By Dovydas Kersys · Chapter 20 of 40 in Hydraulic Troubleshooting.
Before you begin: Hydraulic equipment can contain lethal stored energy. Only trained and authorized personnel should perform hydraulic work. Isolate energy, support loads, verify the pressure state, use pressure-rated instruments and follow site-specific lockout/tagout procedures. The diagrams are simplified training illustrations; use the actual machine schematic and manufacturer instructions.
Hydraulic systems overheat when input power is not being converted into useful work or removed by the cooler. Continuous relief flow, throttling, internal leakage and high return backpressure are common sources.
In practice, the fastest diagnosis comes from deciding what the component is supposed to control and then measuring the variable on both sides of that component. Pressure tells you about load and restriction; flow tells you about speed; temperature tells you where power is being lost; leakage tells you whether clearances or seals are allowing oil to bypass the intended path.
Figure 20. Simplified diagnostic view for 20. Overheating and Excessive Power Loss.
What the technician may observe
- oil exceeds normal operating temperature
- motor current high even when machine is idle
- relief valve or manifold is very hot
- machine loses strength as temperature rises
- cooler inlet hot but outlet nearly same temperature
Diagnostic sequence
- Check standby pressure and pump unloading condition. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Measure pressure drops across valves and filters at normal flow. Record the result before changing the next variable so the test actually narrows the fault.
- Check internal leakage by comparing hot and cold performance. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Verify cooler fan, airflow, water flow or thermostat operation. Record the result before changing the next variable so the test actually narrows the fault.
- Inspect relief valve for continuous flow. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Calculate approximate hydraulic power being lost when a large pressure drop carries significant flow. Record the result before changing the next variable so the test actually narrows the fault.
How to interpret the measurements
Begin by asking whether the symptom affects the whole machine or only this function. If other functions sharing the same supply remain normal, a common pump or reservoir fault becomes less likely and the local valve, actuator, plumbing and mechanical load deserve more attention.
Next, compare pressure upstream and downstream while oil is actually flowing. A large pressure difference across a component means that component or passage is consuming hydraulic energy. If flow is low everywhere, investigate pump delivery, inlet conditions, speed and major internal leakage before blaming a local valve.
Finally, repeat the test at the condition that creates the complaint: hot oil, full load, a particular direction, low speed or rapid cycling. Hydraulic clearances, viscosity, pilot pressures and valve forces change with operating condition, so a static test may look normal while the production fault remains.
Corrective actions after the cause is proven
- Eliminate unnecessary throttling and relief flow.
- Repair high internal leakage.
- Restore cooler performance.
- Correct viscosity after temperature control is stable.
Common mistakes
- Adjusting the relief or compensator before recording the original setting and proving that the setting is wrong.
- Replacing a component because it is hot, even though the heat may be arriving from an upstream restriction or continuous bypass flow.
- Calling a problem “low pressure” when the actual complaint is low flow or excessive mechanical load.
- Testing only with cold oil when the production fault appears after the system reaches operating temperature.
Field rule: For 20. overheating and excessive power loss, do not replace a component until one measurement or isolation test shows how it fails to perform its intended function.
Symptom-to-next-test table
| Observed condition | First useful test |
|---|---|
| Oil exceeds normal operating temperature | Measure temperature and pressure drop at the suspected heat source. |
| Motor current high even when machine is idle | Measure the controlled variable before and after the suspected component. |
| Relief valve or manifold is very hot | Measure temperature and pressure drop at the suspected heat source. |
| Machine loses strength as temperature rises | Measure temperature and pressure drop at the suspected heat source. |
Field practice
Before leaving the machine, write down three numbers that would let another technician judge whether the repair truly restored normal operation. Good choices are usually operating temperature, a pressure at a defined machine state, and either flow or cycle time. This creates the baseline that makes the next fault faster to diagnose.
PART V – VALVE TROUBLESHOOTING
Practical diagnosis from symptom to measurement to root cause.
Continue the series
- Previous chapter: Hydraulic Low Pressure or No Pressure
- Next chapter: Hydraulic Relief Valves: Setting, Instability and Leakage
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Adapted from Hydraulic Troubleshooting for Maintenance Technicians, by Dovydas Kersys. © 2026 Dovydas Kersys. Featured cover image is an AI-generated editorial illustration.