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John, Master A.S.E Automotive technician
Category: Chevy
Satisfied Customers: 9176
Experience:  A.S.E MasterTechnician A1-A8 w/L1,38 years experience General Motors Master Technician
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Duramax sits for a while

Customer Question

My duramax diesel (LB7) is hard to start. especially,when it sits for a while. but,0nce it runs,there no problem. It has 101,0000 miles on it. When I check the oil,It,s not over full. I heard that would mean the injectors are bad. But the truck runs real good at idle and while driving it. Any possible reason, it"s doing this?
Submitted: 7 years ago.
Category: Chevy
Expert:  John replied 7 years ago.


First thing to make sure is that the glow plug system is operating properly.Diagnosing this,a class 2 scan tool is needed to view computer codes & view operating data.


The glow plug system is used to assist in providing the heat required to begin combustion during engine starting at cold ambient temperatures. The glow plugs are heated before and during cranking, as well as during the engine operation. The engine control module (ECM) controls the glow plug ON times by monitoring coolant temperatures and glow plug voltage. This system check will check the glow plugs and the glow plug feed circuit coming from the relay/controller.

If the glow plug relay or controller is stuck in the ON position, check for proper operation of the glow plugs. When the glow plugs are commanded ON by the scan tool, an internal engine control module (ECM) timer protects the glow plugs from damage by cycling them ON for 3 seconds and then OFF for 12 seconds. Most glow plug system failures are covered by DTC P0380. If no DTCs are stored, the vehicle is hard to start and white smoke is present during cranking or after the vehicle is started, the most likely cause of failure is the glow plugs.


  1. Turn OFF the ignition.
  2. Disconnect all the glow plugs.
  3. With a test lamp connected to battery voltage, probe the spade terminal on each glow plug.

Do all glow plugs illuminate the test lamp?


  1. Turn ON the ignition, with the engine OFF.
  2. Verify the glow plugs are still disconnected.
  3. Probe each glow plug supply voltage circuit with a test lamp connected to ground. With a scan tool, command the glow plugs ON.

Does each circuit illuminate the test lamp?



Does the test lamp illuminate all the time for each glow plug terminal, even when the glow plugs are not commanded ON?


  1. Disconnect the glow plug supply voltage connection from the glow plug relay.
  2. Probe each glow plug terminal with a test lamp connected to ground.

Does the test lamp illuminate for all terminals?


Repair the short to voltage on the glow plug supply voltage circuit.



Replace the glow plug relay/controller.

John and 3 other Chevy Specialists are ready to help you
Customer: replied 7 years ago.
I don't have a Tech 2 scanner. I do have a scanner that reads and clear codes. So how can I command to come on?
Expert:  John replied 7 years ago.
You will need a scan tool that has output tests for vehicle systems.Usually when these systems quit working it is the glow plug relay( On the left side of the engine above the valve cover) .Make sure the IGN FUSE 1 fuse(in under hood electrical center) is good & there is voltage to the relay when key is on.Also check the 175 amp battery fuse(near the battery) that powers glow plugs.The glow plug controller for your vehicle is incorporated into the ECM.

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