Five Mechanical Faults of Diesel Forklifts

When a diesel engine experiences a mechanical failure during operation, it may cause minor damage to basic components in light cases, or lead to mechanical accidents in severe cases. Before a diesel engine fails, certain abnormal signs will appear in its speed, sound, exhaust, water temperature, oil pressure, and other aspects—these are the premonitory characteristics of a fault. Therefore, operators should make quick and accurate judgments based on these premonitory characteristics and take timely measures to avoid accidents. 1. "Runaway" Fault Premonitory Characteristics The speed suddenly increases and keeps accelerating, uncontrollable by the throttle; abnormal, harsh sounds are emitted; large amounts of blue or black smoke pour from the exhaust pipe. "Runaway" refers to overspeed idling, which does not occur under load, but only when just starting or when the load suddenly disappears or decreases during operation. The main causes are a malfunction in the governor system, causing fuel supply to lose control, or excessive oil in the oil pan. When premonitory signs of "runaway" appear, if emergency measures are not taken immediately, it can ultimately lead to major accidents such as cylinder breakage or shaft fracture. Handling Measures The goal is mainly to achieve flameout by cutting off fuel or air. Emergency measures that can be taken include: first, closing the throttle to stop fuel supply and depressing the brake; second, blocking the intake pipe to cut off air intake; third, quickly loosening the high-pressure fuel lines to stop fuel supply; fourth, for vehicle diesel engines in motion, engaging a high gear (braking) to stall the engine due to insufficient torque. 2. Piston Seizure Fault Premonitory Characteristics The speed drops, operation becomes weak, the sound is muffled, water temperature is excessively high, and if equipped with a water temperature gauge, its reading exceeds 100°C. At this point, sprinkling some water on the components will cause rapid evaporation. The main causes are severe water shortage or failure of the water cooling system. If the engine is immediately shut down and stopped, piston seizure may occur. Handling Measures Run at idle speed for a period of time or shut down and crank the crankshaft to assist cooling, until the water temperature drops to around 40°C, then slowly add cooling water. Be careful not to add cooling water immediately, as this may cause components to deform or crack due to an overly rapid local temperature drop. 3. Cylinder Breakage Fault Premonitory Characteristics A "tapping" sound is heard in the crankcase area, like a small hammer lightly striking a hard floor—commonly known as a "knocking sound." Oil pressure drops, and this becomes more pronounced during rapid acceleration. Cylinder breakage is a highly destructive mechanical fault, mainly caused by loosening or stretching and necking of the connecting rod bolts, or excessive clearance in the connecting rod journal fit, resulting in bolt loosening or fracture. If there is a knocking sound but oil pressure is normal, it is not a premonition of cylinder breakage; instead, consideration should be given to adjusting the fuel injection advance angle. Handling Measures Immediately shut down for inspection and replace with new parts. 4. Bearing Burnout Fault Premonitory Characteristics The sound becomes muffled, as if the load suddenly increases, speed drops, oil pressure decreases, and black smoke is emitted from the exhaust. Bearing burnout can cause the bearing shell and journal to seize together, stalling the engine. The main cause is the destruction of the oil film on the bearing shell surface, leading to direct metal-to-metal contact between the bearing shell and journal, with friction temperature rising rapidly and resulting in adhesion. Handling Measures Immediately shut down, remove the cover to inspect the connecting rod and bearing shells, identify the cause, and repair or replace as needed. 5. Shaft Fracture Fault Premonitory Characteristics A muffled knocking sound occurs inside the crankcase, which grows louder when the throttle is increased; severe vibration of components occurs during idle operation, and black smoke is emitted from the exhaust. Shaft fracture results from mechanical fatigue at the crankshaft journal shoulder, and always follows a gradual process from cracking to breaking. During hidden crack development, the premonitory characteristics are not obvious, but as the crack expands, the signs become increasingly apparent, ultimately leading to shaft fracture and engine stall. If the throttle vibrates or knocking frequency coincides and the engine is stopped, it may be at the critical point of shaft fracture. At this time, remove the cover for inspection and push the flywheel by hand; if the axial clearance is large and pushing requires little effort, it indicates that the crankshaft has already broken. Handling Measures Upon detecting premonitory signs, shut down immediately for inspection, and if cracks are found, replace the crankshaft promptly.