Troubleshooting the Zexel Injection Pump on the Hitachi EX400

[MikePhua]

The Legacy of the Hitachi EX400
The Hitachi EX400 hydraulic excavator is a product of Japan’s post-war industrial renaissance. Developed by Hitachi Construction Machinery Co., Ltd., a subsidiary of the Hitachi Group founded in 1970, the EX400 was introduced in the late 1980s as part of the EX series—a line that helped solidify Hitachi’s global reputation for reliability and innovation in heavy equipment. The EX400, with an operating weight of approximately 40 metric tons and a bucket capacity ranging from 1.6 to 2.5 cubic meters, was designed for large-scale earthmoving, mining, and infrastructure projects.
Sales of the EX series surged through the 1990s, with thousands of units exported to North America, Europe, and Southeast Asia. The EX400 became a staple on construction sites due to its robust build, efficient hydraulic system, and compatibility with a range of diesel engines, including the Nissan PE6 and Isuzu 6SD1. One of its most critical components is the Zexel inline fuel injection pump, a precision-engineered device responsible for metering and delivering fuel to the engine’s cylinders.
Understanding the Zexel Injection System
Zexel, originally a division of Diesel Kiki Co., Ltd., was rebranded in the 1990s after Bosch acquired a controlling interest. The Zexel inline injection pump used on the EX400 is a mechanical, camshaft-driven unit that relies on precise timing and pressure to atomize diesel fuel for combustion. Key components include:

• Plunger and Barrel Assembly: Controls fuel delivery volume based on camshaft rotation and rack position.
  
• Delivery Valve: Prevents backflow and maintains pressure in the injector lines.
  
• Governor: Regulates engine speed by adjusting fuel quantity.
  
• Hand Primer Pump: Allows manual fuel system priming during maintenance or after fuel system repairs.
  
• Supply Pump: Mounted on the side of the injection pump, it draws fuel from the tank and feeds it to the high-pressure section.
  

Terminology annotation:

• Bleeding the system: Removing air from fuel lines to ensure uninterrupted fuel flow.
  
• Banjo bolt: A hollow bolt used to connect fluid lines, often containing a fine mesh screen to trap debris.
  
• Cracking injector lines: Loosening fuel lines at the injector to allow trapped air to escape during bleeding.
  

Common Failure Symptoms and Diagnostic Path
A typical issue encountered with the EX400’s Zexel pump is failure to deliver fuel to the injectors after maintenance. This often occurs after replacing O-rings or resealing injectors. Despite fuel reaching the pump and filter, the injectors remain dry. This can be misleading, as the engine may have run fine before the repair.
Recommended diagnostic steps include:

• Confirm fuel delivery to the injection pump inlet and outlet.
  
• Check for clogged banjo bolt screens on the supply pump inlet.
  
• Use the hand primer pump vigorously before and during cranking.
  
• Loosen all six injector lines and observe for fuel discharge.
  
• Inspect injector orientation—incorrect installation can block flow.
  
• Verify that the rack and governor linkage are not seized or misaligned.
  

In one case, the breakthrough came when the operator used the hand primer simultaneously with cranking. This dual action allowed fuel to reach the injectors, and the engine began to fire. Once the hand primer was screwed down, cranking alone failed to restart the engine. Returning to the combined method restored ignition.
Field Anecdotes and Lessons Learned
In British Columbia, a contractor shared his experience with an EX400 that refused to start after injector resealing. Despite fuel at the pump, the injectors remained dry. The culprit was not debris in the screen but insufficient priming pressure. The lesson: mechanical pumps require active engagement, especially after air enters the system.
A similar story unfolded in South Carolina, where a master pump rebuilder recalled dozens of cases where clogged banjo screens caused premature starter failure. He emphasized that many operators overlook the hand primer’s role, treating it as optional rather than essential.
During the 2008 infrastructure boom, EX400s were deployed across North America for highway expansion. Mechanics often faced fuel system issues due to contaminated diesel, especially in colder climates where paraffin crystallization clogged screens. The Zexel pump’s resilience was praised, but only when maintained with diligence.
Preventative Measures and Maintenance Tips
To avoid fuel delivery issues, implement the following:

• Replace fuel filters every 250 hours or sooner in dusty environments.
  
• Clean banjo bolt screens quarterly.
  
• Use diesel additives to prevent microbial growth and wax formation.
  
• Prime the system manually after any fuel line disconnection.
  
• Train operators on proper bleeding techniques and injector installation.
  

For long-term reliability, consider retrofitting with an electric lift pump to assist the mechanical supply pump. This reduces starter wear and improves cold-start performance. Additionally, installing transparent fuel lines can help visually confirm flow during diagnostics.
Conclusion
The Zexel injection pump on the Hitachi EX400 is a marvel of mechanical engineering, but it demands respect and understanding. Its quirks—like the need for simultaneous priming and cranking—are not flaws but features rooted in its design philosophy. With proper technique and attention to detail, even a stubborn EX400 can be coaxed back to life, purring like the workhorse it was built to be.