Really need help fuel rail pressure

[Cwmartinjr]

I really need help with this issue. I am showing a spn code 1239 stating fuel leak but when plugged in to a computer it states fuel rail pressure loss. I don't belive it's an injector because of no fuel in the oil.i have replaced the high relief value that changed nothing. Took truck to the shop they said high pressure fuel pump and suppose to have diagnose the problem and guess what still doing the same thing. I have lost around 100hp 6 percent boost and my pyrometer is about 300 degrees off where she should be while pulling. I have a 2014 Cummings isx 15 litter 550hp 2250 torque.