Comprehensive Guide to Diagnosing and Resolving Fuel Issues After Cleaning the Banjo Screen Filter on Hitachi EX120-2 Excavators

[MikePhua]

Introduction to the Hitachi EX120-2 Fuel System Issue
The Hitachi EX120-2 excavator is a reliable medium-sized hydraulic machine widely used in construction. Occasionally, fuel system troubles arise that can significantly affect machine performance. One such issue involves symptoms that occur after cleaning or replacing the banjo screen filter, a crucial fuel filter component. Operators have reported that after removing clogging deposits from the banjo filter, the machine may struggle to idle properly, sputter, or spit despite initially running better with a partially clogged filter. Understanding this paradox requires deeper knowledge of the fuel system, common causes, diagnostics, and practical repair strategies.
This article provides an in-depth discussion of the Hitachi EX120-2 fuel system focusing on banjo filter issues, supplemented with terminology explanations, maintenance tips, case stories, and proven solutions to ensure smooth engine operation.

Understanding the Banjo Screen Filter and Its Role

• Banjo Screen Filter:
  A small yet vital fuel filter element located typically near the fuel injection pump inlet, designed to trap debris and contamination from fuel before it reaches critical injection components.
  
• Fuel Flow Impact:
  When clogged, the banjo filter restricts fuel flow, causing power loss or rough engine running. However, a sudden removal or cleaning can introduce air or dislodge particles downstream, triggering idling and sputtering problems.
  

Common Symptoms After Cleaning the Banjo Screen Filter

• Engine starts and runs initially but struggles to idle smoothly.
  
• Spitting, sputtering, or uneven fuel delivery during low-speed or idle conditions.
  
• Machine running better with a partially clogged filter but worse when fully cleaned.
  
• Difficulty maintaining stable engine speed or irregular power delivery.
  

Typical Causes of Post-Cleaning Fuel Issues

• Air Entrapment in Fuel Lines:
  Removing or cleaning the banjo screen filter can allow air bubbles to enter the fuel system, causing erratic injection pump operation or injector firing irregularities.
  
• Loose or Improperly Seated Filter Installation:
  Incorrect reassembly can result in fuel leaks or insufficient filtering, disturbing pressure balance.
  
• Debris Migration:
  Cleaning the filter may dislodge particles that migrate downstream, clogging finer injector nozzles or sensitive valves.
  
• Fuel Supply Restrictions Elsewhere:
  Other clogged filters, such as main fuel tank filters or fine secondary filters, can compound the problem.
  
• Suction or Vacuum Leaks:
  Leaks in fuel lines or connections can cause inconsistent fuel delivery amplified after system disturbance.
  

Diagnostic and Troubleshooting Procedures

• Bleed Air From the Fuel System:
  Perform careful bleeding sequences starting at the fuel tank, through supply lines, filters, and injection pump to purge trapped air fully.
  
• Inspect Filter and Connections:
  Verify banjo filter is installed correctly with tight seals and no damaged O-rings or gaskets.
  
• Check Other Fuel Filters and Lines:
  Examine primary and secondary filters for contamination or blockages, replacing if necessary.
  
• Test Fuel Flow:
  Disconnect lines after the banjo filter to observe fuel flow rate and pressure consistency during priming.
  
• Monitor Engine Behavior:
  Use diagnostics or manual observation to check injector firing and idle stability once bleeding is complete.
  
• Inspect Return and Drain Lines:
  Ensure no clogging or return flow restrictions are causing backpressure.
  

Maintenance Recommendations

• Regularly replace banjo screen filters and all fuel filters as per Hitachi maintenance schedules to prevent clogging.
  
• Avoid cleaning filters on-site repeatedly; replacement is preferred to avoid damage and contamination.
  
• Use clean, quality fuel and maintain fuel tank cleanliness to minimize filter clogging.
  
• Schedule fuel system inspections focusing on all filters, fuel lines, and injector condition.
  
• Train operators and technicians on proper fuel system bleeding techniques after filter changes or maintenance.
  

Practical Solutions and Repair Tips

• After filter cleaning or replacement, immediately bleed air from the entire fuel system following manufacturer procedures to restore fuel delivery smoothness.
  
• Replace the banjo screen filter if symptoms persist; partial clogging may sometimes offer smoother flow than a damaged or improperly cleaned filter.
  
• Use fuel additives designed to clean injectors and fuel lines, which may reduce debris accumulation downstream.
  
• If multiple filters exist, replace or clean all sequentially, as clogging in any filter affects system performance.
  
• Monitor fuel line fittings and replace sealing elements to avoid air leaks creating vacuum instability.
  

Terminology Explained

• Banjo Filter: A small cylindrical filter element with a "banjo" fitting style designed for compact fuel filter installations.
  
• Injection Pump: High-pressure pump delivering precise fuel quantities to the engine injectors.
  
• Priming: The process of removing air from the fuel system and filling it with fuel to enable proper operation.
  
• Sputtering: Engine irregular combustion due to unstable fuel delivery or air contamination.
  
• Vacuum Leak: Unintended air ingress in fuel or intake systems disrupting pressure balance.
  
• Diesel Injector Clogging: Blockage of the fine nozzle openings in injectors reducing atomization and engine smoothness.
  

Real-World Case Stories and Insights

• An EX120-2 operator noted improved running after cleaning a severely clogged banjo filter but then experienced idling issues until fully bleeding the fuel system multiple times over a workday. Regular bleeding and careful filter installation solved the problem permanently.
  
• A technician shared that partial clogging of the banjo filter sometimes stabilizes low flow conditions; cleaning might temporarily increase air entry and downstream debris flushing, causing transient rough running.
  
• Another user had fuel line connectors replaced after spotting minor suction leaks, which dramatically improved engine smoothness post-filter maintenance.
  
• In certain cases, users found that replacing the banjo filter altogether was necessary as repeated cleaning degraded the element, allowing particulate passage and injector wear.
  

Additional Recommendations

• Always keep spare banjo filters and complete fuel filter service kits on-site for quick replacements.
  
• Document fuel filter changes and bleeding procedures in maintenance logs to track trends and predict potential fuel system issues early.
  
• Conduct regular injector testing and cleaning as part of fuel system preventive care.
  
• Coordinate with Hitachi authorized service centers for complex fuel system diagnostics or if problems persist after standard troubleshooting.
  
• Consider fuel polishing or filtration systems for fuel tanks to further protect sensitive fuel components and prolong filter life.
  

Summary
Fuel issues following cleaning of the banjo screen filter on the Hitachi EX120-2 excavator often arise from air entrapment, improper filter installation, debris migration, or related fuel system complications. Careful bleeding of the fuel system, thorough inspection of all filters and lines, and replacement of worn components are essential steps to restore smooth running and idle stability.
Partial clogging of the banjo filter may mask other fuel system weaknesses; thus, replacing the filter is preferable to repeated cleaning. Vigilant maintenance, proper fuel handling, and adherence to manufacturer procedures prevent recurrence and protect the injection system.
Real-world experiences confirm that understanding the banjo filter’s role, combined with disciplined service and troubleshooting approaches, ensures optimal engine performance and equipment reliability for Hitachi EX120-2 operators and maintenance teams.
This guide offers a practical resource to navigate this common fuel issue effectively, enhancing machine uptime and operator confidence.