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6.6L Duramax "cranks but doesn't fire" diagnostic flow chart.


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Old 09-16-2007, 12:10 AM   #1
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Here is a diagnostic flow chart direct from GM for problem solving a crank but doesn't fire condition on a 6.6 Duramax. It should be applicable for the LB7, LLY, LBZ and LLM.

Engine Cranks but Does Not Run
Circuit Description

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

The Engine Cranks But Does Not Run diagnostic table assumes the following:
The batteries are completely charged. Refer to Battery Inspection/Test in Engine Electrical.
The cranking speed is within specifications. Refer to Engine Cranks Slowly in Engine Electrical.
There is adequate fuel in the fuel tanks.
Diagnostic Aids

If the cause of an engine cranks but will not run condition has not been found, inspect for the following conditions:
Hard starting only in cold ambient temperatures. These may cause an intermittent condition that may not occur in the service bay:
- Fuel heater inoperative, refer to Fuel Heater Inoperative .
- Ice blockage at the fuel pickup in the fuel tank. This will be a high vacuum in the supply lines while cranking, and the problem will disappear after the vehicle is brought in the service bay. It may also exhibit a start and stall condition or a starting condition with no acceleration.
The correct cranking speed is 100 RPM cold and 180 RPM hot. A scan tool can be used to inspect cranking speed by pulling the ignition 1 relay and monitoring engine speed on the scan tool while cranking.
Water or foreign material in fuel system
A basic engine problem
If the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor are disconnected or malfunctioning at the same time, an Engine Cranks But Does Not Run condition will exist.
Test Description

The numbers below refer to the step numbers on the diagnostic table.

5.

This step tests for an ignition 1 voltage supply to the engine control module (ECM).
6.

This step tests the 12-volt reference circuit to the CKP sensor and the CMP sensor. The 12-volt reference circuit for the CKP sensor and the CMP sensor is connected internally within the ECM. If the 12-volt reference circuit is shorted to ground, the engine will not start and no engine speed will be indicated on the scan tool.
7.

If there is fuel in the engine oil, fuel may be leaking from the fuel injector or fuel injection pump into the crankcase.
8.

If the fuel system will not even briefly prime to 10 psi and will not start, the check valve in the fuel filter/heater element housing is stuck open.
9.

This step determines if the fuel system is bleeding down causing a no start. The fuel pressure will slowly drop to 0 psi, but should still be above 8 psi 10 seconds after pressurizing the fuel system.
11.

This step determines if the problem is a stuck open or broken fuel injector. If fuel vapors come out of any of the glow plug holes, excessive fuel is being sent into a cylinder, and not being distributed to all of the fuel injectors.
12.

The engine will not run without an actual fuel rail pressure more than 10 MPa (1,450 psi).
14.

If the no-start condition takes place in cold operating temperatures, 2-4C (35-40F) or colder, a glow plug system failure may cause the no-start condition.
17.

In some cases, no compression, possibly with excessive fuel, in a single cylinder can cause a no start
1-

Did you perform the Diagnostic System Check - Engine Controls?

yes-Go to Step 2
no-Go to Diagnostic System Check - Engine Controls

2-


1. Turn On the ignition, with the engine OFF.
2. Observe the DTC Information with a scan tool.

Does the scan tool display DTC P0090, P0335, P0336, P0340, P0370, P0374, P0380, P0601, P0602, P0603, P0604, P0611, P0612, P1621, P1626, P1631, P1683, or U1800?

yes-Go to Diagnostic Trouble Code (DTC) List
no-Go to Step 3

3-

Is the customer's concern with a fuel smell or fuel leak?

yes-Go to Fuel Leaks
no-Go to Step 4

4-

Observe the Actual Fuel Rail Pressure parameter with a scan tool.

Is the pressure within the specified range? 1-1.8 MPa

yes-Go to Step 5
no-Go to Step 18

5-

Observe the Ignition 1 signal parameter with a scan tool.

Is the Ignition 1 signal parameter at the specified value? B+

yes-Go to Step 6
no-Go to Step 24

6-
1. Attempt to start the engine.
2. Observe the Engine Speed parameter with a scan tool while the engine is cranking.

Is the Engine Speed parameter more than the specified value? 0 RPM

yes-Go to Step 7
no-Go to Step 19

7-
Inspect for the following conditions:
Excessive fuel in the engine oil--Refer to Fuel in Engine Oil in Engine Mechanical.
Contaminated Fuel--Refer to Contaminants-in-Fuel Diagnosis .

Did you find and correct the condition?

yes-Go to Step 34
no-Go to Step 8

8-
1. Install the J 44638 Vacuum Gage to the fuel system service port on the right front side of the engine.
2. Attempt to hand prime the fuel manager 30 times or until the specified pressure is reached.

Can you prime the system to the specified value? 8 psi

yes-Go to Step 9
no-Go to Step 31

9-

Does the pressure measure more than the specified value for more than 2 minutes? 2 psi

yes-Go to Step 10
no-Go to Step 13

10-

Perform the Engine Compression Test. Refer to Engine Compression Test in Engine Mechanical - 6.6L. Repair the engine as necessary.

Did you find and correct the condition?

yes-Go to Step 34
no-Go to Step 11

11-

Did any of the cylinders emit any fuel vapor during the Engine Compression Test?

yes-Go to Step 28
no-Go to Step 12

12
1. Crank the engine for 15 seconds.
2. Observe the Actual Fuel Rail Pressure parameter with a scan tool.

Is the Actual Fuel Rail Pressure parameter more than the specified value? 10 MPa (1,450 psi)

yes-Go to Step 14
no-Go to Step 13

13
1. Reprime the fuel manager to 10 psi.
2. Attempt to start the engine while the prime is still above 8 psi.

