10-23-2025, 01:02 AM
A Komatsu D65PX-15 dozer showing rough idle and worsening black exhaust smoke at operating temperature may be suffering from fuel-air imbalance, injector calibration issues, or deeper combustion inefficiencies. Despite recent rebuilds and component replacements, persistent symptoms suggest the need for advanced diagnostics beyond routine service.
Komatsu D65PX-15 Overview
The Komatsu D65PX-15 is a mid-size crawler dozer designed for grading, land clearing, and heavy earthmoving. Introduced in the early 2000s, it features a Komatsu SAA6D114E engine producing approximately 205 horsepower. The PX variant includes a wider track frame for reduced ground pressure, making it ideal for soft terrain. Komatsu, founded in 1921, is one of the world’s leading construction equipment manufacturers, with millions of machines sold globally.
This model is known for its hydrostatic steering, electronically controlled fuel delivery, and robust undercarriage. With proper maintenance, D65PX-15 units often exceed 15,000 hours of service life.
Terminology Notes
The machine idles smoothly when cold but develops a rough, cam-like lope at operating temperature. Below idle, it sounds like a cylinder is misfiring. When revved, it emits thick black smoke that partially clears at constant RPM but remains visible. The smoke has worsened over time.
The engine was rebuilt 1,500 hours ago, and new injectors were installed 1,000 hours ago. Fuel lines, throttle potentiometer, filters, and air intake have all been replaced. A top-end tune was performed, but no improvement followed. The machine runs daily for 8 hours and is serviced every 250 hours.
Field Observations and Diagnostic Clues
In Queensland, a contractor faced similar symptoms on a D65PX-12. After replacing injectors twice, the issue persisted until a Komatsu technician discovered a cracked exhaust manifold causing backpressure and poor scavenging. Once replaced, idle stabilized and smoke cleared.
In Alberta, a forestry operator found that a rebuilt engine had mismatched injector nozzles—two were high-flow units intended for a different Komatsu model. After installing the correct set, fuel economy improved and black smoke disappeared.
Final Thoughts
Persistent rough idle and black smoke on a Komatsu D65PX-15, despite recent rebuilds, often point to subtle calibration or airflow issues. These machines are robust but sensitive to injector timing, turbo efficiency, and sensor alignment. A methodical diagnostic approach—combining mechanical inspection with electronic analysis—can restore full performance and extend engine life.
Komatsu D65PX-15 Overview
The Komatsu D65PX-15 is a mid-size crawler dozer designed for grading, land clearing, and heavy earthmoving. Introduced in the early 2000s, it features a Komatsu SAA6D114E engine producing approximately 205 horsepower. The PX variant includes a wider track frame for reduced ground pressure, making it ideal for soft terrain. Komatsu, founded in 1921, is one of the world’s leading construction equipment manufacturers, with millions of machines sold globally.
This model is known for its hydrostatic steering, electronically controlled fuel delivery, and robust undercarriage. With proper maintenance, D65PX-15 units often exceed 15,000 hours of service life.
Terminology Notes
- Rough Idle: Irregular engine rhythm at low RPM, often caused by uneven cylinder firing or fuel delivery issues.
- Black Smoke: Indicates incomplete combustion, typically due to excess fuel, poor air intake, or injector malfunction.
- Throttle Potentiometer: An electronic sensor that translates pedal position into fuel delivery signals.
- Top-End Tune: Adjustment of valve lash, injector timing, and turbo inspection to optimize combustion.
The machine idles smoothly when cold but develops a rough, cam-like lope at operating temperature. Below idle, it sounds like a cylinder is misfiring. When revved, it emits thick black smoke that partially clears at constant RPM but remains visible. The smoke has worsened over time.
The engine was rebuilt 1,500 hours ago, and new injectors were installed 1,000 hours ago. Fuel lines, throttle potentiometer, filters, and air intake have all been replaced. A top-end tune was performed, but no improvement followed. The machine runs daily for 8 hours and is serviced every 250 hours.
Field Observations and Diagnostic Clues
- Cold vs Hot Behavior: Smooth cold idle suggests mechanical integrity. Rough hot idle points to thermal expansion affecting injector spray pattern or valve seating.
- Injector Calibration: New injectors may not be properly matched or timed. Komatsu engines require precise calibration to avoid over-fueling.
- Turbocharger Inspection: A sticky wastegate or worn bearings can reduce air intake, leading to rich fuel mixtures and black smoke.
- ECU Mapping Drift: Older electronic control units may develop calibration drift, especially if throttle sensors were replaced without full reprogramming.
- Compression Imbalance: A cylinder with lower compression may misfire under load but appear normal when cold.
- Perform Cylinder Cut-Out Test: Use diagnostic software to disable each injector briefly and observe RPM drop. This identifies weak cylinders.
- Check Turbo Boost Pressure: Use a gauge to verify boost under load. Low pressure confirms air starvation.
- Inspect Valve Lash Hot: Recheck valve clearances at operating temperature to catch thermal distortion.
- Scan ECU for Fault Codes: Even if no warning lights are present, hidden codes may reveal sensor mismatches.
- Test Fuel Return Flow: Excessive return flow may indicate injector leakage or pump bypass.
In Queensland, a contractor faced similar symptoms on a D65PX-12. After replacing injectors twice, the issue persisted until a Komatsu technician discovered a cracked exhaust manifold causing backpressure and poor scavenging. Once replaced, idle stabilized and smoke cleared.
In Alberta, a forestry operator found that a rebuilt engine had mismatched injector nozzles—two were high-flow units intended for a different Komatsu model. After installing the correct set, fuel economy improved and black smoke disappeared.
Final Thoughts
Persistent rough idle and black smoke on a Komatsu D65PX-15, despite recent rebuilds, often point to subtle calibration or airflow issues. These machines are robust but sensitive to injector timing, turbo efficiency, and sensor alignment. A methodical diagnostic approach—combining mechanical inspection with electronic analysis—can restore full performance and extend engine life.
