A construction site reported that a SANY SY215C excavator was emitting heavy black smoke and the engine RPM was surging (hunting) up and down when the hydraulic load was applied. At idle, the engine ran rough but stable.
The technician checked the fuel filters, which were clean. Suspecting an air restriction, the technician inspected the air intake system. The rubber hose connecting the air filter housing to the turbocharger was found to be collapsed and had a large tear on the underside. This tear allowed unfiltered, hot air to bypass the filter, but more importantly, it disrupted the Mass Air Flow (MAF) sensor reading. The engine control unit (ECU) was receiving incorrect data about the air volume, causing it to over-fuel, resulting in black smoke.
After replacing the intake hose, the black smoke reduced but the surging persisted. The technician then performed a cylinder cut-out test using the diagnostic panel. The test revealed that cylinders 2 and 4 were contributing less power than the others. The fuel injectors were removed and tested on a bench. The injector nozzles were found to be "dripping" instead of atomizing the fuel properly, likely caused by dirty fuel over time. This poor atomization caused incomplete combustion, leading to the rough idle and surging.
The repair involved replacing the torn intake hose and installing four new fuel injectors. The injector coding was programmed into the ECU. After the repair, the excavator ran smoothly with clear exhaust and stable RPM under heavy digging loads.
