Komatsu D37P-5 Stalling Under Load and Fuel System Diagnostics

[MikePhua]

A 1999 Komatsu D37P-5 dozer with 3700 hours began stalling intermittently under load after 20 minutes of operation. The issue was traced to fuel delivery restrictions, highlighting the importance of inspecting hidden filters and venting systems in older machines.
Machine Overview and Fuel System Design
The Komatsu D37P-5 is a mid-size hydrostatic dozer designed for fine grading, land clearing, and slope work. Komatsu, founded in 1921 in Japan, has produced millions of machines globally and is known for its robust undercarriage and reliable diesel engines. The D37P-5 features a low-ground-pressure track system and a fuel system that includes a mechanical lift pump, hand primer, inline filters, and a return line.
Terminology Note

• Banjo Bolt: A hollow bolt used to connect fuel lines, often containing a hidden mesh screen.
  
• Inline Pump: A fuel injection system where the pump delivers fuel directly to each cylinder in sequence.
  
• Fuel Cap Vent: A small passage that allows air into the tank to replace consumed fuel.
  
• Water Separator: A filter that removes water from diesel fuel to prevent injector damage.
  
• Priming: The process of manually filling the fuel system to remove air and restore pressure.
  

Symptoms and Early Observations
The dozer would run normally for the first 20 minutes, then begin to bog down under load, as if starved for fuel. The operator had to stop, reduce engine speed, wait 20–30 seconds, and then resume work. Over time, the frequency of stalling increased. Initial repairs included:

• Replacing all fuel lines
  
• Flushing the fuel tank
  
• Installing a 24V electric fuel pump near the engine
  
• Adding a secondary filter with water separator near the tank
  

Despite these upgrades, the stalling persisted.
Critical Diagnostic Insights
Two overlooked components emerged as likely culprits:

• Banjo bolt screen blockage: Located below the hand primer at the supply pump, this fine mesh screen can clog with debris over time. It’s often undocumented in service manuals and missed during routine maintenance.
  
• Fuel cap vent obstruction: If the vent is blocked, a vacuum forms in the tank, restricting fuel flow. This is most noticeable when the tank is full and the machine is under heavy load.
  

In similar cases, operators found that removing the fuel cap temporarily restored performance, confirming vent blockage. One technician in South Carolina noted that the banjo bolt screen was completely clogged in a D39 dozer, causing identical symptoms.
Recommended Solutions

• Remove and inspect the banjo bolt at the supply pump. Clean or replace the internal screen.
  
• Test the fuel cap vent by loosening the cap during operation. If performance improves, replace or clean the vent.
  
• Ensure the electric fuel pump is rated for continuous duty and does not over-pressurize the system.
  
• Check for air leaks at hose clamps and fittings, especially near the water separator.
  
• Monitor fuel pressure at the injection pump inlet. It should remain stable under load.
  

Preventive Maintenance Tips

• Inspect hidden filters every 500 hours, even if not listed in manuals.
  
• Replace fuel caps annually to ensure venting integrity.
  
• Use biocide additives in diesel to prevent microbial growth in the tank.
  
• Keep a log of stalling incidents to identify patterns related to temperature, load, or fuel level.
  
• Train operators to recognize early signs of fuel starvation, such as hesitation or surging.
  

Conclusion
Stalling under load in older Komatsu dozers often stems from overlooked fuel system restrictions. The banjo bolt screen and fuel cap vent are small components with outsized impact. By addressing these hidden points of failure and maintaining clean, well-vented fuel delivery, operators can restore full performance and avoid unnecessary downtime. In legacy machines, experience and attention to detail often outperform parts replacement alone.