Pump · Medium
The pump growls, whines or knocks
Sound is the cheapest sensor on the machine. A growl that disappears when the machine is cold is air; a knock that follows load is metal. Identify which before you take the pump off.
What it usually is
| Share of cases | Cause and how to check it |
|---|---|
| 40% | Cavitation: suction strainer clogged or air ingress Vacuum gauge at the pump inlet: over −0.02 MPa means a restricted suction. Look for foam in the tank. |
| 28% | Worn piston shoes / slippers (metal on metal) Cut the filter open. Brass-coloured particles mean shoes; steel flakes mean cylinder block or valve plate. |
| 18% | Shaft seal letting air in Oil weep at the shaft, or a whistle at idle that stops when the case is pressurised. |
| 14% | Coupling or alignment Hand-turn the pump with the engine off: roughness in a quarter-turn means coupling, not hydraulics. |
Field tests
1. Cold start listening
Record the first 30 seconds after start with a phone. Cavitation noise is loudest when the oil is thick.
2. Inlet vacuum
Tee a vacuum gauge before the pump. A new strainer should read under −0.01 MPa at full flow.
3. Case drain and filter cut
Both together separate air from metal in five minutes.
What actually fixes it
A pump that is cavitating is not a worn pump — replacing it without fixing the suction kills the new one too. That is the conversation this page exists to start.
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