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Gm Tech Cam
Gm Tech Cam, Service Technician
Category: Transmissions
Satisfied Customers: 29520
Experience:  11 years experience, specializing in Trans, Elec and Dmax
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I have an early 2004 Duramax 3500 that is going into limp

Customer Question

I have an early 2004 Duramax 3500 that is going into limp mode. I have had the transmission, transfer case and torque converter rebuilt, all the transmission sensors changed, also new injectors and wiring harness, and the computer reflashed by the dealer. I have taken it to the dealer and several mechanics and this is still happening. It runs fine until you get into the throttle (just getting on freeway) and it happens again (usually within 60 miles). When you pull over and turn it off it resets. Any ideas on what's going on.
Submitted: 1 year ago.
Category: Transmissions
Expert:  Gm Tech Cam replied 1 year ago.

Hi there, I am Cam. Can you tell me if you mean limp mose is 2000 RPM?

Customer: replied 1 year ago.
When the engine is given fuel it will not shift. It will rev to 2000 RPM's
Expert:  Gm Tech Cam replied 1 year ago.

Ok, that is what I was suspecting, that is engine limp mode. You said you have the early LB7 druramax?

Customer: replied 1 year ago.
Hello, are you there?
Expert:  Gm Tech Cam replied 1 year ago.

Yes I am, I was in the parking lot looking at a vehicle.

Customer: replied 1 year ago.
I was waiting at my pc and my phone just notified me of a text.
Expert:  Gm Tech Cam replied 1 year ago.

Ok so I need to know if you have the LB7 and if so, have you ever replaced the injectors? We need to do this test. These use hydraulic pressure from the fuel system to operate the injector. SO a small amount of fuel is supposed to bypass to open the main part of the injector to make it operate. When these start to fail, this causes to much fuel to bypass and return to the tank. This resulting fuel loss will lower the fuel rail pressure and at a time when the pressure is needed most, during cranking, it takes longer to build the required 1450 psi to operate the injector. SO the test we do is this. Find the 2 fuel lines on the rear drivers side valve cover. Take the smaller one off and attach your hose to the metal pipe coming from the engine. Put the other end into the container and run the engine to fill that line. Then shut it off and remove the IGN 1 relay from beside the starter relay. If the engine will not start at that time, crank it to fill the line, then empty out the container and keep the line full. Crank the engine for 15 seconds fully and measure the amount of fuel that comes out of that line. More than 24cc means the injectors have failed and they need to be replaced. The code 2 LLY engines can test each injector individually, and the spec is 3ml or 3cc each for the same cranking period.

Customer: replied 1 year ago.
As I stated in the initial question, this has all been done, injectors, harness, trans, transfer case, torque converter, and computer reflashed.
Expert:  Gm Tech Cam replied 1 year ago.

OK, so the next test requires the scan tool. What I am suspecting are the following. First that you re getting air in the system, which would do the same thing. This is checked by putting a clear section of hose in the place of the line out of the filter housing. Then watch for a build up of air or steady stream of bubbles. IF so then the areas to look are the filter housing, or flex lines back to the tank. If there are no bubbles then we need to look at the actual output of the pump. With it running we need to command the maximum pressure up to 160 MPA or 23300 psi and see how close the actual fuel rail pressure is. It must be within 2 map or 300 psi of desired. If this falls short and there are no kinks in the rubber lines to the pump, then it means the injection pump is failing to build the pressure needed and that causes limp mode.

Expert:  Gm Tech Cam replied 1 year ago.

Limp mode is simply the inability of the fuel system to meet the demands and dropping down. 99% of the time it is injectors, but the odd time its hoses or the pump . I have changed a couple pumps for this type of issue, but I was always able to find it using the method I outlined.

Customer: replied 1 year ago.
Ok. I'll check this out and get back with you. Thanks
Expert:  Gm Tech Cam replied 1 year ago.

Please bookmark this page so you can easily navigate back to it so we can continue the discussion. This is done by pressing (CTRL D on PC and CMD D on Mac). You can reply at any time for follow up information regarding this issue, even after you rate the site and I will reply.

Customer: replied 1 year ago.
This has all been tried ( there is 25,000 psi, lines are good) and it is still going into limp mode. Not sure what to even ask by this point.
Expert:  Gm Tech Cam replied 1 year ago.

If this has all been done and the issue has not been found then you need to find a different shop that will do the test correctly. They're missing something!!