Here's my problem. I have an 04 GMC with an LB7. I recently replaced the injectors, cups, CP3, high pressure lines and fuel filter housing. After putting everything back together. I now have a crank no start issue. I have 2 codes one is p0641 for a 5v reference and the other is p1125 for app 1 sensor. I have replaced the fuel rail pressure sensor, thinking that was my issue but no. I also replaced the throttle pedal. Still won't start. A friend of mine seems to think that the fuel pressure relief valve could be bad causing the fuel to just totally bypass the injectors and go back to the tank. I am at my wits end with this and would like some other ideas to check. Thanks
No start issue
- Thread starter usafmorgan
- Start date
P0641 can set if a terminal was damaged in the connector C107. It’s the one on the driver side valve cover between upper coolant pipe and charge air pipe. Just unplug both those main connectors there and look for bent/damaged terminals. If you didn’t have the codes prior to the work, it’s probably something that happened during service. Does that scanner you are using allow you to see data? If so you need to look at rail pressure while cranking engine over. If it’s not building up it won’t fire. My only other advice would be to stop throwing parts at it. LB7 crank no start search on this forum will provide you with countless diagnostic tips but you do need a scan tool that can read data parameters.
I have checked both the connectors you are talking about. All the pins are good. According to prodemand if only one of the app sensors is bad it won't throw a code. As the other two sensors will compensate for it. So there was no dtc before I started working on it. I have checked all the wires for the throttle and they check good. Yes the scanner im using shows live data. It does show rail pressure but its not buulding. Leaving me to believe that the fuel pressure relive valve is bad.
pressure relief valves are rarely bad on lb7s especially at cranking pressure. you need to see if you are even getting fuel to the CP3. if so then you need to see if the CP3 is getting the fuel to the injectors. you can loosen one of the injector lines at the rail and see if you get fuel when cranking. if so then you can also check the returns crom the CP3, injectors and relief valve to see if any are returning too much thus dropping pressure