Troubleshooting Guide: Overheating Hydraulic Systems – Root Cause Analysis by Thermography, Flow/Pressure Diagnosis and Cooling Circuit Check

1. Problem Description and Scope

Overheating of a hydraulic system is a critical indicator of malfunction, threatening component longevity, energy efficiency and operational safety. This diagnostic guide is specifically developed for service technicians experiencing hydraulic fluid temperatures exceeding rated operating ranges (typically > 60°C to 70°C, depending on fluid type and OEM recommendations), which can result in accelerated oil degradation, decreased viscosity, increased cavitation and premature wear of pumps, motors and cylinders. It applies to hydraulic equipment present in the aerospace (e.g.: test benches, ground flight controls) and energy (e.g.: hydroelectric power plants, wind turbines, pressure lubrication systems) sectors. Overheating can be classified as:

• Critical: Temperatures exceeding the upper limit specified by the manufacturer by 20°C, resulting in emergency shutdowns or rapid irreversible damage.
• Major: Temperatures exceeding the upper limit by 10-20°C, requiring rapid intervention to avoid significant damage.
• Minor: Temperatures exceeding the upper limit by 5-10°C, indicating an emerging problem requiring planned investigation.

2. Safety Precautions

WARNING: Hydraulic systems operate under high pressures and hot fluids. Failure to follow safety procedures can result in serious injury, burns, fluid injection under the skin, or death.

• Lockout/Tagout: ALWAYS isolate the energy source and lockout the devices before any intervention on the hydraulic system, in accordance with the NF standard EN 1037. Check the absence of voltage and residual pressure.
• Stored Energy: ALWAYS purge the energy stored in the accumulators and cylinders before dismantling any component. Use appropriate pressure gauges and relief valves.
• Personal Protective Equipment (PPE): Wearing chemical and heat resistant gloves (NF EN 388, NF EN 407), safety glasses or face shield (NF EN 166), long-sleeved work clothing and safety shoes (NF EN ISO 20345) is ESSENTIAL.
• Fluid Leaks: NEVER use bare hands to detect leaks. Use a piece of cardboard or a leak detector. Hydraulic fluid under pressure can pierce the skin.
• High Temperatures: The surfaces of hydraulic components (tank, lines, pump, motor) can be extremely hot. Use an infrared thermometer to check the temperature before touching.

3. Required Diagnostic Tools

4. Initial Assessment Checklist

Before initiating in-depth diagnosis, a methodical assessment of the initial conditions is essential.

5. Systematic Diagnostic Flowchart

1. Initial Symptom: Abnormally high hydraulic oil temperature (> 70°C).
1. Quick Check Oil Level and Cooler:
   1. IF Oil Level < MIN :
      • Probable Cause: Insufficient level, reducing heat dissipation surface.
      • Action: Add the fluid specified by the OEM. Go to 9.
   2. IF Chiller (air/water) is not active/functional (fan off, water off):
      • Probable Cause: Cooling system failure.
      • Action: Diagnose the cooler control circuit (thermostat, motor, valve). Go to 9.
   3. ELSE: Carry out thermography.
2. Diagnosis by Thermography:
   1. Perform a complete thermal scan of the system in operation.
   2. IF Localized hot spots (>90°C) on specific components (e.g. pump, relief valve, line restriction):
      • Probable Cause: Excessive pressure drop, internal friction, restriction.
      • Action: Go to the flow/pressure diagnosis (5.c).
   3. IF High temperature uniformly throughout the system, with no specific hot spots:
      • Probable Cause: Insufficient heat dissipation or excessive overall heat generation.
      • Action: Proceed to checking the cooling circuit (5.d) and analyzing the fluid (5.e).
3. Flow/Pressure Diagnosis:
   1. Measure the pump discharge pressure and the main relief valve setting pressure.
      • IF Discharge Pressure > OEM Specified Pressure:
        • Probable Cause: Improperly adjusted or stuck discharge valve, generating heat.
        • Action: Adjust/Clean the valve. Go to 9.
      • IF Abnormal pressure drop (> 5 bar) across a filter or pipe:
        • Probable Cause: Clogging or restriction.
        • Action: Inspect/Replace the component. Go to 9.
   2. Measure the flow rate of the pump under vacuum and load.
      • IF Actual flow < Débit théorique (calculé via cylindrée et régime) de > 10%:
        • Probable Cause: Significant internal pump leaks, generating heat.
        • Action: Repair/Replace the pump. Go to 9.
   3. Measure the flow in the tank return circuit.
      • IF Return Flow > Expected flow without useful work (cylinders stationary):
        • Probable Cause: Internal leaks in cylinders, motors or control valves, converting energy to heat.
        • Action: Isolate and repair the faulty component. Go to 9.
4. Cooling Circuit Check:
   1. Air/Oil Cooler:
      • IF Low or no air flow through the radiator:
        • Probable Cause: Obstruction of the fins (dust, debris) or failure of the radiator fan.
        • Action: Clean the radiator. Diagnose/Replace fan motor (check power supply with DMM). Go to 9.
   2. Water/Oil Cooler:
      • IF Cooler outlet water temperature close to that of the oil, with low thermal gradient:
        • Probable Cause: Internal clogging (scale, sludge) of the tubes or low water flow.
        • Action: Clean the cooler. Check valve and cooling water flow. Go to 9.
   3. Cooling Circuit Thermostat:
      • IF Cooler does not activate at the preset temperature:
        • Probable Cause: Defective or incorrectly calibrated thermostat.
        • Action: Test the thermostat with a heating oil bath. Replace if defective. Go to 9.
5. Hydraulic Fluid Analysis:
   1. Take an oil sample for laboratory analysis.
   2. IF Analysis result indicates:
      • Kinematic viscosity outside OEM specification (eg: ±10% of nominal value).
      • Water content > 100 ppm (for oils without special additives).
      • Cleanliness ISO 4406 Class > 18/16/13.
      • Total acidity index (IAN) significantly increased.
      • Probable Cause: Thermal degradation of the oil, contamination by water or particles, incorrect viscosity.
      • Action: Replace the hydraulic fluid, check the cause of the contamination (tank sealing, filtration). Go to 9.

