Troubleshooting Hydraulic Overload and Engine Stall in a 2008 Kubota U35-S

[MikePhua]

Understanding the Problem: Engine Stalls Under Load
A common and deeply frustrating issue faced by operators of compact excavators like the 2008 Kubota U35-S is engine stalling during specific hydraulic operations—particularly when swinging the boom or attempting multiple functions simultaneously. While the engine runs fine under idle or light single-function loads, engaging in combined movements such as boom raise with swing or track movement may cause the engine to bog down or stall entirely.
This issue isn’t just a nuisance; it directly affects productivity and can signal a deeper hydraulic or control system problem that could worsen if neglected.
Key Terminology

• Hydraulic Load Sense (LS) System: A system that adjusts hydraulic output based on demand. A faulty LS signal can overload the engine.
  
• Priority Valve: A valve that controls hydraulic flow to prioritize certain functions, like swing or travel, over others.
  
• Load-Induced Stall: Engine stalling that occurs when the hydraulic system demands more power than the engine can deliver.
  
• Pilot Pressure: Low-pressure hydraulic signal used to control high-pressure actuators. If this is inconsistent, controls may behave erratically.
  
• Hydraulic Bypass: When hydraulic fluid flows through the system without doing work, often due to faulty valves or incorrect pressure settings.
  

Likely Causes of the Stall Issue
There are several possible causes behind this behavior, particularly in a machine like the U35-S which integrates electronic and hydraulic systems.

1. Weak or Contaminated Fuel Supply
   • Clogged fuel filters or an underperforming fuel pump can reduce engine power.
     
   • Air bubbles in fuel lines can lead to inconsistent power delivery.
     
2. Hydraulic Pump Overload
   • Excessive flow demand during simultaneous operations may exceed engine output.
     
   • If load sensing is not functioning properly, the pump can place unnecessary demand on the engine.
     
3. Incorrect Relief Valve Settings
   • Relief valves are set to allow hydraulic fluid to bypass at a specific pressure to protect components.
     
   • If set too high, the engine may be overworked before the relief kicks in.
     
4. Load Sensing Line Blockage or Leak
   • The LS system tells the hydraulic pump how much pressure is needed.
     
   • If the line is blocked or leaking, the pump can go to full stroke unexpectedly, creating overload.
     
5. Defective Hydraulic Control Valve
   • The main valve block may have a stuck spool, allowing too much flow.
     
   • Poor internal sealing can also result in improper distribution of flow during multi-function use.
     
6. Engine Governor Issues
   • A mechanical or electronic governor may fail to increase throttle appropriately under load.
     
   • If the governor response is sluggish, the engine may not maintain RPM during heavy demand.
     

Real-World Case: Farm Excavator Blues
In rural Tennessee, a farm operator reported his U35-S stalling whenever trying to swing and dig at the same time. After several costly visits from mobile mechanics, the culprit turned out to be a failing pilot pressure solenoid. It would intermittently lose signal during multi-function use, reducing the flow to critical valves. Replacing the solenoid—and flushing the pilot lines—resolved the issue entirely.
Diagnostic Checklist for Owners and Mechanics
To troubleshoot effectively, operators can follow this logical approach:

• Inspect the Fuel System
  • Replace fuel filters.
    
  • Check for water or debris in the tank.
    
  • Test fuel pump pressure.
    
• Examine the Hydraulic System
  • Measure pump output pressure during function engagement.
    
  • Check if relief valves open at factory-set pressures.
    
  • Inspect hydraulic oil for contamination or aeration.
    
• Test Engine Response
  • Engage swing or travel with no load and watch RPM drop.
    
  • Listen for governor lag or stumble.
    
  • Use diagnostic tools if electronically controlled.
    
• Evaluate Load Sensing Circuit
  • Ensure LS signal lines are intact and not kinked or blocked.
    
  • Check for internal leaks in the main valve body.
    

Design Considerations and Weak Points
Compact excavators, particularly those in the under-4-ton class, are often used for a wide range of tasks from trenching to landscaping. Their hydraulic systems are built to be responsive, but not always robust under extreme simultaneous loads. The Kubota U35-S, while reliable, can fall victim to:

• Debris in pilot circuits due to infrequent filter replacement
  
• User-overload from trying to move multiple actuators simultaneously
  
• Wear in high-cycle components like control valve spools and swing motors
  

Preventive Maintenance and Best Practices

• Replace hydraulic filters every 500 hours or as specified in the manual.
  
• Clean the fuel tank annually to prevent microbial growth and sludge buildup.
  
• Always warm up the hydraulic oil before full operation in cold weather.
  
• Avoid full-speed multi-function tests before engine reaches operating temperature.
  
• Don’t ignore early signs like sluggish swing or slow track engagement—they often precede full stalls.
  

Conclusion
The 2008 Kubota U35-S remains a popular machine in the compact excavator segment, but when the engine stalls under combined hydraulic loads, it’s a sign of underlying imbalance between engine output and hydraulic demand. Through targeted troubleshooting—fuel system, hydraulics, load sensing, and control valves—these issues can often be resolved without major repairs. Understanding how these systems interact allows both owners and technicians to keep the machine running smoothly and extend its working life.