A contractor reported that a Komatsu PC200-8 excavator was producing excessive black smoke from the exhaust and would stall whenever the operator tried to dig in hard clay. The engine ran fine at idle but produced a heavy "lug" sound and stalled when the hydraulic load increased.
The technician first suspected a hydraulic pump issue, as the stalling happened under load. The pump flow was measured, but the stalling occurred even when the hydraulic pressure was relieved (free flow). This pointed to an engine issue. The intake manifold pressure was checked using a gauge. At full throttle, the boost pressure was only 15 psi, compared to the standard 30 psi.
The air filter was inspected and found to be completely blocked with dust. After replacing the air filter, the engine ran slightly better but still lacked power and smoked. The technician then focused on the turbocharger. The turbo wastegate actuator-a diaphragm that controls the exhaust bypass-was tested using compressed air. The diaphragm was found to be ruptured and leaking.
The ruptured wastegate actuator meant the turbocharger was bypassing exhaust gas around the turbine wheel instead of driving it. This resulted in low boost pressure. The engine computer (ECU), seeing low air mass flow, reduced the fuel injection rate to protect the engine, causing the stalling. However, with the blocked air filter compounding the issue, the engine was suffocating. The black smoke was unburnt fuel caused by the lack of air.
The repair involved replacing the air filter element and installing a new turbocharger wastegate actuator. The turbocharger was also checked for shaft play, which was within limits. After the repair, the black smoke cleared, and the excavator had full power to dig in hard ground without stalling.
