Hydraulic Stall Issues on the Hitachi EX100-2 and How to Resolve Them

[MikePhua]

Hitachi’s EX100-2 and Its Global Footprint
The Hitachi EX100-2 is a mid-sized hydraulic excavator that emerged during the early 1990s as part of Hitachi’s second-generation EX series. Built for reliability and mechanical simplicity, the EX100-2 was widely adopted across Asia, Europe, and Oceania. Its popularity stemmed from its balance of power, compact footprint, and ease of service—especially in regions where electronic diagnostics were impractical.
Powered by the Isuzu 4BG1T turbocharged diesel engine, the EX100-2 delivers around 70 horsepower and features a robust open-center hydraulic system. With an operating weight of roughly 10,000 kg and a digging depth exceeding 4 meters, it was designed for trenching, site prep, and utility work.
Understanding Hydraulic Stall Symptoms
Hydraulic stall refers to a condition where the engine bogs down or stalls when hydraulic functions are engaged. In the EX100-2, this typically occurs when operating the boom, arm, or bucket under load. Common symptoms include:

• Engine RPM drops sharply during hydraulic actuation
  
• Machine stalls when multiple functions are used simultaneously
  
• Hydraulic response feels sluggish or delayed
  
• Audible strain from the engine when controls are engaged
  

These symptoms suggest that the hydraulic system is demanding more power than the engine can supply, or that flow regulation is compromised.
Terminology Notes

• Open-Center Hydraulic System: A system where hydraulic fluid flows continuously through the control valves and returns to the tank unless diverted by operator input.
  
• Hydraulic Stall: A condition where hydraulic load exceeds available engine torque, causing the engine to bog or shut down.
  
• Load Sensing Valve: A component that adjusts hydraulic flow based on demand, improving efficiency and preventing overload.
  

Root Causes of Hydraulic Stall in the EX100-2
Several factors can contribute to hydraulic stall:

• Sticking or misadjusted pilot control valves: If pilot pressure is inconsistent, main valves may open too aggressively, demanding excessive flow.
  
• Faulty pressure compensator or relief valve: If the system fails to regulate pressure properly, it can overload the pump and engine.
  
• Weak or misfiring engine: A worn turbocharger, clogged fuel filters, or injector issues can reduce available torque.
  
• Hydraulic pump wear: Internal leakage or worn pistons can reduce efficiency, forcing the engine to work harder to maintain flow.
  
• Improper throttle control linkage: If the engine doesn’t ramp up RPM in response to hydraulic demand, stall is likely.
  

In one case from Queensland, an operator noticed his EX100-2 would stall when lifting heavy loads. After weeks of trial-and-error, the issue was traced to a sticky pilot valve that was sending full flow prematurely. Cleaning and re-lubricating the valve resolved the problem.
Diagnostic Steps and Field Solutions
To isolate the cause of hydraulic stall:

• Check engine RPM response when engaging hydraulics
  
• Inspect pilot pressure at control valve banks
  
• Test main pump output pressure and flow rate
  
• Verify relief valve settings against factory specs (~3000 psi typical)
  
• Examine throttle linkage and governor response
  

If pilot valves are suspected, remove and inspect for contamination or wear. Replacing O-rings and cleaning spool surfaces can restore proper modulation.
For engine-related causes, perform a compression test, inspect turbocharger boost, and replace fuel filters. A weak engine may not be able to sustain hydraulic load, especially under simultaneous function use.
Preventive Maintenance and Upgrade Suggestions
To prevent hydraulic stall:

• Replace hydraulic filters every 250 hours
  
• Use high-quality hydraulic oil with proper viscosity index
  
• Inspect pilot lines and valve spools annually
  
• Adjust throttle linkage to ensure RPM rise during hydraulic demand
  
• Install a hydraulic flow meter for real-time diagnostics
  

For machines operating in dusty or tropical environments, consider adding external cooling fans or upgrading to synthetic hydraulic fluid to reduce heat-related viscosity loss.
Hitachi’s Design Philosophy and Serviceability
Hitachi designed the EX100-2 with mechanical simplicity in mind. Unlike newer models with electronic load sensing and CAN bus diagnostics, the EX100-2 relies on analog systems and manual adjustment. This makes it ideal for remote operations and owner-operator maintenance.
Hitachi’s global support network ensures that parts for the EX100-2 remain available, including valve kits, pump rebuilds, and engine components. Many aftermarket suppliers in Southeast Asia and the Middle East offer compatible parts, making long-term ownership viable.
Conclusion
Hydraulic stall in the Hitachi EX100-2 is a solvable issue rooted in the interplay between pilot control, engine torque, and flow regulation. By methodically diagnosing each subsystem and understanding the machine’s mechanical logic, operators can restore full functionality without resorting to guesswork. In the world of mid-sized excavators, the EX100-2 continues to prove that simplicity and reliability are a powerful combination—especially when paired with field wisdom and a well-calibrated hydraulic system.