Stand next to an excavator with pump cavitation and you hear it before you see anything: a rattling, gravelly sound from the pump bay, sometimes described as marbles in a tin can, worst at startup and under load. Owners often mistake it for a mechanical knock or a loose guard. It is neither. It is vapor bubbles forming and collapsing inside the hydraulic pump — and left alone, it machines the internals of a good pump into scrap.
What cavitation actually is
A hydraulic pump needs oil delivered to its inlet faster than it takes oil away. When inlet supply falls short, pressure at the inlet drops below the oil's vapor pressure, bubbles form, travel into the high-pressure side and collapse violently. Each collapse is microscopic, but millions of them sandblast the surfaces they touch — the cylinder block, the valve plate, the pistons. On the Kawasaki K3V and K5V pumps that power most of the SK range, cavitation damage shows up first on the soft surfaces and then as a pump that can no longer hold pressure when hot.
The usual causes, in the order we see them
- A clogged suction strainer. The strainer in the hydraulic tank is the classic culprit — it silts up slowly, and nobody notices until the pump starts singing. On SK machines it is a service item, not a lifetime part.
- A collapsing suction hose. Old hose softens, and under inlet suction the inner liner can delaminate and close like a valve. The hose looks fine from outside.
- Air leaks on the suction side. A tired O-ring or a loose clamp admits air instead of oil-tightness; aeration and cavitation sound similar and often arrive together.
- Cold oil. Thick winter oil cannot feed the pump fast enough in the first minutes. Machines that work hard immediately after a cold start cavitate briefly every morning — and every morning takes a little more metal.
- A restricted tank breather. The tank must breathe as oil level cycles; a blocked breather pulls a vacuum on every cycle.
Confirm it before you buy anything
Cavitation is cheap to confirm and expensive to guess at. Check the suction strainer and the breather first — both are ten-minute jobs. Pinch-test the suction hose warm and cold. Listen with a screwdriver or stethoscope at the pump body while a helper works the machine gently. And pull a small oil sample: glittering metal in the sample tells you how long the noise has been running. If the strainer fixes the noise, you caught it early. If the noise persists with a clean supply path, the pump needs to come off and be measured — at that point the regulator and the rotating group are both suspects.
What it costs if ignored
A strainer and a hose cost very little. A pump that has cavitated for a season does not come back with new oil — the valve plate and cylinder block are lapped surfaces, and once they are frosted, pressure loss is permanent. On the SK200-8 the main pump is the Kawasaki K3V112DT, quoted as 2924610-0030 / YN10V00005S156; its set plate is 2924110-0056 / LC10V00010S153. Those are the numbers a year of gravel-noise turns into. The cheap hearing test is the one you do this week.
Send the numbers, get the quote
If the pump has to come off, send the machine model and serial number plus the pump nameplate, and we will quote the pump, the rotating group or the repair kit within 24 hours with photographs of the actual units. Guangzhou Shenyue Machinery Technology Co., Ltd. holds 3,000+ part types in stock across four Guangzhou warehouses, with 100,000+ part numbers available to order. Email kobelco-parts@foxmail.com or WhatsApp +86-18144825432.
We are an independent supplier of Kobelco parts and not an authorized Kobelco dealer. We are authorized distributors for Mitsubishi, Garrett, TONGMYUNG, Marusan and NIKURA.