Hitachi EX100-3 Excavator Overheating and Hydraulic Heat Management

[MikePhua]

The EX100-3 and Hitachi’s Mid-Class Excavator Legacy
The Hitachi EX100-3 was introduced in the 1990s as part of Hitachi’s third-generation excavator series, designed to improve hydraulic efficiency, operator comfort, and service access. With an operating weight of approximately 10.5 metric tons and a dig depth exceeding 18 feet, the EX100-3 became a popular choice for contractors in Asia, Africa, and Latin America. Its reputation for mechanical simplicity and robust hydraulic performance made it a staple in fleets handling trenching, demolition, and utility work.
Hitachi Construction Machinery, founded in 1970, has sold millions of excavators globally. The EX100-3 was powered by the Isuzu 4BG1T turbocharged diesel engine, paired with a load-sensing hydraulic system and a stacked valve bank. Its cooling system includes a multi-core radiator, hydraulic oil cooler, and a belt-driven fan assembly.
Overheating Symptoms and Initial Observations
Operators may notice the following signs:

• Engine temperature climbs rapidly after 30–40 minutes of work
  
• Cooling system appears clean but heat persists
  
• Hydraulic functions remain responsive but generate excess heat
  
• Radiator and oil cooler show no visible clogging
  
• Ambient temperature above 35°C worsens the issue
  

In one fleet in northern Australia, an EX100-3 overheated consistently during summer trenching. Despite cleaning the radiator and replacing the thermostat, the issue persisted until deeper hydraulic diagnostics were performed.
Bypass Pipe Blockage and Thermostat Function
One overlooked cause of overheating is the bypass pipe between the water pump housing and the thermostat base. This pipe allows coolant to circulate through the engine block when the thermostat is closed, maintaining even temperature distribution during warm-up.
If this pipe is blocked or removed:

• Coolant stagnates in the block during startup
  
• Uneven heating causes localized hot spots
  
• Thermostat may open late or erratically
  
• Overall cooling efficiency drops
  

Recommended actions:

• Inspect bypass pipe for blockage or missing connections
  
• Reinstall with OEM hose and clamps
  
• Replace thermostat with factory-rated unit (typically 82°C opening)
  
• Pressure test the cooling system to verify flow paths
  

In one rebuild in Chile, a technician discovered the bypass pipe had been blanked off by a previous owner. Reconnecting it restored proper coolant circulation and reduced startup temperature spikes.
Hydraulic Heat Transfer and Cooling System Load
Hydraulic systems generate heat during operation, especially when relief valves are activated or pilot pressure is unstable. In the EX100-3, the hydraulic oil cooler is mounted in front of the radiator, meaning hot hydraulic air can recirculate through the radiator if airflow is disrupted.
Key contributors to hydraulic heat:

• Low pilot pressure causing main pump overcompensation
  
• Relief valve activation due to excessive flow
  
• Blocked hydraulic cooler fins reducing heat dissipation
  
• Fan shroud gaps allowing hot air recirculation
  

Diagnostic strategy:

• Use infrared thermometer to measure hydraulic cooler outlet temperature
  
• Check pilot pressure against factory spec (typically 400–600 psi)
  
• Inspect main pump solenoids for coil resistance and spring integrity
  
• Verify relief valve settings and spool movement
  
• Seal gaps around radiator and cooler to prevent air recirculation
  

In one fleet in Texas, a JD 490E with similar symptoms was found to have low pilot pressure. The main pump overcompensated, sending excess flow through relief valves and generating heat. Replacing the pilot control valve resolved the issue.
Water Pump and Radiator Efficiency
The water pump circulates coolant through the engine and radiator. If the pump impeller is worn or the belt is loose, flow drops and heat accumulates.
Inspection checklist:

• Remove pump and inspect impeller blades for erosion
  
• Check bearing play and seal condition
  
• Replace pump if impeller clearance exceeds spec
  
• Inspect radiator core for internal scale or external debris
  
• Use radiator flush solution to remove internal buildup
  
• Pressure test radiator cap (target: 13–16 psi)
  

In one rebuild in France, a technician replaced the water pump and flushed the radiator with citric acid solution. The machine operated 20°C cooler under load.
Preventive Measures and Long-Term Solutions
To prevent overheating:

• Clean radiator and hydraulic cooler monthly
  
• Replace coolant every 1,000 hours or annually
  
• Use 50/50 ethylene glycol mix with corrosion inhibitors
  
• Inspect fan belt tension and pulley alignment quarterly
  
• Monitor pilot pressure and relief valve settings annually
  
• Seal radiator compartment to prevent hot air recirculation
  

Recommended upgrades:

• Install hydraulic temperature gauge in cab
  
• Add auxiliary electric fan for hydraulic cooler
  
• Use synthetic hydraulic fluid with high thermal stability
  
• Retrofit bypass pipe with quick-disconnect for inspection
  

In one municipal fleet in Maine, adding a hydraulic temp gauge helped detect early signs of relief valve activation, reducing heat-related downtime by 40%.
Conclusion
Overheating in the Hitachi EX100-3 excavator often stems from a combination of cooling system inefficiencies and hydraulic heat transfer. Blocked bypass pipes, low pilot pressure, and poor airflow management can all contribute to rising temperatures. With thorough diagnostics, targeted repairs, and preventive upgrades, operators can restore thermal balance and extend machine life—even in extreme climates.