A construction crew reported that a Komatsu PC200-7 excavator was lifting the boom very slowly, and the engine emitted heavy black smoke whenever the boom was raised. The machine had over 12,000 hours of operation.
The technician first checked the engine air filter and fuel injectors, which were in good condition. He then connected a pressure gauge to the main hydraulic pump outlet. When the boom was raised, the pressure reached 25 MPa (normal), but the pump output flow was significantly lower than spec. The engine RPM dropped excessively during the lift, causing the black smoke.
He suspected a hydraulic pump issue. He removed the hydraulic tank filler cap. There was a loud "hiss" of air sucking into the tank. The "Hydraulic Tank Breather" (vent valve) was completely plugged with mud. The breather allows air to enter the tank as the oil level drops (when cylinders extend). When plugged, a vacuum formed in the tank. This vacuum "starved" the pump suction line, causing cavitation (bubbles in the oil). The cavitating pump could not deliver enough oil, causing the slow boom speed.
The black smoke was caused by the engine trying to compensate for the pump's inefficient operation. The main pump's "Regulator" (controller) sensed the low pressure and tried to increase the pump's displacement (swash plate angle) to maximum. Because the pump was cavitating, this increased the mechanical drag on the engine, overloading it.
The repair involved cleaning the breather valve with a solvent. The main pump was removed and disassembled. The swash plate and piston shoes showed signs of cavitation erosion (pitted surface). The rotating group was replaced. After refilling the tank with fresh oil and purging the air, the boom lifted at normal speed, and the engine smoke cleared.
