Caterpillar D6H Scavenge Pump Failures and Field Solutions

[MikePhua]

The Legacy of the D6H Dozer
The Caterpillar D6H was introduced in the mid-1980s as part of CAT’s evolution of the D6 series, which dates back to the 1940s. Designed for mid-range dozing, grading, and site preparation, the D6H featured a torque converter drive, elevated sprocket undercarriage, and a robust hydraulic system. It quickly became a favorite among contractors and municipalities for its balance of power and maneuverability. By the late 1990s, thousands of D6H units had been deployed globally, many still operating today thanks to their rugged design and rebuild-friendly architecture.
One of the less-discussed but critical components of the D6H’s engine system is the scavenge pump—a part of the lubrication circuit responsible for returning oil from the crankcase and turbocharger areas back to the sump. When this pump fails, it can trigger a cascade of engine issues, from oil starvation to turbo damage.
Terminology Notes

• Scavenge pump: A pump that collects oil from low-pressure zones (like the turbocharger or crankcase) and returns it to the oil pan or reservoir.
  
• Sump: The bottom part of the oil system where lubricating oil collects before being recirculated.
  
• Oil starvation: A condition where engine components do not receive adequate lubrication, leading to accelerated wear or failure.
  
• Elevated sprocket: A design feature that lifts the final drive above the track frame, reducing shock loads and improving serviceability.
  

Symptoms of Scavenge Pump Failure
Operators of aging D6H units have reported several telltale signs of scavenge pump malfunction:

• Excessive oil consumption without visible leaks
  
• Blue smoke from the exhaust, especially under load
  
• Oil pooling in the turbocharger housing
  
• Low oil pressure readings despite full sump levels
  
• Engine derating or shutdown due to lubrication alarms
  

These symptoms often appear gradually, making early detection difficult. In some cases, the turbo seals are blamed prematurely, when the root cause is actually oil backing up due to poor scavenge flow.
Root Causes and Contributing Factors
Scavenge pump failure in the D6H is typically mechanical, though several contributing factors accelerate wear:

• Debris ingestion from poor filtration
  
• Worn pump gears or housing due to high operating hours
  
• Seal degradation from thermal cycling
  
• Misalignment during rebuilds or improper gasket installation
  
• Use of incorrect oil viscosity, especially in cold climates
  

The D6H’s pump is gear-driven and mounted externally, making it accessible but also vulnerable to contamination during service. If the pump’s internal clearances exceed tolerance, oil flow drops and scavenging becomes ineffective.
A Story from the Yukon
In 2011, a road-building crew in the Yukon Territory noticed their D6H was burning oil at an alarming rate. After replacing the turbo twice, they discovered the scavenge pump had worn gears and a cracked housing. The cold climate had thickened the oil, and the pump couldn’t keep up. After installing a rebuilt pump and switching to a multi-grade synthetic oil, the machine ran another 3,000 hours without incident. This case illustrates the importance of matching oil viscosity to ambient conditions and inspecting beyond the obvious.
Inspection and Diagnosis Tips
To confirm scavenge pump issues, technicians should:

• Remove the pump and inspect gear teeth for wear or pitting
  
• Check housing for cracks or scoring
  
• Measure gear backlash and shaft play against factory specs
  
• Pressure test the oil return line for restrictions
  
• Inspect turbo drain ports for oil pooling
  

If the pump is marginal, it may still function under light loads but fail during high RPM or incline operation. A borescope inspection of the turbo drain cavity can reveal oil accumulation, a strong indicator of poor scavenging.
Recommended Solutions and Upgrades
When replacing or rebuilding the scavenge pump, consider the following:

• Use OEM or high-quality aftermarket parts with hardened gear sets
  
• Replace all associated seals and gaskets during installation
  
• Flush the oil system to remove debris and sludge
  
• Upgrade to synthetic oil with better cold-flow properties
  
• Install an inline oil temperature sensor to monitor viscosity behavior
  

For fleets operating in extreme conditions, adding a pre-lube system can reduce startup wear and improve oil circulation before ignition.
Preventive Maintenance Strategies
To avoid future failures, implement a proactive maintenance schedule:

• Change oil and filters every 250 hours or as per duty cycle
  
• Inspect scavenge pump during major service intervals (1,000–2,000 hours)
  
• Monitor oil pressure and consumption trends monthly
  
• Train operators to recognize smoke patterns and pressure anomalies
  
• Keep detailed service logs including oil type and ambient conditions
  

These steps can extend engine life and reduce unplanned downtime, especially in remote or high-load applications.
Final Thoughts
The Caterpillar D6H remains a cornerstone of mid-size dozing operations, but its longevity depends on attention to detail—especially in the lubrication system. Scavenge pump failure is a silent killer, often misdiagnosed or overlooked. By understanding its role, recognizing early symptoms, and applying thoughtful solutions, operators and technicians can keep these machines running strong for decades. The D6H’s legacy is built not just on horsepower, but on the wisdom of those who maintain it.