Why Does the Bucket Tilt When Lowering the Lift on a Bobcat 873

[MikePhua]

Bobcat 873 and Its Hydraulic Control System
The Bobcat 873 skid steer loader was introduced in the late 1990s as part of Bobcat’s high-flow series, designed for demanding attachments and heavy lifting. With a rated operating capacity of 2,400 lbs and an optional high-flow hydraulic system delivering up to 30.5 GPM, the 873 became popular in construction, agriculture, and rental fleets. Bobcat, a division of Doosan Group since 2007, has sold millions of skid steers globally, with the 873 remaining a respected model for its mechanical simplicity and robust frame.
The 873 uses a pilot-controlled hydraulic system with proportional valves that regulate lift and tilt functions. These valves are actuated by joystick inputs, and their behavior is governed by internal spool movement and pressure feedback. When functioning correctly, the lift and tilt circuits operate independently, allowing precise control of bucket position during arm movement.
Unintended Tilt During Lift or Lower
A common issue reported by operators is unintended bucket tilt when lowering or raising the lift arms. In one case, the bucket tilted forward during both lift and descent, despite the tilt function working normally when operated directly. This behavior suggests a hydraulic cross-bleed or valve leakage, where fluid intended for one circuit migrates into another due to internal wear or contamination.
Possible Causes and Diagnostic Path
Several factors can contribute to this issue:

• Spool valve wear: Internal leakage between lift and tilt spools can cause pressure crossover.
  
• Pilot pressure imbalance: If pilot lines are contaminated or restricted, valve response may become erratic.
  
• Check valve failure: These valves prevent backflow and maintain circuit isolation. A failed check valve can allow unintended movement.
  
• Cylinder bypass: Internal seal failure in the tilt cylinder may allow fluid to bypass the piston, causing drift during unrelated operations.
  

To diagnose:

• Observe tilt behavior during lift and descent without joystick input.
  
• Check hydraulic fluid for contamination or aeration.
  
• Inspect pilot lines and valve block for leaks or loose fittings.
  
• Test cylinder integrity using a pressure hold test.
  

Stress Events and System Fatigue
In one documented case, the issue began after the machine was used to lift the front end of a triple-axle fifth-wheel camper—an operation that pushed the loader to its mechanical limits. Such stress events can cause:

• Microfractures in valve seats
  
• Seal deformation in cylinders
  
• Pressure spikes that damage spool tolerances
  

Even if symptoms don’t appear immediately, latent damage may manifest in subsequent operations. It’s advisable to inspect all hydraulic components after extreme loads, especially if the machine was operated near or beyond its rated capacity.
Solutions and Preventive Measures
To resolve the issue:

• Replace or rebuild the main control valve block if internal leakage is confirmed.
  
• Flush the hydraulic system and replace filters to remove contaminants.
  
• Repack or replace tilt cylinders if bypass is detected.
  
• Upgrade pilot line fittings to prevent pressure loss.
  

Preventive steps include:

• Avoid lifting loads that exceed 2,400 lbs, even momentarily.
  
• Use auxiliary supports or jacks for non-standard lifting tasks.
  
• Perform regular hydraulic fluid analysis every 250 hours.
  
• Inspect joystick and valve response monthly for drift or lag.
  

A Story from the Midwest
A contractor in Iowa used his Bobcat 873 to lift a buried propane tank during a winter emergency. Though the machine succeeded, it later developed tilt drift similar to the case above. After replacing the valve block and repacking the tilt cylinders, the issue was resolved. The contractor now uses a load cell to monitor lifting force and avoids exceeding 85% of rated capacity.
Conclusion
Unintended bucket tilt during lift or descent on a Bobcat 873 is typically caused by internal hydraulic leakage, often triggered by overloading or component wear. With careful diagnosis and targeted repairs, the issue can be resolved without full system replacement. The 873 remains a capable machine, but like all hydraulic equipment, it demands respect for its design limits and proactive maintenance to ensure safe and precise operation.