Koehring 6625 Hydraulic Excavator Slow Hydraulics Problems

[MikePhua]

Model Overview
The Koehring 6625 hydraulic excavator is a mid-sized machine known for its durable steel construction and strong hydraulic capabilities, widely used from the late 1970s to the early 1990s. Koehring, a respected name in the construction and excavation equipment industry, offered this model for heavy-duty utility across various terrains. The 6625 is equipped with a robust hydraulic system driving all boom, stick, bucket, and swing functions, designed for efficient earthmoving with smooth operation.
Common Causes of Slow Hydraulic Movements

• Hydraulic Fluid Contamination
  Dirt, water, or sludge in the hydraulic oil reduces flow rates, causing sluggish movements and wear on components. Regular fluid testing and filtration prevent this common issue.
  
• Worn or Damaged Hydraulic Pumps
  Internal leakage or worn pump components decrease pressure and flow, leading to slow boom, stick, or bucket actions.
  
• Control Valve Wear or Sticking
  Spool valves that do not shift properly or have worn seals can restrict fluid flow, producing uneven or slow actuator response.
  
• Hydraulic Cylinder Wear or Binding
  Cylinders with worn seals, bent rods, or internal scoring increase resistance, slow movement, and fluid leakage.
  
• Low System Pressure or Improper Relief Valve Settings
  Relief valves set too low will limit maximum pressure, reducing hydraulic power output.
  
• Air in the Hydraulic System
  Entrapped air causes spongy controls and delayed hydraulic response.
  

Diagnostic Steps

• Inspect and replace hydraulic fluid with OEM-recommended grade, typically ISO VG 46 anti-wear oil.
  
• Check hydraulic filter condition and replace if clogged.
  
• Measure pump output pressure against factory specifications, usually around 2500–3000 psi for boom and stick.
  
• Examine control valve spool movement and repair or replace if binding or leaking.
  
• Inspect hydraulic cylinders for smooth rod movement; repair or rebuild as needed.
  
• Bleed hydraulic system thoroughly to remove air pockets.
  

Maintenance Recommendations

• Conduct scheduled fluid and filter changes every 1000 hours or per manufacturer guidelines.
  
• Use clean, proper-grade hydraulic fluid to reduce component wear.
  
• Grease all pivot points regularly to prevent mechanical binding and strain on hydraulics.
  
• Monitor hoses and connectors for leaks or damage, replace aging parts promptly.
  
• Train operators on smooth hydraulic functions to minimize abrupt load changes.
  

Technical Parameters

• Hydraulic system operating pressures: ~2500-3000 psi
  
• Hydraulic fluid volume: Approximately 55 gallons (210 liters)
  
• Pump flow rate: Estimated 70-80 gallons per minute (265-300 liters per minute)
  

User Experiences
A longtime operator reported slow stick and boom motions after years of heavy use on the 6625. After an oil analysis showed contamination, a complete hydraulic fluid flush and new filtration restored smooth, responsive hydraulics. Another maintenance crew discovered worn spool valve seals causing sluggish bucket curl response; replacing valve seals and adjusting pressure settings resolved the issue.
Terminology

• Spool Valve: A valve component that directs hydraulic fluid flow for actuator movement.
  
• Relief Valve: Hydraulic pressure control valve preventing overpressurization.
  
• Bleeding: Removing trapped air from hydraulic circuits to ensure consistent fluid transmission.
  
• Hydraulic Pump Output: Pressure and flow rate generated by the pump feeding hydraulic actuators.
  
• ISO VG 46: A standard viscosity grade for anti-wear hydraulic oils.
  

Summary
Hydraulic slowness in Koehring 6625 excavators is commonly attributed to fluid contamination, control valve wear, or pump degradation. Timely diagnostics focused on hydraulic fluid condition, valve operations, and cylinder functionality can restore system responsiveness. Adherence to routine maintenance schedules and using correct hydraulic fluids ensure the 6625 remains productive and reliable through extended service periods.