Diagnosing and Resolving Fuel Leaks in the Bobcat 773 Skid Steer

[MikePhua]

The Bobcat 773 and Its Role in Compact Equipment History
The Bobcat 773 skid steer loader was introduced in the late 1990s as part of Bobcat’s 700-series lineup, which aimed to deliver more power and hydraulic capacity in a compact footprint. Bobcat, originally founded in North Dakota in 1947, revolutionized the compact equipment industry with its first skid steer in 1960. The 773 model featured a vertical lift path, making it ideal for loading trucks and handling palletized materials. Powered by a Kubota V2203 diesel engine, the 773 offered around 46 horsepower and a rated operating capacity of 1,750 pounds. It became one of Bobcat’s best-selling models, with tens of thousands of units deployed across construction, landscaping, and agricultural sectors.
Identifying the Source of a Fuel Leak
Fuel leaks in skid steers can originate from multiple points, including:

• Fuel tank seams or welds
  
• Overflow or vent tubes
  
• Fuel lines and fittings
  
• Injection pump seals
  
• Return lines from injectors
  

In one case, a recently purchased Bobcat 773 began leaking diesel fuel intermittently—both while running and during refueling. Upon inspection, the overflow tube at the top of the tank was found disconnected. Reconnecting it reduced the severity of the leak, but a persistent drip remained. This raised questions about residual fuel trapped in the belly pan or a deeper structural issue with the tank itself.
Understanding Overflow and Venting Systems
The fuel tank on the Bobcat 773 includes an overflow or vent tube designed to equalize pressure and prevent vacuum lock. When disconnected, this tube can allow fuel to escape—especially during filling, when pressure builds rapidly. However, under normal operating conditions, gravity alone should prevent fuel from exiting through the top of the tank unless the system is pressurized or the tank is overfilled.
Key terminology:

• Overflow Tube: A line that allows excess fuel or vapor to escape during filling.
  
• Vent Line: Maintains atmospheric pressure inside the tank to prevent vacuum lock.
  
• Belly Pan: The lower cavity of the machine where fluids and debris can accumulate.
  

If fuel has spilled into the belly pan, it can continue to drip for days, especially if mixed with dirt and hydraulic residue. This creates a sponge-like environment that slowly releases trapped diesel.
Residual Fuel vs. Active Leak
Distinguishing between residual fuel and an active leak requires thorough cleaning and observation. Operators should:

• Lift the cab and inspect all visible fuel lines and fittings.
  
• Use absorbent pads to soak up pooled diesel in the belly pan.
  
• Pressure wash the interior cavity to remove grime and fuel residue.
  
• Monitor the machine after cleaning to identify fresh leaks.
  

One technician noted that after reconnecting a vent tube, residual fuel continued to drip for nearly a week. Only after a full pressure wash and drying did the leak stop—confirming that the tank itself was intact.
Tank Integrity and Signs of Structural Failure
If the leak persists after cleaning and reconnecting hoses, the fuel tank may be cracked or punctured. Common causes include:

• Impact damage from debris or tools
  
• Stress fractures near mounting points
  
• Corrosion in older tanks, especially steel variants
  

To inspect for cracks:

• Drain the tank completely and remove it from the machine.
  
• Use a flashlight and mirror to inspect welds and seams.
  
• Apply low-pressure air and soapy water to detect bubbles from pinholes.
  

Plastic tanks may be repairable using plastic welding techniques, while steel tanks often require brazing or replacement. A cracked tank is a serious issue, as diesel fuel poses fire risks and environmental hazards.
Preventative Measures and Maintenance Tips
To avoid future fuel leaks:

• Inspect vent and overflow tubes monthly.
  
• Replace brittle or cracked fuel lines with OEM-grade hose.
  
• Use thread sealant on fittings to prevent seepage.
  
• Avoid overfilling the tank, especially in hot weather when fuel expands.
  
• Clean the belly pan regularly to prevent buildup of flammable residue.
  

Installing a fuel shutoff valve near the tank outlet can also help isolate leaks and simplify maintenance.
Anecdotes from the Field
One operator shared a story of a Bobcat 763 that leaked fuel during winter. After chasing the leak for days, he discovered a cracked vent fitting hidden behind the engine shroud. Replacing the $12 part resolved the issue. Another technician recalled a 773 that had been pressure-washed without sealing the fuel cap—water entered the tank, causing injector issues and a false leak diagnosis.
These stories highlight the importance of methodical inspection and understanding the machine’s fuel system layout.
Conclusion
Fuel leaks in the Bobcat 773 are often caused by disconnected vent tubes, residual diesel in the belly pan, or minor fitting failures. However, persistent leaks may indicate structural damage to the tank itself. By combining visual inspection, cleaning, and pressure testing, operators can pinpoint the source and restore the machine to safe operation. In compact equipment, where every hour counts, resolving fuel issues quickly and thoroughly ensures productivity and protects both the operator and the environment.