IVECO CLOSE PROBLEM 2.3 HPI

Hi everyone, I have a problem with my Iveco. It starts but then stalls after about two seconds. We've checked the fuel pressure, injectors, regulator, and piston compression, and everything seems fine! The thing is, when I open one of the injectors or loosen the nut on the common rail inlet pipe, the truck starts! So we're guessing it's excess pressure and a problem with the high-pressure fuel pump. Can anyone help?