Komatsu D31P-18 Oil Pressure Drop After Warm-Up and Internal Flow Disruption

[MikePhua]

The D31P-18 and Its Mechanical Profile
The Komatsu D31P-18 is a mid-size crawler dozer designed for grading, clearing, and light earthmoving. Powered by the Komatsu 6D95L diesel engine, it delivers around 75 horsepower and features a low ground pressure undercarriage ideal for soft terrain. Introduced in the late 1980s, the D31P-18 became popular in forestry, agriculture, and municipal work due to its compact footprint and mechanical reliability.
Komatsu, founded in 1921, has produced thousands of D31-series machines globally. The 6D95L engine is known for its mechanical fuel injection, robust cast block, and straightforward serviceability. However, like many aging machines, it can develop oil pressure inconsistencies that challenge even experienced technicians.
Terminology Note

• Oil Pressure Regulating Valve: A spring-loaded valve in the oil pump that maintains system pressure by bypassing excess oil.
  
• Cam Bearing: A sleeve or shell that supports the camshaft and allows oil flow through galleries.
  
• Oil Gallery: Internal passageways that distribute pressurized oil to bearings and moving components.
  
• Mechanical Gauge: A direct-reading oil pressure gauge using a capillary tube and Bourdon mechanism.
  

Symptoms of Pressure Loss After Warm-Up
Operators have reported the following behavior:

• Cold start oil pressure reaches 100 psi
  
• After 15 minutes of operation, pressure drops to 10 psi at full throttle and 3–5 psi at idle
  
• Restarting the engine while warm shows continued low pressure
  
• After cooling, pressure returns to high levels
  
• Engine rebuild included cam bearing replacement and oil pump inspection
  

This pattern suggests a thermal expansion issue, internal bypassing, or a misfit component affecting oil flow once the engine reaches operating temperature.
Root Causes and Diagnostic Pathways
Several factors can contribute to this pressure drop:

• Incorrect Cam Bearing Installation: If the bearing blocks or misaligns oil galleries, flow to the top end and pressure sensors may be disrupted once thermal expansion occurs.
  
• Weak or Stuck Relief Valve Spring: The oil pump’s pressure regulating valve may open prematurely when hot, dumping oil back into the sump.
  
• Excessive Bearing Clearance: Rebuilt engines with improperly sized bearings may allow oil to bleed off under heat, reducing pressure.
  
• Faulty Gauge or Sender: Mechanical gauges are more reliable, but diaphragm-type senders can fail under heat, giving false low readings.
  
• Oil Viscosity Breakdown: Using low-grade or incorrect viscosity oil can result in thinning under heat, reducing pressure.
  

In one case, a technician in Louisiana rebuilt a D31P-18 engine and installed a new cam bearing. The engine showed high pressure cold but dropped dangerously low when warm. After pulling the engine again, they discovered the bearing had blocked a critical oil gallery. Replacing it with a properly machined unit restored pressure stability.
Recommended Diagnostic Sequence

• Install a mechanical gauge with a tee fitting to verify readings
  
• Inspect oil pressure relief valve spring tension and valve seating
  
• Measure bearing clearances with plastigage during rebuild
  
• Confirm oil gallery alignment through cam bearing ports
  
• Use high-quality 15W-40 diesel-rated oil with proper additives
  

If the engine is out, bench test the oil pump with a pressure gauge and simulate warm conditions using heated oil. Monitor pressure over time to detect valve creep or bypassing.
Preventive Maintenance and Oil System Integrity
To maintain consistent oil pressure:

• Change oil and filters every 250 hours or quarterly
  
• Use OEM or precision-machined bearings during rebuilds
  
• Flush oil galleries with solvent during engine overhaul
  
• Replace pressure relief valve springs every 2,000 hours
  
• Avoid mixing oil brands or viscosities without compatibility checks
  

Operators should monitor pressure during warm-up and avoid high RPM until the system stabilizes. Installing a temperature-compensated gauge can help track pressure trends more accurately.
Anecdote from the Field
In 2023, a forestry crew in Florida restored a D31P-18 for trail maintenance. After rebuilding the engine, they noticed oil pressure dropped sharply after warm-up. Suspecting a faulty sender, they installed a mechanical gauge and confirmed the drop was real. Upon inspection, they found the oil pump relief valve spring had weakened over time. Replacing it with a new OEM spring restored pressure to 45 psi hot at full throttle, and the dozer returned to service without further issue.
Manufacturer History and Engine Evolution
Komatsu’s D31 series has evolved through multiple generations, with the D31P-18 representing a balance between mechanical simplicity and hydraulic refinement. The 6D95L engine was used across dozers, loaders, and excavators, making parts widely available. Later models introduced electronic monitoring and improved oil routing, but the mechanical systems of the D31P-18 remain favored by independent operators for their serviceability.
Sales of the D31 series peaked in the 1990s, with thousands still operating in Asia, North America, and South America. Their longevity depends heavily on oil system integrity and disciplined maintenance.
Recommendations for Technicians and Fleet Managers

• Always verify oil pressure with a mechanical gauge before teardown
  
• Document bearing sizes and clearances during rebuilds
  
• Replace cam bearings only with verified OEM or precision aftermarket units
  
• Inspect oil pump relief valve for wear and spring fatigue
  
• Use oil analysis to monitor viscosity and contamination over time
  

Conclusion
Low oil pressure after warm-up in the Komatsu D31P-18 is often a symptom of internal misalignment, thermal expansion, or component fatigue. By understanding the oil flow path, verifying gauge accuracy, and inspecting critical components like cam bearings and relief valves, technicians can restore pressure and prevent engine damage. With proper care, the D31P-18 remains a reliable and capable machine in demanding environments.