For decades, the first step a mechanic took when a diesel engine ran out of fuel or had a hard-start issue was to crack the injector lines at the cylinder head, crank the engine until fuel squirted out, and tighten them back up. On a Tier 4 Final engine with a High-Pressure Common Rail (HPCR) fuel system, doing that will permanently maim or kill you.
An older mechanical injection system pushed fuel at around 3,000 to 5,000 PSI. HPCR systems push fuel at 30,000 to 40,000 PSI. At that pressure, diesel fuel is no longer a liquid; it is a cutting torch. If you crack an injector line on a running HPCR engine, the fuel escaping through the microscopic gap does not spray. It exits as an invisible, highly focused stream of high-velocity vapor that can easily penetrate human skin and inject fuel into the tissue.
Fuel injection injury looks like a minor puncture wound on the surface, but underneath, the toxic diesel is destroying tissue and blood vessels. If not surgically debrided within hours, it leads to amputation or fatal sepsis. OEMs now seal the high-pressure rail and lines under the valve cover specifically to prevent mechanics from touching them. Bleeding the system is done via a laptop command that cycles the transfer pump, or by using the manual priming pump on the fuel filter base. The old-school mechanic's habit of cracking a line is a death sentence on modern iron.
