Case 850B Dozer Engine Troubles and Mechanical Recovery Strategies

[MikePhua]

The 850B and Its Role in Mid-Sized Earthmoving
The Case 850B crawler dozer was produced between 1976 and 1981, during a period when Case was expanding its construction equipment lineup to meet growing demand for versatile, mid-sized machines. Powered by the CASE A336BD diesel engine rated at 75 horsepower, the 850B was designed for land clearing, grading, and light-to-medium dozing tasks. With an operating weight of approximately 15,800 lbs and a powershift transmission, it offered a balance of maneuverability and pushing power that made it popular among contractors and municipalities.
The A336BD engine is a naturally aspirated, four-cylinder diesel known for its simplicity and torque delivery. However, like many engines of its era, it requires diligent maintenance and careful troubleshooting when performance issues arise.
Common Engine Symptoms and Field Diagnoses
Operators encountering engine trouble on the 850B often report:

• Hard starting or failure to start
  
• Loss of power under load
  
• Excessive white or black smoke
  
• Engine stalling after warm-up
  
• Unusual knocking or vibration
  
• Overheating during sustained operation
  

These symptoms typically point to issues in fuel delivery, air intake, compression, or cooling. In one case from Alberta, a contractor noted that his 850B would start cold but stall after 15 minutes of grading. The root cause was traced to a clogged fuel return line causing injector pressure imbalance.
Fuel System and Combustion Challenges
The A336BD uses a mechanical injection pump and inline injectors. Fuel system problems are often the first suspects in engine trouble.
Key failure points:

• Clogged fuel filters or water contamination
  
• Air leaks in suction lines
  
• Weak lift pump or failing injection pump
  
• Injector tip erosion or carbon buildup
  

Recommended actions:

• Replace both primary and secondary fuel filters
  
• Bleed air from system using manual primer
  
• Inspect return line for blockage or backpressure
  
• Test injector spray pattern and pop-off pressure
  

In one rebuild in Maine, a technician discovered that the fuel tank vent was plugged, causing vacuum lock and starving the pump. After cleaning the vent and replacing the lift pump, the engine ran smoothly under full load.
Air Intake and Compression Integrity
Restricted airflow or poor compression can mimic fuel issues. The 850B’s air intake system includes a dry element filter and pre-cleaner.
Inspection steps:

• Remove and inspect air filter for clogging or collapse
  
• Check intake hose for cracks or loose clamps
  
• Perform compression test (target: 350–400 psi per cylinder)
  
• Inspect valve lash and timing gear wear
  

A farmer in Wisconsin reported sluggish performance and black smoke. The air filter had collapsed internally, starving the engine of oxygen. After replacing the filter and adjusting valve lash, the dozer regained full power.
Cooling System and Overheating Risks
Overheating is a common issue, especially during summer grading or when pushing heavy loads uphill.
Cooling system components:

• Belt-driven water pump
  
• Thermostat (typically 180°F)
  
• Radiator with top and bottom tanks
  
• Fan shroud and belt
  

Troubleshooting tips:

• Flush radiator and inspect for scale buildup
  
• Replace thermostat and test in boiling water
  
• Check fan belt tension and pulley alignment
  
• Inspect water pump weep hole for leakage
  

In one municipal fleet in Texas, an 850B overheated during ditch shaping. The radiator core was 40% blocked with sediment. After chemical flushing and replacing the thermostat, the engine maintained safe operating temperature.
Electrical and Starting System Checks
Starting issues may stem from weak batteries, poor connections, or worn starter motors.
Checklist:

• Battery voltage: 12.6V minimum at rest
  
• Cranking voltage: ≥10.5V under load
  
• Starter draw: ≤400 amps
  
• Ground strap integrity and terminal corrosion
  

A technician in Oregon found that a slow crank was due to a corroded ground strap hidden under the battery tray. Replacing the strap and cleaning terminals restored reliable starts.
Engine Rebuild Considerations and Parts Availability
If diagnostics point to internal wear—such as low compression, excessive blow-by, or bearing knock—a rebuild may be necessary.
Rebuild components:

• Pistons and rings
  
• Cylinder liners
  
• Main and rod bearings
  
• Gasket set
  
• Injector nozzles and pump calibration
  

Parts for the A336BD remain available through aftermarket suppliers and salvage yards. In one restoration in Pennsylvania, a retired operator rebuilt his 850B engine using a kit sourced from Ohio, completing the job in 10 days with full recovery of torque and fuel efficiency.
Preventive Maintenance and Longevity Tips
To avoid engine trouble:

• Change oil and filters every 250 hours
  
• Use high-detergent diesel oil (SAE 15W-40)
  
• Drain water from fuel tank monthly
  
• Clean air filter weekly in dusty conditions
  
• Monitor coolant level and condition
  

A contractor in North Dakota logs engine hours and fluid changes on a whiteboard mounted inside the shop. His 850B has run over 6,000 hours with only one injector replacement.
Conclusion
Engine trouble on the Case 850B dozer is often solvable with methodical diagnostics and attention to fuel, air, and cooling systems. Whether the issue is a clogged return line, collapsed air filter, or worn injector, the A336BD engine responds well to targeted repairs and preventive care. With parts still available and a robust mechanical design, the 850B remains a dependable workhorse for those willing to listen to its symptoms and act decisively.