10-18-2025, 02:27 PM
The Komatsu 650 and Its Fuel System Design
The Komatsu D65 series, including the 650 variant, has long been a staple in mid-size dozer fleets across forestry, mining, and civil construction. With an operating weight around 20 tons and a diesel engine producing over 200 hp, the 650 was engineered for reliability in remote and rugged terrain. Komatsu, founded in Japan in 1921, has sold hundreds of thousands of dozers globally, and the D65 lineage remains one of its most enduring.
The fuel system on the 650 includes a primary and secondary filter, a lift pump, a fuel strainer, and a pickup assembly inside the tank. These components work together to deliver clean diesel to the injection pump. When any part of this system fails or becomes obstructed, fuel starvation can occur—often misdiagnosed as engine or pump failure.
Terminology Notes
In one case, a Komatsu 650 received a contaminated batch of diesel containing emulsified water. After draining the tank and replacing the fuel, the machine ran briefly before plugging both filters. Over 10–15 filters were changed before the fuel ran clean, but the engine continued to die intermittently—especially when the tank was below half full or the machine was descending steep grades.
These symptoms suggest:
One technician in Alaska noted that his 650 ran fine uphill but died every time it descended steep terrain. After pulling the pickup tube, he found a hairline crack near the top—allowing air to enter when fuel sloshed away from the tube. Another operator in Wisconsin discovered that his fuel strainer was packed with grit from a rusting tank interior, despite clean filters.
A mechanic in Oregon emphasized that emulsified water can leave behind sticky residue that clogs screens and corrodes fittings. He recommended flushing the entire system with a diesel-compatible cleaner after contamination.
Preventive Measures and Recommendations
Running filters dry on a Komatsu 650 is often the result of upstream fuel system issues—not the filters themselves. Cracked pickup tubes, plugged strainers, and weak lift pumps can all mimic fuel starvation. By inspecting each component methodically and understanding how terrain and fuel level affect suction, operators can restore reliable performance. The 650’s rugged design deserves a clean and sealed fuel system to match its legacy in the field.
The Komatsu D65 series, including the 650 variant, has long been a staple in mid-size dozer fleets across forestry, mining, and civil construction. With an operating weight around 20 tons and a diesel engine producing over 200 hp, the 650 was engineered for reliability in remote and rugged terrain. Komatsu, founded in Japan in 1921, has sold hundreds of thousands of dozers globally, and the D65 lineage remains one of its most enduring.
The fuel system on the 650 includes a primary and secondary filter, a lift pump, a fuel strainer, and a pickup assembly inside the tank. These components work together to deliver clean diesel to the injection pump. When any part of this system fails or becomes obstructed, fuel starvation can occur—often misdiagnosed as engine or pump failure.
Terminology Notes
- Lift Pump: A low-pressure pump that draws fuel from the tank and feeds it to the injection system.
- Fuel Strainer: A coarse mesh basket that traps debris before fuel reaches the filters.
- Pickup Tube: A pipe inside the tank that draws fuel from the bottom and delivers it to the lift pump.
- Emulsified Water: A mixture of water and diesel that appears cloudy and can clog filters rapidly.
In one case, a Komatsu 650 received a contaminated batch of diesel containing emulsified water. After draining the tank and replacing the fuel, the machine ran briefly before plugging both filters. Over 10–15 filters were changed before the fuel ran clean, but the engine continued to die intermittently—especially when the tank was below half full or the machine was descending steep grades.
These symptoms suggest:
- Air intrusion into the fuel system
- Cracked or loose pickup tube
- Plugged fuel strainer
- Weak or damaged lift pump
- Inspect Pickup Assembly: Remove the fuel gauge and pickup unit from the tank. Look for cracks, loose fittings, or collapsed hoses.
- Check Fuel Strainer: Trace the fuel line from the tank to the strainer. Clean or replace the basket inside the cast housing.
- Test Lift Pump Output: Use a pressure gauge to verify consistent flow. A weak pump may fail under load or low fuel conditions.
- Prime Filters Manually: If the engine starts after priming but dies later, the system is losing prime—likely due to air leaks or suction-side restrictions.
- Monitor Fuel Level Behavior: If the machine dies below half a tank, the pickup tube may be cracked above the fuel line, drawing air instead of diesel.
One technician in Alaska noted that his 650 ran fine uphill but died every time it descended steep terrain. After pulling the pickup tube, he found a hairline crack near the top—allowing air to enter when fuel sloshed away from the tube. Another operator in Wisconsin discovered that his fuel strainer was packed with grit from a rusting tank interior, despite clean filters.
A mechanic in Oregon emphasized that emulsified water can leave behind sticky residue that clogs screens and corrodes fittings. He recommended flushing the entire system with a diesel-compatible cleaner after contamination.
Preventive Measures and Recommendations
- Inspect Fuel After Delivery: Use a clear jar to check for cloudiness or water separation.
- Replace Pickup Tubes Every 5 Years: Plastic and rubber degrade over time, especially in cold climates.
- Clean Fuel Strainer Annually: Include in scheduled maintenance to prevent hidden blockages.
- Use Biocide Additives: Prevent microbial growth in stored diesel.
- Install Inline Sight Glass: Allows visual confirmation of air bubbles or flow issues.
Running filters dry on a Komatsu 650 is often the result of upstream fuel system issues—not the filters themselves. Cracked pickup tubes, plugged strainers, and weak lift pumps can all mimic fuel starvation. By inspecting each component methodically and understanding how terrain and fuel level affect suction, operators can restore reliable performance. The 650’s rugged design deserves a clean and sealed fuel system to match its legacy in the field.
