Measure delivered flow at operating temperature and at rated pressure, then repeat the measurement cold. A large drop between cold and hot readings is the signature of a worn pump. If flow is within specification hot, the pump is not your problem no matter how the machine feels.
How to confirm it
- Confirm what flow the pump should deliver
From the service documentation, establish rated flow at rated engine speed and at operating temperature. Some specifications quote flow at a specific pressure too — the pressure matters, because a pump that delivers rated flow at zero pressure may deliver half of it at working pressure.
- Bring the oil to operating temperature
As with pressure testing, a cold flow reading is misleading in the flattering direction. Run the machine until hydraulic oil is genuinely at operating temperature before taking the measurement that will decide anything.
- Install a flow meter in series at the pressure line
Connect a flow meter rated for the system's flow and pressure into the pump delivery line, using the machine's own test ports where they exist. Follow the meter manufacturer's instructions on orientation and on loading — a flow meter installed backwards gives a plausible but wrong result.
- Load the circuit and read delivered flow
Take the reading at rated pressure by loading the meter. Flow measured at zero pressure is not useful: a worn pump can look healthy with no load and collapse under it. This is the single most common mistake in flow testing.
- Compare delivered flow against specification
Note the shortfall. A modest shortfall explains sluggishness; a large one explains a machine that barely works. Record the number — it is the evidence for or against replacing a pump.
- Repeat the reading cold and compare
The difference between cold and hot flow is the diagnostic signal. A pump that delivers near specification cold and much less hot has clearance-dependent wear and needs repair or replacement.
- Check suction side before condemning the pump
Inspect the suction strainer, the suction hose for internal collapse, and the oil level. A starved pump delivers low flow at every temperature, and the fix costs a fraction of a pump.
What to do about it
Hot flow within specification: the pump is not the fault. Return to pressure testing and to the circuit-based checks rather than buying a pump.
Hot flow well below specification with a clean suction side and correct relief pressure: plan on pump repair or replacement. Confirm the pump variant against the machine serial number before ordering, because many models were built with more than one pump.
Low flow at both temperatures: check the suction side and the pump drive coupling first. A starved pump or a slipping coupling produces low flow without the pump being worn.
Low flow with fine metal in the return filter: expect pump and possibly motor wear together. The whole circuit should be treated as contaminated and flushed.
Need the parts this covers?
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Request a quoteFrequently asked
Can I test flow without a flow meter?
You can estimate it with a timing test — raise a function and time how long a known cylinder takes to stroke. That gives a comparative figure good enough to confirm a fault, though not precise enough to quote as a specification figure.
Why does my flow reading at zero pressure look fine?
Because a worn pump leaks internally in proportion to pressure. With no load, the leakage is small and the reading flatters the pump. Always load the circuit to rated pressure when measuring flow.
Is measuring flow worth the cost of a flow meter?
On a fleet, yes — it prevents pump replacements that were never justified. For a single machine, hire or borrow one. The alternative is replacing an expensive component on the strength of a pressure reading that cannot actually detect the fault.