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i have an 02 duramax with the lb7 and allison 5 spd. i frequently change the fuel filter and air filter, and have replaced the fuel pressure solenoid on the variable speed pump. but my truck does not have the power it should. it builds boost but when it gets arouns 2800 rpm it likes to stay there and kinda fall on its face. the truck doesnt take off like it should! i have chrcked the wastegate and its working and it builds about 23 pounds of boost. im completely clueless on this thing and so is the mechanic in my area. the truck has 180000 on it. and has had the injectors replaced. from time to time it blows black smoke and sometimes greyish smoke but not always. any help would be greatly appreciated!!!!!
 

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check for fuel in the oil

put a drop of oil on a paper towel if you see rings it has fuel

its free to test and easy good place to start to see if injectors are goin or bad
 

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Discussion Starter #3
thanks for the advice, however, i have checked that and no fuel in the oil! i took the truck to a guy in my area, sposedly one of the best around, and he says everything is working properly and he doesnt see anything that is wrong with it. however he agrres it does not have the power it should!! is it possible the computer is junk?? he thinks that may be the next thing to try but i really dont have the money to jus throw parts at it and guess! i have tried an edge j/a programmer and it made no difference jus blew black smoke. and would sometimes cut out like it wasnt getting enough fuel! it acted as if it were tryin to backfire so i took the programmer off! the maf sensor has been cleaned. made no difference either! thanks again for any help!!!!
 

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cp3 is getting weak i bet is the edge hooked up if so unhook the whole thing
 

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Discussion Starter #5
No i took it off completely when it kept tryin to backfire. If the cp3 was weak would it throw a code? It had a low fuel press code and thats y i replaced the fuel press solenoid. And the code hasn come back!!
 

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Discussion Starter #6
No i took it off completely when it kept tryin to backfire. If the cp3 was weak would it throw a code? It had a low fuel press code and thats y i replaced the fuel press solenoid. And the code hasn come back!!
 

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My buddies 02 is doing the same thing. Won't really rev above 2k rpm. He's been driving in limp mode as his programmer had a limp code. Top speed of about 55-60...
 

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There were actually 3 codes on the DS. The only one I remembered was some type of power limiting code...I'll ask him to pull them and post.
 

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Discussion Starter #11
I dont remember the code it was last yr bout this time i replaced it. The truck runs good . Itll rev as high as u want and go as fast as any other truck. But doesnt have the power! My truck wont even turn the tires over doin a burnout.
 

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Discussion Starter #12
I had the computer hooked to it and its got the fuel press. Boost and everything reads as it should. Jus no power. My buddy jus bought an 04 with the lb7 and it dusts mine with his at half throttle. My trucks been this way ever since i bought it 2 yrs ago! Could it be a bad program in the computer? Seems no matter what i do it doesnt gain anything!
 

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I know this thread is a few months old, but are there any updates to your problem? I am having the same issues/symtoms.
 

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I have seen this dozens of times. The fuel pressure relief valve is opening prematurely. This was a big problems on the early LB7 engines. You have to replace the pressure relief valve. This is why you have fuel pressure when the pressure is low, but under load when the pressure is high, the valve is opening. Once the valve opens, it weakens and will open more readily until is fails all together.
 

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I have seen this dozens of times. The fuel pressure relief valve is opening prematurely. This was a big problems on the early LB7 engines. You have to replace the pressure relief valve. This is why you have fuel pressure when the pressure is low, but under load when the pressure is high, the valve is opening. Once the valve opens, it weakens and will open more readily until is fails all together.
I this the same as fuel pressure regulator or somthing differrant?

Also not to doubt your knowledge but is there a way to check pressure a wide open throttle?
 

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The fuel pressure regulator is on the high pressure pump. The fuel pressure relief valve is part of the junction block. You can check pressure at WOT using a scan tool. Should be around 23,000 PSI or 160 MPa.
 

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Thanks for the information. I plan on taking this truck to a shop and having the fuel system checked out after Christmas. I don't (fingers crossed) there are any fuel problems other than lack of it under load or upper RPM range. Is this valve seviceable or do you have to buy the whole Block? I will assume its dealer only?
 

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Thanks for the information. I plan on taking this truck to a shop and having the fuel system checked out after Christmas. I don't (fingers crossed) there are any fuel problems other than lack of it under load or upper RPM range. Is this valve seviceable or do you have to buy the whole Block? I will assume its dealer only?
I have only serviced them as part of the block. You can look around for an aftermarket solution. I am sure someone out there has something to fix this.
 

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Yea I have been reading. The newer trucks it looks as if you can just change the valve. The even offer a block off if you will. Would this be a bad thing to do on a stock truck? Is this valve a pop of if you will in case the actual fuel pressue regulator fails?
 

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In my opinion, someone would be crazy to block this off. It is a safety device. On the newer trucks, the fuel pressure goes as high as 26,000 PSI or 180 MPa. I do not want to be around if something goes wrong at that pressure. I know the guys who race or pull with their trucks block off the pressure relief valve. I would not recommend it on a stock truck.
 
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