A construction-materials distribution yard reported that a Cat DP50N 5-ton diesel counterbalance forklift had developed a progressive cooling-system fault over a two-week period. The operator first noticed a small puddle of coolant forming beneath the truck after each shift, and the coolant level in the expansion tank required topping up every two to three days. As the fault progressed, the engine's operating temperature climbed noticeably above its normal range during extended loaded operation, and the cab display would intermittently illuminate an engine overtemperature warning. The operator also reported a faint grinding sound from the front of the engine during idle - a noise that had not been present when the truck was running normally.
The technician first understood the DP50N's cooling architecture. The diesel engine's cooling system uses a centrifugal "Water Pump" mounted on the engine's front cover, driven by the engine's accessory belt. The pump's impeller circulates coolant through the engine block, cylinder head, and radiator, maintaining the engine at its designed operating temperature. The pump's shaft is supported on a sealed bearing assembly, and a mechanical "Shaft Seal" prevents coolant from leaking outward along the pump's shaft. Most water pumps include a "Weep Hole" - a small drainage port positioned below the shaft seal - that allows coolant leaking past a failing seal to escape externally rather than contaminating the pump's bearing. A small amount of coolant weeping from the weep hole is normal during the pump's early service life; a persistent or growing leak indicates the shaft seal has failed and the pump requires replacement.
He verified the symptom by inspecting the water pump's weep hole. Fresh coolant residue was visible around the weep hole's exterior, and a droplet of coolant was actively forming at the port during idle - confirming the shaft seal had failed. He then listened to the pump's bearing with a mechanic's stethoscope during idle. A faint grinding noise was clearly audible from the pump's bearing area - indicating the bearing had begun to degrade, likely from coolant contamination having migrated past the failed seal. He further inspected the pump's pulley for lateral play by attempting to rock it by hand - the pulley showed measurable lateral movement, confirming bearing degradation.
The root cause was traced to water pump shaft seal failure compounded by bearing degradation. The pump's shaft seal had hardened and cracked over extended service (the truck had approximately 6,500 hours), allowing coolant to leak past the seal and out through the weep hole. The leaking coolant had migrated into the pump's bearing assembly, washing away the bearing's grease and introducing abrasive wear - producing the grinding noise and the lateral play the technician observed. The progressive coolant loss had reduced the cooling system's effective coolant volume, causing the engine's operating temperature to climb and eventually triggering the overtemperature warning the operator described.
The repair involved replacing the complete water pump with a new unit (the failed shaft seal and degraded bearing are not field-repairable), replacing the accessory drive belt with a new belt (the old belt showed glazing from the degraded bearing's drag), and inspecting the cooling system's remaining components. The radiator's external fins were cleaned with compressed air (moderate dust accumulation found and removed), the radiator cap was pressure-tested (within specification), and the cooling system's hoses and hose clamps were inspected for condition (all within service limits). The cooling system was flushed with a chemical flush solution to remove any degraded coolant and scale, and the system was refilled with fresh specified coolant to the correct level. The system was bled to remove trapped air, and the coolant level was checked after a warm-up cycle and topped up as needed. After reassembly, a 60-minute loaded operational test showed the engine's operating temperature stable within its normal range, no coolant leakage from the weep hole, and the grinding noise eliminated. Water pump weep-hole inspection and accessory belt condition checks were added to the truck's 500-hour preventive maintenance checklist.