Does the engine start?

yes-Go to Fuel System Diagnosis
no- Go to Fuel System Diagnosis - High Pressure Side

14-

Observe the Glow Plug System Type parameter with a scan tool.

Is the Glow Plug System Type Federal?

yes-Go to Step 15
no-Go to Step 16

15-

Important: Repeat this procedure on both banks of the engine. This test may be repeated as many times as necessary.

1. Turn OFF the ignition.
2. Connect a test lamp to a glow plug harness connector and a good ground on one side of the engine.
3. Turn ON the ignition, with the engine OFF.

Does the test lamp turn ON and OFF?

yes-Go to Step 16
no-Go to Glow Plug System Diagnosis

16

Important: If there is high resistance in the signal or low reference circuits of the crankshaft position (CKP) sensor the Engine Speed parameter of the scan tool will display a value more than 0. It will not be an accurate measure of engine speed, and can cause an Engine Cranks but does Not Run condition.

Test the CKP sensor signal and low reference circuits for high resistance. Refer to Circuit Testing in Wiring Systems.

Did you find and correct the condition?

yes-Go to Step 34
no- Go to Step 17

17

Inspect for the following conditions:
A plugged air filter
A collapsed air intake duct
The fuel heater is inoperative. If the customer concern is that the engine will not start when ambient temperatures are less than 2-4C (35-40F), refer to Fuel Heater Inoperative .
A restricted exhaust system--Refer to Symptoms - Engine Exhaust in Engine Exhaust.

Did you find and correct the condition?

yes- Go to Step 34
no- Go to Diagnostic Aids

18-
1. Disconnect the fuel rail pressure (FRP) sensor.
2. Observe the Actual Fuel Rail Pressure parameter on the scan tool.

Does the Actual Fuel Rail Pressure parameter measure more than the specified value? 175 MPa


yes-Go to Step 23
no- Go to Step 22

19.
1. Turn OFF the ignition.
2. Disconnect the CKP sensor.
3. Turn ON the ignition, with the engine OFF.
4. Probe the 12-volt reference circuit of the CKP sensor with a DMM connected to a good ground.

Is the voltage more than the specified value? 10 Volts yes-Go to Step 29 no- Go to Step 20

20-
1. Turn OFF the ignition.
2. Disconnect the camshaft position (CMP) sensor.
3. Turn ON the ignition, with the engine OFF.
4. Probe the 12-volt reference circuit of the CMP sensor with a DMM connected to a good ground.

Is the voltage more than the specified value? 10 Volts yes- Go to Step 30. no-Go to Step 21

21 Test the 12-volt reference circuit of the CKP sensor and the CMP sensor for a short to ground. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition? yes- Go to Step 34. no-Go to Step 33

22- Test the FRP sensor signal circuit for a short to ground. Refer to Circuit Testing in Wiring Systems.

Did you find and correct the condition? yes- Go to Step 34. No- Go to Step 33

23- Test the FRP sensor circuits for high resistance. Refer to Circuit Testing in Wiring Systems.
Did you find and correct the condition? yes- go to step 34. no go to step 25

24-
1. Test the ignition 1 voltage circuit of the engine control module (ECM) for a short to ground, a high resistance, or an open. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.
2. Replace the fuse if necessary.

Did you find and correct the condition?

yes-Go to Step 34
no-Go to Step 26

25-
Test for an intermittent or for a poor connection at the FRP sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

yes-Go to Step 34
no-Go to Step 32

26-
1. Clean and tighten the ECM shared ground. Refer to Power and Grounding Component Views in Wiring Systems.
2. Attempt to start the engine.

Does the engine start?

yes-Go to Step 34
no-Go to Step 27

27-

Test for an intermittent and for a poor connection at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

yes-Go to Step 34
no-Go to Step 33

28-

Important: When the fuel injector pressure lines are removed, debris will fall on the fuel injector inlet fitting. Vacuum the debris from the area to prevent the debris from falling in the fuel injector.

Replace the fuel injectors on the affected cylinders. Refer to Fuel Injector Replacement .

Did you complete the replacement?

yes-Go to Step 34


29-

Replace the CKP sensor. Refer to Crankshaft Position (CKP) Sensor Replacement .
Did you complete the replacement?

yes-Go to Step 34

30-

Replace the CMP sensor. Refer to Camshaft Position (CMP) Sensor Replacement .

Did you complete the replacement?

yes-Go to Step 34

31-

Important: Before replacing the fuel filter/heater element housing, inspect the fuel vent screw for damage or cross threading. Replace the vent screw w/"O" ring if either condition is found.

Replace the fuel filter/heater element housing. Refer to Fuel Filter/Heater Element Housing Replacement .

Did you complete the replacement?

yes-Go to Step 34


32-

Replace the FRP sensor. Refer to Fuel Rail Pressure (FRP) Sensor Replacement .

Did you complete the replacement?

yes-Go to Step 34

33-

Replace the engine control module (ECM). Refer to Engine Control Module (ECM) Replacement .

Did you complete the replacement?

yes-Go to Step 34

34-
1. Clear any DTCs with a scan tool.
2. Attempt to start the engine.

Does the engine start and continue to run?

yes-Go to Step 35
no-Go to Step 2

35-
1. Allow the engine to idle until normal operating temperature is reached.
2. Observe the DTC Information with a scan tool.

Are any DTCs displayed?

yes- Go to Diagnostic Trouble Code (DTC) List
no-System OK

_____________________________________________
Brian

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Old 09-17-2007, 11:34 PM   #2
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nice info

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Old 07-18-2012, 03:41 PM   #3
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How to do number 21?? I hate electrical problems

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Old 12-09-2012, 07:45 AM   #4
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I think I may have a bad CMP sensor where do I get information on how to replace it.

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