignition has been turned OFF for longer than 10 seconds.

Inspect for the following conditions:

    • Poor connection at the engine control module (ECM)
    • Harness connectors for backed-out terminals
    • Improper mating
    • Broken locks
    • Improperly formed or damaged terminals
    • Poor terminal to wire connection

Test Description

The number below refers to the step number on the diagnostic table.

  1. If the fuel rail pressure (FRP) regulator low control circuits between the ECM and the FRP regulator are shorted to ground, Actual FRP Regulator will be approximately 500 mA lower as compared with Desired FRP Regulator current.

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2
  1. Install a scan tool.
  2. Crank the engine for the specified amount of time.
  3. Monitor the DTC information with a scan tool.

Does the scan tool display any DTCs that failed this ignition?

15 seconds

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3
  1. Turn OFF the ignition.
  2. Wait 2 minutes for the fuel pressure to bleed down from the fuel rail.
  3. Turn ON the ignition, with the engine OFF. DO NOT start the engine.
  4. Observe the Actual Fuel Rail Pressure (FRP) parameter with a scan tool.

Is the Actual FRP parameter the specified value?

0 psi (0 MPa)

Go to Step 4

Go to Step 6

4

Important: If the vehicle has run out of fuel, air may be trapped in the fuel system.

  1. Ensure the fuel tanks have adequate fuel and the fuel quality is good. Take a sample.
  2. Observe the Actual Fuel Rail Pressure parameter on the scan tool while cranking over the engine for 5 seconds.

Does the Actual Fuel Rail Pressure reach the minimum specified value during crank?

3630 psi (25 MPa)

Go to Step 9

Go to Step 5

5

Observe the Actual FRP Regulator parameter on the scan tool while cranking over the engine for 5 seconds.

Does the Actual FRP Regulator Current reach the desired value during crank?

900 mA

Go to Fuel System Diagnosis

Go to Step 8

6
  1. Turn OFF the ignition.
  2. Disconnect the FRP sensor harness connector.
  3. Inspect for an intermittent, poor connection, and corrosion at the harness connector of the FRP sensor.
  4. Disconnect the ECM harness connector.
  5. Inspect for an intermittent, poor connection, and corrosion on each FRP sensor circuit at the harness connector of the ECM.
  6. Test for high resistance on each FRP sensor circuit.
  7. Repair the circuits as necessary.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 7

7

Replace the FRP sensor. Refer to Fuel Injection Fuel Rail Assembly Replacement.

Did you complete the replacement?

--

Go to Step 12

--

8
  1. Test the FRP regulator low control circuits between the ECM and the FRP regulator for a short to ground.
  2. Repair the circuits as necessary.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

9
  1. Inspect for normal readings at key up for the following sensor inputs. Use the Scan Tool Data List or a known good vehicle to determine nominal values.
    2. • Engine coolant temperature (ECT) sensor
    • Mass air flow (MAF) sensor
    • Exhaust gas recirculation (EGR) valve position sensor
    • Cam signal present
    • Crank signal present
  2. Repair the circuits or replace the sensor if necessary.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 10

10
  1. Other possible causes for the no start condition:
    2. • Engine mechanical timing
    • Flywheel installed incorrectly causing the CKP sensor to be incorrectly timed to the engine. Disconnect sensor and attempt to start engine to verify.
    • Heavily restricted intake, exhaust or catalytic converter plugged solid
    • Poor engine compression
    • Water or gasoline contamination in fuel
  2. Repair as necessary.

Did you find and correct the condition?

--

Go to Step 12

--

11

Important: The replacement ECM must be programmed.

Replace the ECM. Refer to Engine Control Module Replacement/Restore Fuel Rate Data.

Did you complete the replacement?

--

Go to Step 12

--

12
  1. Reconnect all previously disconnected harness connectors.
  2. Turn OFF the ignition for 30 seconds.
  3. Attempt to start the engine.

Does the engine start and continue to run?

--

Go to Step 9

Go to Step 2

13

Observe the stored information, Capture Info with a scan tool.

Are there any DTCs that you have not diagnosed.

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK