A lumber yard reported that a Mitsubishi 4DQ25 diesel forklift emitted thick black smoke, lacked power to lift logs, and the exhaust manifold was glowing red hot after 15 minutes of operation.
The technician first performed a "Differential Pressure Test" across the "Diesel Particulate Filter (DPF)." The pressure differential reading was 250 mbar (the limit is 30 mbar). He removed the DPF and found the internal ceramic core had melted into a glass-like plug, blocking the exhaust flow. The engine couldn't expel exhaust, causing the black smoke and glowing manifold.
To find the root cause of the DPF melting, he checked the "Turbocharger Wastegate Lever." The lever was seized solid by rust. The wastegate is designed to open when boost pressure gets too high. Because it was seized, the turbo spun uncontrollably, over-boosting the engine. The engine ECU saw the high boost and injected excessive fuel to match, but the fuel couldn't burn completely due to the blocked exhaust, creating extreme exhaust temperatures that melted the DPF.
The repair involved replacing the melted DPF unit and installing a new turbocharger assembly (the seized wastegate had damaged the internal seals). The exhaust manifold and ports were cleaned of carbon. The engine ECU was updated with the latest calibration to prevent over-fueling. After reassembly, the engine ran clean with full power and normal exhaust backpressure.
