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The Hitachi EX120-2 and Its Role in Compact Excavation
The Hitachi EX120-2 hydraulic excavator was introduced in the early 1990s as part of Hitachi’s second-generation lineup of compact and mid-size machines. With an operating weight of around 12 metric tons and powered by the reliable Isuzu 4BG1 engine, the EX120-2 became a popular choice for contractors working in urban environments, utility trenching, and light demolition. Hitachi Construction Machinery, founded in 1970 as a division of Hitachi Ltd., quickly gained global recognition for its smooth hydraulic systems and durable undercarriage designs.
By the late 1990s, the EX120 series had sold tens of thousands of units worldwide, with the -2 variant introducing improved hydraulic control valves and a more refined swing motor assembly. Despite its reputation for reliability, some aging units now exhibit performance quirks—especially under simultaneous hydraulic load.
Stalling During Swing Operation Indicates Hydraulic Overload or Fuel Starvation
One recurring issue with older EX120-2 machines is engine stalling when the operator swings the upper structure in either direction. The machine runs fine during travel, digging, and lifting, but stalls momentarily when initiating a swing. After the stall, the swing resumes, suggesting a brief interruption in engine power or hydraulic balance.
Common symptoms:
Potential Causes Include Fuel Delivery Restriction or Weak Hydraulic Pump
When an excavator stalls under hydraulic load, two systems must be considered: fuel delivery and hydraulic output. If the fuel system cannot supply adequate diesel under load, the engine will bog. Similarly, if the hydraulic pump demands excessive torque from the engine, it may cause a stall—especially if the pump is worn or the relief valves are misadjusted.
Diagnostic checklist:
Swing Motor Load May Exceed Engine Torque Under Certain Conditions
The swing motor on the EX120-2 is powered by hydraulic flow diverted from the main pump. If the swing motor’s internal resistance is high—due to worn bearings, tight seals, or contaminated fluid—it may demand more torque than the engine can deliver, especially at idle. This can cause the engine to stall momentarily before recovering.
Swing system considerations:
Electronic Control and Throttle Response May Contribute
Although the EX120-2 is largely mechanical, some units feature electronic throttle control. If the throttle actuator or sensor is slow to respond, the engine may not ramp up RPM quickly enough to meet hydraulic demand. This delay can mimic a stall, especially when transitioning from idle to full swing.
Electronic checks:
Preventive Maintenance and Long-Term Solutions
To prevent stalling during swing, regular maintenance of both fuel and hydraulic systems is essential. Older machines like the EX120-2 benefit from proactive filter changes, fluid sampling, and relief valve calibration. Swing motors should be inspected annually, especially if the machine operates in dusty or humid environments.
Maintenance tips:
Conclusion
Stalling during swing on a Hitachi EX120-2 is often caused by a combination of fuel delivery restriction, hydraulic overload, or delayed throttle response. By inspecting filters, relief valves, and swing motor components, operators can restore smooth operation and avoid unnecessary downtime. In compact excavation, even a brief stall can disrupt productivity—making proactive diagnostics a smart investment.
The Hitachi EX120-2 hydraulic excavator was introduced in the early 1990s as part of Hitachi’s second-generation lineup of compact and mid-size machines. With an operating weight of around 12 metric tons and powered by the reliable Isuzu 4BG1 engine, the EX120-2 became a popular choice for contractors working in urban environments, utility trenching, and light demolition. Hitachi Construction Machinery, founded in 1970 as a division of Hitachi Ltd., quickly gained global recognition for its smooth hydraulic systems and durable undercarriage designs.
By the late 1990s, the EX120 series had sold tens of thousands of units worldwide, with the -2 variant introducing improved hydraulic control valves and a more refined swing motor assembly. Despite its reputation for reliability, some aging units now exhibit performance quirks—especially under simultaneous hydraulic load.