6. Cause-Fault Matrix

7. Root Cause Analysis for Each Defect

7.1. Internal Pump Leaks

Why this happens: Normal wear and tear of moving pump components (pistons, barrel, distributor for piston pumps; gears for gear pumps; vanes for vane pumps) results in increased operating clearances. These clearances allow oil to move from high pressure zones to low pressure zones within the pump itself without doing any useful work. This loss of volume generates direct heating of the fluid.

How to confirm: Thermography will reveal a high temperature on the pump body. The most reliable method is flow measurement. Comparing the actual flow rate measured at the pump outlet with the theoretical flow rate (calculated from pump displacement and input shaft rotation speed), a difference greater than 10% indicates significant internal leaks. A no-load pressure build-up test can also highlight abnormal slowness, a sign of a leak.

Damage if not resolved: Overheating accelerates the degradation of the oil, reducing its lubricating capacity and causing pump and other system components to wear out even more quickly. This leads to loss of efficiency, slow system response and, ultimately, catastrophic pump failure.

7.2. Discharge Valve Improperly Adjusted or Blocked

Why this happens: The relief valve (or pressure relief valve) is designed to protect the system from overpressure by redirecting fluid back to the reservoir when the pressure reaches a set threshold. If this valve is stuck in the partially open position all the time, or if its set pressure is too high for the system's needs, a significant portion of the pump's flow will constantly be discharged to the reservoir. Oil passing through the restricted orifice of the valve experiences a pressure drop and converts its energy into heat.

How to confirm: A thermography will reveal a very clear hot spot on the wastegate body. Measuring the pressure upstream of the valve will show abnormally high or unstable pressure. Listening may reveal a characteristic hissing sound of fluid passing through an orifice. A test by isolating the valve and measuring the reopening/closing pressure using a test bench is ideal.

Damage if not resolved: Continuous generation of heat, accelerated degradation of the oil. This forces the pump to continually work against excessive resistance, reducing its lifespan and increasing energy consumption. Loss of precise control of the system.

7.3. Insufficient Cooling Capacity

Why this happens: A cooler that is undersized for the system heat load, external (dust, debris) or internal (scale, sludge, gum) radiator fouling, cooling fan (motor, blades) failure for air/oil coolers, or insufficient/blocked cooling water flow for water/oil coolers. The temperature control thermostat may also be faulty and prevent the cooler from operating.

How to confirm: Measure the temperature gradient (ΔT) of the oil at the inlet and outlet of the cooler. A low ΔT (less than 5°C) indicates low heat transfer efficiency. For an air/oil cooler, check the cleanliness of the fins and the operation of the fan (air flow, power supply). For a water/oil cooler, check the flow rate and temperature of the cooling water, and the absence of blockages in the water lines. Thermography can locate cold spots on the clogged cooler.

Damage if not resolved: Chronic inability to maintain oil temperature within acceptable limits, leading to all of the oil degradation and component wear issues previously mentioned.

7.4. Incorrect or Degraded Fluid Viscosity

Why this happens: Using an oil whose kinematic viscosity does not meet OEM specifications (too high or too low), or thermal degradation and oxidation of existing oil. Oil that is too viscous (cold) increases losses through internal friction and pump effort. Oil that is too thin (hot or degraded) increases internal component leaks, generating heat.

How to confirm: The only reliable method is laboratory analysis of oil samples (ASTM D445 for viscosity, ASTM D6304 for water, IAN for acidity, Cleanliness ISO 4406). Visual indicators (burning odor, dark color, emulsion) may suggest degradation but are not quantitative.

Damage if not resolved: Accelerated wear of pumps and motors (loss of lubricating film), increased cavitation, malfunction of servo valves, corrosion and varnish deposits in the system. Reduced system lifespan.

8. Step-by-Step Resolution Procedures

WARNING: ALL work must be carried out with the system logged out and purged of all stored energy.

