A landscaping materials supplier reported a frustrating issue with a Cat 950M wheel loader: the engine would spontaneously shut down when climbing a hill with a full bucket of gravel. The machine would restart immediately after sitting for five minutes, but the failure was becoming more frequent. The ECM was not triggering any active check engine lights or fault codes at the time of the stall.
The technician connected Caterpillar's Electronic Technician (ET) software to monitor live engine parameters during a road test. While driving on flat ground, the fuel rail pressure held steady at 4,500 psi. However, when the loader hit the incline and the engine torque demand increased, the fuel rail pressure plummeted to 2,000 psi just before the engine stalled.
The initial suspicion was a failing high-pressure fuel injection pump. However, before replacing the pump, the technician checked the low-pressure fuel supply side. A vacuum gauge was teed into the fuel line between the tank and the transfer pump. Under heavy load, the vacuum reading spiked to 12 inHg, indicating a severe restriction in the fuel supply. The fuel tank was drained, and the pickup tube assembly was removed. The fine mesh screen on the pickup tube was heavily clogged with a black, tar-like substance-a mixture of degraded diesel fuel and biological growth from water contamination. When the machine hit an incline, the fuel sloshed away from the partially blocked screen, causing the transfer pump to starve and the rail pressure to collapse.
Additionally, the fuel rail pressure sensor was tested and found to be providing erratic feedback. The sensor was falsely reporting a high-pressure condition to the ECM right before the stall, which temporarily shut down the fuel metering valve, compounding the starvation issue.
The repair involved boiling out the fuel tank to remove the biological sludge, installing a new fuel pickup tube and screen assembly, and replacing the faulty fuel rail pressure sensor. The fuel filters were replaced, and the system was primed. The loader was then driven up the incline under a full load, maintaining a rock-solid 4,500 psi rail pressure with zero stalling.