Stalling During Swing Operation Indicates Hydraulic Overload or Fuel Starvation
One recurring issue with older EX120-2 machines is engine stalling when the operator swings the upper structure in either direction. The machine runs fine during travel, digging, and lifting, but stalls momentarily when initiating a swing. After the stall, the swing resumes, suggesting a brief interruption in engine power or hydraulic balance.
Common symptoms:
- Engine bogs or stalls when swinging left or right
- No fault codes or warning lights
- Swing resumes after brief hesitation
- No issues during travel or boom operation
- Swing Motor: A hydraulic motor that rotates the upper structure of the excavator.
- Hydraulic Overload: A condition where multiple hydraulic functions demand more flow or pressure than the pump can deliver.
Potential Causes Include Fuel Delivery Restriction or Weak Hydraulic Pump
When an excavator stalls under hydraulic load, two systems must be considered: fuel delivery and hydraulic output. If the fuel system cannot supply adequate diesel under load, the engine will bog. Similarly, if the hydraulic pump demands excessive torque from the engine, it may cause a stall—especially if the pump is worn or the relief valves are misadjusted.
Diagnostic checklist:
- Inspect fuel filters for clogging or water contamination
- Test lift pump pressure and flow rate
- Check swing motor relief valve settings
- Monitor engine RPM during swing initiation
- Inspect hydraulic pump for internal leakage or wear
- Lift Pump: A low-pressure pump that delivers fuel from the tank to the injection pump.
- Relief Valve: A hydraulic valve that limits system pressure to prevent overload or damage.
Swing Motor Load May Exceed Engine Torque Under Certain Conditions
The swing motor on the EX120-2 is powered by hydraulic flow diverted from the main pump. If the swing motor’s internal resistance is high—due to worn bearings, tight seals, or contaminated fluid—it may demand more torque than the engine can deliver, especially at idle. This can cause the engine to stall momentarily before recovering.
Swing system considerations:
- Check swing motor case drain for excessive flow
- Inspect swing bearing for binding or wear
- Test swing brake release pressure
- Monitor hydraulic fluid temperature during operation
- Case Drain: A low-pressure return line that carries leakage fluid from hydraulic motors back to the tank.
- Swing Brake: A hydraulic or mechanical brake that holds the upper structure in place when not swinging.
Electronic Control and Throttle Response May Contribute
Although the EX120-2 is largely mechanical, some units feature electronic throttle control. If the throttle actuator or sensor is slow to respond, the engine may not ramp up RPM quickly enough to meet hydraulic demand. This delay can mimic a stall, especially when transitioning from idle to full swing.
Electronic checks:
- Inspect throttle actuator linkage and sensor
- Test voltage response during throttle changes
- Clean electrical connectors and grounds
- Verify idle speed setting matches factory spec
- Throttle Actuator: A motor or solenoid that adjusts engine RPM based on operator input.
- Idle Speed Setting: The baseline RPM at which the engine runs without load.
Preventive Maintenance and Long-Term Solutions
To prevent stalling during swing, regular maintenance of both fuel and hydraulic systems is essential. Older machines like the EX120-2 benefit from proactive filter changes, fluid sampling, and relief valve calibration. Swing motors should be inspected annually, especially if the machine operates in dusty or humid environments.
Maintenance tips:
- Replace fuel filters every 250 hours
- Sample hydraulic fluid every 500 hours
- Inspect swing motor and bearing annually
- Adjust relief valves to factory specs
- Monitor engine RPM under load monthly
- Fluid Sampling: The process of analyzing oil for contamination, wear metals, and viscosity.
- Relief Valve Calibration: Adjusting valve settings to ensure proper pressure limits.
Conclusion
Stalling during swing on a Hitachi EX120-2 is often caused by a combination of fuel delivery restriction, hydraulic overload, or delayed throttle response. By inspecting filters, relief valves, and swing motor components, operators can restore smooth operation and avoid unnecessary downtime. In compact excavation, even a brief stall can disrupt productivity—making proactive diagnostics a smart investment.