8.1. Fixing Internal Pump Leaks

1. Deposit: Apply the deposit/deconsignment (LOTO) procedure.
2. Depressurization: Check the absence of residual pressure.
3. Draining: Drain the hydraulic circuit if major disassembly of the pump is necessary.
4. Disassembly: Release and disassemble the pump.
5. Inspection: Visually inspect the condition of the contact surfaces, pistons, barrel, gears. Look for signs of wear, deep scratches or cavitation.
6. Repair/Replacement: Consider repair by a certified specialist or replacement of the pump with a new model or reconditioned by UNITEC.
7. Reassembly: Reassemble the pump with new seals and tightening torques specified by the OEM (e.g.: fixing bolts at 80 Nm, fittings at 40 Nm, check standard NF E 48-005 for hydraulic fittings).
8. Filling and Bleeding: Fill the reservoir with the recommended fluid. Bleed air from system per OEM procedure.
9. Check: Start the system at low load. Measure pump flow and temperature. Confirm the absence of abnormal hot spots.

8.2. Relief Valve Adjustment/Repair

1. Deposit: Apply the deposit/deconsignment (LOTO) procedure.
2. Depressurization: Check the absence of residual pressure.
3. Pressure Measurement: Connect a precision pressure gauge (Class 1.0) to the pressure line upstream of the valve.
4. Adjustment (if accessible): Loosen the locknut. Adjust the relief valve adjustment screw to the pressure specified by the OEM (eg: 200 bar ±5 bar). Tighten the locknut (torque specified by OEM).
5. Cleaning/Replacement (if blocked/contaminated): If the valve is blocked, disassemble it (after local draining), inspect its drawer and seat for the presence of contamination (particles, gum) or damage. Clean or replace the drawer and/or seat if necessary. Reassemble with new seals.
6. Check: Start the system. Check the discharge pressure on the pressure gauge. The temperature on the valve should be in sync with the rest of the system and not have any hot spots.

8.3. Improved Cooling Capacity

1. Deposit: Apply the deposit/deconsignment (LOTO) procedure.
2. Depressurization: Check the absence of residual pressure.
3. Cleaning the Air/Oil Cooler: Stop the fan. Use clean, dry compressed air (max 6 bar) or a high pressure cleaner (at a close distance so as not to damage the fins) to remove debris and dust from the fins.
4. Fan Diagnosis:
   1. Electrical Check: With a DMM, check the fan motor supply voltage (ex: 400V AC ±10%). Measure the resistance of the motor windings (compare to OEM values).
   2. Mechanical Check: Inspect the fan blades for damage or imbalance. Check the motor bearing (noise, play).
   3. Replacement: Replace the motor or blades if defective.
5. Cleaning the Water/Oil Cooler: Insulate the water circuit. Drain the water. Carry out chemical (according to the cooler manufacturer's recommendations) or mechanical cleaning of the tubes to remove limescale and sludge. Rinse thoroughly.
6. Checking Water Flow: Measure the cooling water flow (with a flow meter) and check the chiller inlet/outlet pressure. Make sure the valves are fully open.
7. Thermostat Test: Remove the thermostat from the circuit. Immerse it in a heating oil or water bath and check its opening/closing temperature using a calibrated thermometer. Replace if out of specification.
8. Check: Start the system. Measure the cooler ΔT and tank temperature. The ΔT must comply with OEM specifications (typically > 5°C).

8.4. Hydraulic Fluid Replacement and Inspection

1. Deposit: Apply the deposit/deconsignment (LOTO) procedure.
2. Depressurization: Check the absence of residual pressure.
3. Complete Drain: Completely drain the hydraulic tank and all low points of the circuit.
4. Cleaning the Tank: Clean the inside of the tank, including the walls, bottom and suction screens, to remove sludge and deposits.
5. Replacement of Filters: Replace all filters (suction, pressure, return, breather) with new elements conforming to OEM specifications (cleanliness ISO 4406).
6. Filling: Fill the tank with the volume and type of hydraulic fluid EXACTLY specified by the manufacturer (e.g. ISO VG 46, NF EN ISO 3448). Use a filtration/filling unit to ensure initial fluid cleanliness (typically 18/16/13 or better).
7. Purge: Purge air from the system.
8. Check: Start the system. Complete a complete operating cycle. Measure the temperature. Perform an oil analysis after 100 hours of operation to confirm cleanliness and viscosity.

9. Preventive Measures

10. Spare Parts and Components

To order spare parts, please consult our E-Catalog UNITEC-D.

11. References

• NF EN 1037: Machine safety - Prevention of unintended start-up.
• NF EN ISO 4406: Hydraulic fluids - Method for coding the level of solid particle contamination.
• NF EN ISO 11158: Hydraulic fluids - Specification of hydraulic oils of type HM, HV, HL, HG and HH.
• NF EN ISO 20345: Personal protective equipment - Safety shoes.
• NF EN 388: Protective gloves against mechanical risks.
• NF EN 407: Protective gloves against thermal risks.
• NF EN 837-1: Manometers - Part 1: Bourdon tube manometers.
• NF ISO 10816: Measurement and evaluation of mechanical vibrations of non-rotating machines.
• Original Equipment Manufacturer (OEM) Operation and Maintenance Manuals.
• Additional UNITEC-D maintenance guides (e.g.: Hydraulic filtration guide, Pump noise diagnosis).