A steel service center reported that a Linde H25D forklift (Cummins QSB3.3 engine) ran perfectly while driving, but whenever the operator attempted to lift a heavy 2.5-ton steel coil, the engine would sputter, lose RPM, and stall completely. The engine would restart immediately after cranking, but the stall recurred as soon as the hydraulic load was applied.
The technician initially suspected a fuel lift pump failure, but the pump delivered a steady 4 psi to the high-pressure fuel pump. He then checked the engine fault codes and found a "Fuel Pressure Too Low - Bank 1" code. He connected a clear section of fuel hose between the high-pressure pump outlet and the common rail. When the operator engaged the lift, the fuel in the clear hose was full of tiny bubbles, and the rail pressure dropped from 250 bar to 80 bar, stalling the engine.
The air ingress was occurring on the suction side of the high-pressure pump. He traced the fuel lines and found the "O-Ring" sealing the high-pressure fuel injector return line to the rail was cracked and brittle. While driving, the engine's mechanical fuel lift pump provided enough pressure to overcome the slight air leak. However, when the hydraulic pump engaged, the engine RPM dipped momentarily. This RPM drop caused the mechanical fuel pump output to momentarily decrease, allowing air to be sucked past the cracked O-ring directly into the common rail. The air bubbles caused a vapor lock, starving the injectors and stalling the engine under the sudden load.
The repair involved replacing the high-pressure fuel line O-rings (all of them, as a preventive measure) and bleeding the air from the common rail system. The fuel filter/water separator was also replaced to ensure no debris was contributing to the restriction. After reassembly, the forklift lifted full capacity continuously without any RPM drop or stalling.
