31 EXPERT HYDRAULIC REPAIR & DIAGNOSTIC FIXES

• Restricted Inlet (Cause 1): Always check the line from the reservoir to the pump. Be sure filters and strainers are not clogged.
• Air Leaks (Cause 2): Check pump intake piping joints and the pump shaft seal. Test by pouring oil on joints or the seal while listening for a change in the sound of operation.
• Air Entrainment (Cause 3): This occurs if the oil level is low or the return line is installed above the oil level. Check the oil level and the position of the return line.
• Reservoir Air Vent (Cause 4): Air must be allowed to be breathed into the reservoir. Clean or replace the breather.
• Filter or Strainer (Cause 8): Keep filters clean enough to permit adequate flow. Be sure the original filter has not been replaced with one of smaller capacity.

Trouble Area 2: Mechanical Pump Failures

• Pump Rotation or Speed Incorrect (Cause 5): First find out the speed recommended by the manufacturer, then check the pump speed. Reversed leads on 3-phase motors are a common cause of wrong rotation.
• Pump Out of Line with Motor (Cause 9): Check the alignment. Misalignment may be caused by temperature variation or incorrect belt tension.
• Pump Head Too Loose (Cause 10): Test by pouring oil over the pump head. Replace the faulty gasket or tighten the head as necessary.
• Pump Head Too Tight (Cause 11): If a pump was recently overhauled, follow manufacturer instructions. Too tight reduces clearances and increases rubbing friction.
• Stuck Pump Vane (Cause 12): Inspect the vane-type pump for wedged chips or sticky oil, then clean and re-assemble.
• Pump Malfunction (Cause 14): Look for mechanical trouble in the pump and replace worn and broken parts if appreciable pressure is not developed after blocking the system.
• Dirt in Pump (Cause 20): Dismantle and clean the pump.

Section B: Fluid Quality and System Component Issues
This section focuses on oil quality, valve operation, and overall system integrity.

Trouble Area 3: Valve and Pressure Control

• Relief Valve Not Functioning (Cause 15): The valve setting may not be high enough, or the spring inside may be broken. Check the seat for score marks or replace the spring and re-adjust the valve.
• Incorrect Control Valve Setting (Cause 16): If direction control valves are unintentionally set at the neutral position, oil can return to the reservoir without resistance, causing little pressure to develop.
• Internal Leakage (Cause 17): Determine the location of the leakage in valves, cylinders, or pumps by progressively blocking off various parts of the circuit.
• Excessive Pressure (Cause 18): Check the relief or regulator valve’s maximum setting to ensure it is not exceeding the pump rating.
• Valve Deposits (Cause 26): Repair or replace the valves. This is usually caused by a high sediment level or oil oxidation.
• Pilot Pressure Too Low (Cause 27): Consult manufacturer instructions; pilot pressure typically runs about 50 psi.

Trouble Area 4: Leaks, Hoses, and Sealing

• Restricted Lines (Cause 24): Check for crimped lines or internal obstructions.
• Loosened Fittings (Cause 29): Tighten, reseal, or replace O-rings. Check fittings for signs of cracks or improper installation.
• Dried or Worn Seals or O-rings (Cause 30): Replace or tighten worn components. Switching to seal-control oil may help.
• Line Breakage (Cause 31): Check hose configuration for twists, tight bends, or sags, and install hose supports. Inspect for cheap hose quality or operator abuse.

Section C: Fluid Quality and Temperature

• Oil Viscosity Too High (Cause 6): If oil is too heavy, some pump types cannot pick up prime. Drain and refill with oil of correct viscosity.
• Oil Viscosity Too Low (Cause 7): Check pump manufacturer’s recommendations. Oil with anti-wear agents is needed.
• Low Fluid Level (Cause 19): Add recommended oil and check the level. Low supply means less oil to carry away heat, causing a temperature rise.
• Fluid Contamination (Cause 22): Install an adequate filter or replace oil more often. Determine the source of the material and correct it.
• Reservoir Too Small (Cause 25): The reservoir may be too small to provide adequate cooling or may vortex. Replace it with a larger reservoir or install a cooler.

Section D: Other Electrical and Component Wear
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• Packing Worn or Damaged (Cause 21): Tighten the packing gland or replace the packing. Abrasives in the oil may cause this, so check for points where abrasives are entering the system.
• Oil Cooler Clogged (Cause 23): Try to blow it out with compressed air or use solvents.
• Electrical Solenoid Problems (Cause 28): Make necessary electrical repairs.
• Cavitation (Cause 13): Check for clogged or restricted intake line, plugged air vent in reservoir.

Solutions & Next StepsNeed to Pinpoint the Problem? Download Our Expert Cheatsheet.

The guide above details many common issues, but we know diagnostics can be complex. For a quick, scannable reference to identify the most likely cause for your system’s exact symptoms, use our comprehensive Troubleshooting Cheatsheet.