Hitachi EX120 Engine Bogging and Hydraulic Load Response Issues

[MikePhua]

The EX120 and Its Hydraulic-Electronic Integration
The Hitachi EX120 excavator, introduced in the early 1990s, was part of Hitachi’s push to dominate the mid-size excavator market globally. With an operating weight of approximately 12 metric tons and powered by the reliable Isuzu 4BG1 engine, the EX120 became a staple in construction, demolition, and utility work. Hitachi Construction Machinery, founded in 1970, was among the first to integrate electronic engine control with hydraulic load sensing, allowing machines like the EX120 to deliver smooth, efficient performance under varying workloads.
The EX120’s hydraulic system is driven by twin variable-displacement piston pumps, coordinated through a load-sensing control valve and monitored by an engine control unit (ECU). When the machine bogs down—meaning the engine RPM drops sharply under hydraulic load—the issue often lies in the interaction between fuel delivery, hydraulic demand, and electronic feedback.
Terminology annotation:
- Bogging: A condition where the engine RPM drops excessively under load, leading to sluggish or stalled operation.
- Load-sensing hydraulics: A system that adjusts pump output based on the pressure and flow demands of the actuators.
- ECU (Engine Control Unit): An electronic module that manages fuel injection, throttle response, and engine protection parameters.
Symptoms and Operational Behavior
Operators experiencing bogging in the EX120 typically report:

• Engine RPM drops when boom or arm functions are engaged
  
• Excavator stalls or struggles during simultaneous movements
  
• No fault codes displayed on the monitor panel
  
• Fuel system components recently replaced with no improvement
  
• Hydraulic functions feel strong but overwhelm the engine
  

These symptoms suggest a mismatch between hydraulic demand and engine torque output, often caused by incorrect sensor feedback, fuel delivery issues, or pump overcompensation.
Fuel System Inspection and Air Intrusion
The Isuzu 4BG1 engine relies on a mechanical injection pump and a low-pressure lift pump to deliver fuel. Air leaks in the suction side of the fuel system can cause inconsistent fuel delivery, especially under load. Even minor leaks at banjo fittings or cracked hoses can introduce air bubbles that reduce injector pressure.
Checklist:

• Inspect fuel lines from tank to lift pump for cracks or loose clamps
  
• Replace fuel filters and bleed system thoroughly
  
• Check banjo bolt washers for deformation or corrosion
  
• Test lift pump output volume and pressure
  
• Monitor fuel return line for excessive aeration
  

Recommendations:

• Use clear tubing temporarily to observe fuel flow and bubbles
  
• Replace all rubber hoses with diesel-rated reinforced lines
  
• Prime system manually after filter changes to prevent dry starts
  

Terminology annotation:
- Banjo fitting: A hollow bolt and washer assembly used to connect fluid lines, prone to sealing issues under vibration.
- Lift pump: A low-pressure pump that supplies fuel to the injection pump.
Hydraulic Load Control and Pump Compensation
The EX120’s hydraulic pumps are designed to adjust displacement based on control signals. If the pump compensator is stuck or misadjusted, it may deliver full flow regardless of engine RPM, causing overload. Additionally, the main control valve may bypass excessive flow if spool tolerances are worn.
Solutions:

• Inspect pump compensator valve for sticking or contamination
  
• Test pilot pressure and main relief valve settings
  
• Clean or replace hydraulic filters and strainers
  
• Check control valve spool movement and centering springs
  
• Monitor pump swash plate angle during operation
  

Terminology annotation:
- Compensator valve: A hydraulic component that regulates pump output based on system pressure.
- Swash plate: An angled plate inside a piston pump that controls stroke length and flow rate.
Electronic Feedback and Throttle Control
The EX120 uses a throttle motor and position sensor to regulate engine RPM. If the throttle motor is slow, weak, or misaligned, the engine may not respond quickly enough to hydraulic demand. The ECU also relies on feedback from pressure sensors and potentiometers to adjust fuel delivery.
Inspection steps:

• Test throttle motor response and full-range movement
  
• Inspect throttle cable and linkage for binding
  
• Check potentiometer voltage range and calibration
  
• Clean ECU connectors and inspect for corrosion
  
• Verify battery voltage stability under load
  

Recommendations:

• Replace throttle motor if movement is delayed or inconsistent
  
• Use contact cleaner on ECU terminals and reseal with dielectric grease
  
• Calibrate throttle position sensor using service manual procedures
  

Terminology annotation:
- Potentiometer: A variable resistor used to measure position or input level.
- Dielectric grease: A non-conductive lubricant that protects electrical contacts from moisture and corrosion.
Field Anecdotes and Repair Experience
One operator in Queensland reported that his EX120 bogged down during trenching, despite recent fuel system service. After inspecting the hydraulic pump, he found the compensator valve stuck in full-flow position due to debris. Cleaning the valve and adjusting the relief pressure restored normal operation.
Another technician in Ontario traced bogging to a throttle motor that had worn gears. The motor would lag during boom operation, causing the engine to drop RPM. Replacing the motor and recalibrating the throttle sensor resolved the issue.
Preventative Maintenance and System Synchronization
To prevent bogging and maintain performance:

• Replace fuel filters every 250 hours
  
• Inspect throttle motor and linkage quarterly
  
• Flush hydraulic system annually and replace pilot filters
  
• Monitor engine RPM under load and compare to baseline
  
• Use fluid sampling to detect early contamination
  

For machines operating in high-dust or variable load environments, consider upgrading to synthetic hydraulic fluid and installing auxiliary cooling systems.
Conclusion
Bogging in the Hitachi EX120 is often the result of miscommunication between hydraulic demand and engine response. Whether caused by air in the fuel system, stuck hydraulic valves, or sluggish throttle control, the solution lies in methodical inspection and system synchronization. With proper diagnostics and preventative care, the EX120 continues to deliver reliable performance in demanding excavation tasks.