A utility contractor reported that a Komatsu PC210LC-11 excavator's main hydraulic pump emitted a high-pitched whine whenever the boom was raised, and after three hours of trenching the hydraulic oil sight glass showed a distinct milky discoloration. The hydraulic tank temperature had reached 98°C, and the boom raise speed was noticeably slower than specification.
The technician recognized two interlocking symptoms. The milky oil indicated either water ingress or severe aeration - air whipped into the oil under high shear reduces the fluid's bulk modulus, and when it later cools, any condensed atmospheric moisture cannot separate, producing the milky appearance. The pump whine is the textbook acoustic signature of "aeration," distinct from the lower, gravelly "rattle" of cavitation. Cavitation occurs when the pump cannot draw a full charge of oil, producing dissolved-air implosion on the pressure side; aeration occurs when free air is drawn into the suction line through a leak upstream of the pump, producing a steady, high-pitched whine caused by compressed air bubbles passing through the high-pressure chambers. The distinction matters because the failure mechanisms are different.
He confirmed aeration by the classic field test: coating the suspected suction hose joints with a thin film of grease while the engine idled, then revving to rated speed. The whine did not change when he greased the filter housing joint or the pump inlet flange, but it abruptly dropped by 40% when he greased the suction hose itself, mid-span. He removed the hose. The outer cover looked intact, but when he flexed the hose, he found the "Inner Wire Spiral Liner" had cracked longitudinally, creating a flap that opened under suction and ingested atmospheric air. The crack was consistent with age-hardening of the inner nitrile tube after 6,200 hours in a hot, vibration-rich environment.
The aeration had already done structural damage. He removed the main pump regulator and found the "Swashplate Servo Piston Bore" scored along the high-pressure side. The compressed air bubbles, passing through the servo control circuit at 350 bar, had imploded against the bore wall - a hybrid of aeration and cavitation pitting - tearing metal from the cast-iron bore and embedding it in the servo piston seal. The servo piston could no longer stroke smoothly to modulate pump displacement, which explained the slow boom raise and the heat: the pump was dead-heading against the control valve, generating 98°C oil and accelerating the thermal breakdown that produced the milky emulsion.
The repair required replacing both main pump cartridges (the scored servo bore was not field-repairable), the damaged suction hose, and the return-line filter. The hydraulic tank was drained, steam-cleaned to remove the emulsified oil film, and refilled with fresh ISO 46 anti-wear fluid. The tank breather was replaced as well - a clogged breather creates a vacuum that stresses the suction line and accelerates inner-liner cracking. After bleeding all pilot and main lines through the case drains, the pump operated silently, the boom raise returned to specification, and a 50-hour post-repair oil sample came back clean with no water or air-content flags. The site added the suction-hose flex-and-inspect check to its 6,000-hour service, since inner-liner cracking is invisible externally but detectable by hose stiffness.
