Fuel Contamination in Engine Oil on a Case 1150G Dozer

[MikePhua]

The Case 1150G and Its Industrial Legacy
The Case 1150G crawler dozer was introduced in the late 1990s as part of Case Construction’s long-standing commitment to mid-size earthmoving equipment. Case, founded in 1842 and now part of CNH Industrial, has produced generations of dozers known for their mechanical simplicity and field reliability. The 1150G was powered by the Cummins 6T-590 turbocharged diesel engine, a robust inline-six designed for high torque output and long service intervals.
This model was widely adopted across North America and Australia, particularly in forestry, road building, and land clearing. Its hydrostatic transmission and responsive hydraulics made it a favorite among operators who valued maneuverability and pushing power. Though exact production numbers are proprietary, the 1150G was one of Case’s best-selling dozers in its class during its production run.
Symptoms of Fuel in Engine Oil
Fuel contamination in the crankcase is a serious issue that can lead to catastrophic engine failure if not addressed promptly. In the 1150G, the problem typically presents as:

• Rapid increase in oil level, sometimes by several gallons within hours
  
• Thinned engine oil with a strong diesel odor
  
• Reduced oil pressure and increased wear on bearings
  
• Excessive exhaust smoke and poor combustion
  
• Difficulty starting or erratic idle behavior
  

Operators often notice the dipstick reading unusually high or oil appearing unusually thin and dark. In severe cases, the engine may begin to knock due to insufficient lubrication.
Common Sources of Fuel Intrusion
On the Cummins 6T-590 engine, fuel can enter the crankcase through several pathways:

• Injection Pump Front Seal Failure: The most frequent culprit. A worn or damaged seal allows diesel to leak into the timing gear housing and eventually into the oil pan.
  
• Lift Pump Diaphragm Leak: A ruptured diaphragm in the mechanical lift pump can allow fuel to bypass directly into the engine block.
  
• Injector Body Cracks or Seal Failures: Though less common, a cracked injector or failed copper washer can allow fuel to seep past the cylinder head into the oil gallery.
  
• Cold Start Enrichment Malfunction: If the excess fuel device sticks open, it can flood the cylinders and overwhelm the rings, pushing fuel into the crankcase.
  

Terminology Clarification

• Lift Pump: A low-pressure pump that supplies fuel from the tank to the injection pump. Often mechanical and mounted on the engine block.
  
• Injection Pump: A high-pressure pump that meters and delivers fuel to the injectors. On the 6T-590, it’s typically a Bosch or CAV rotary type.
  
• Crankcase: The lower part of the engine housing the crankshaft and oil sump.
  
• Timing Case: The front section of the engine where timing gears and the injection pump drive are located.
  

Diagnostic Approach and Field Techniques
To confirm fuel contamination and identify the source:

• Drain a small sample of engine oil and perform a smell and viscosity test. Diesel-contaminated oil will have a distinct odor and feel thinner than normal.
  
• Remove the lift pump and inspect for wetness or fuel leakage on the diaphragm side.
  
• Check the injection pump front seal by removing the timing cover and inspecting for fuel pooling.
  
• Monitor oil level over time. A rapid increase confirms active leakage.
  
• Use UV dye in the fuel system and inspect the crankcase with a blacklight for confirmation.
  

One technician in Ohio reported a case where the crankcase filled with two extra gallons of fluid after just 90 minutes of operation. The engine ran smoothly, but the oil level was dangerously high. Upon inspection, the injection pump seal had failed, allowing fuel to bypass into the timing case.
Repair Recommendations and Preventive Measures
Once the source is identified, repairs should be immediate:

• Replace the injection pump front seal. This requires removing the pump and resealing with OEM-grade components.
  
• If the lift pump is the issue, replace it entirely. Aftermarket units are available and cost-effective.
  
• Flush the crankcase thoroughly. Run the engine briefly with fresh oil and drain again to remove residual fuel.
  
• Replace the oil filter and refill with high-detergent diesel-rated oil (e.g., SAE 15W-40 CI-4+).
  
• Inspect injectors and copper washers during routine maintenance.
  

Preventive steps include:

• Monitoring oil level weekly during heavy use
  
• Replacing lift pump every 2,000 hours or at signs of leakage
  
• Using fuel additives to reduce injector coking and seal degradation
  
• Keeping service records to track oil changes and fuel system repairs
  

Real-World Impact and Operator Insight
In Australia, a contractor operating a Case 1150G in eucalyptus clearing reported similar symptoms. After noticing a drop in oil pressure and a strong fuel smell, he shut down the machine and discovered the crankcase was overfilled. The lift pump diaphragm had ruptured, and diesel was pouring into the block. A quick replacement and oil flush restored the machine, but the incident highlighted the importance of early detection.
Interestingly, the same contractor had previously run a Case 850B for over 9,000 hours without major issues, underscoring the durability of Case machines when properly maintained.
Conclusion
Fuel contamination in the oil of a Case 1150G dozer is a serious but solvable issue. With a methodical diagnostic approach and timely repairs, operators can prevent long-term damage and keep these rugged machines running strong. The 1150G, backed by Case’s industrial pedigree and Cummins’ engine reliability, remains a workhorse in the field—provided its fuel system is kept in check. Regular inspection, clean fuel, and attention to oil levels are the keys to preserving its legacy.