How To Judge Hydraulic Oil Pump Internal Leakage? Detection & Troubleshooting

  Hydraulic oil pump is the power source of hydraulic system. Internal leakage happens when internal components wear out. Unlike external oil leakage, internal leakage cannot be seen from outside, but it will lead to insufficient system pressure, slow movement and high oil temperature. Many operators mix up pump internal leakage with valve or cylinder faults. This article shares practical diagnosis and testing methods.

Common Symptoms of Hydraulic Pump Internal Leakage

• 1.System pressure cannot reach rated value even adjusting relief valve.

• 2.Actuators move slowly, loss of power under load.

• 3.Hydraulic oil temperature rises rapidly during operation.

• 4.Pump housing gets overheated, abnormal noise occurs under load.

• 5.Large return flow at pump drain port under working pressure.

• Note: These symptoms do not 100% confirm pump damage. Worn relief valve, spool valve or cylinder internal leakage can create same problems. Further testing is required.

Practical Testing Methods

1. Measure drain port flow (Best for piston pump)

Most variable piston pumps come with separate drain port.

1.1.Disconnect drain hose and collect oil into measuring container.

1.2.Run pump to working oil temperature, apply system working pressure.

1.3.Measure drain output flow.

• 1.4.Small dripping: normal condition.

• 1.5.Heavy continuous oil flow: severe internal leakage, worn valve plate / piston assembly.

• Note: This method is not applicable for most gear pumps.

2. Isolation test to eliminate valve & cylinder failure

Disconnect downstream valves and cylinders, connect pressure gauge directly to pump outlet.

• 2.1.If pressure reaches pump rated value: pump is good, fault lies on valves, cylinders or motors.

• 2.2.If pressure remains low under test: pump internal leakage confirmed.

3. Noise & temperature check

Pump runs quiet with low load. Once loaded, pump gets hot and noisy, which indicates worn internal components.

4. Replacement comparison test

Install a known‑good spare pump. If pressure and speed recover, original pump has internal leakage issue.

4.1.Main Causes of Pump Internal Leakage

① Oil contamination: solid particles scratch valve plate, piston and cylinder block. This is the No.1 cause.

② Long‑term over‑pressure operation.

③ Improper hydraulic oil viscosity.

④ Cavitation due to blocked suction filter or air intake.

⑤ Long‑time service and natural wear.

4.2.Solutions For Internal Leakage

① Minor leakage: Replace hydraulic oil and filter, avoid over‑pressure, prepare spare pump for future replacement.

② Severe leakage: Repair by changing worn valve plate and piston parts, or replace whole pump if heavily worn.

③ Prevent recurrence: Solve oil contamination issue after repair or pump replacement. Check suction filter and pipeline.

Common Misunderstandings

1. Low system pressure definitely means bad pump. Relief valve and cylinder leakage can also cause low pressure. Always perform isolation test.

2. No external oil leak means pump is healthy. Internal leakage occurs inside pump housing, invisible externally.

3. Test only under no‑load condition. Internal leakage only shows when pump is under working load.

Conclusion

Internal pump leakage is invisible wear damage. Combine drain‑flow test and pressure isolation test for accurate diagnosis. Always eliminate faults from valves and cylinders before replacing pump. Good oil filtration greatly extends pump service life.