Diagnosing Fuel Prime Loss in the JLG SkyTrak 8042: Causes, Symptoms, and Solutions

[MikePhua]

Overview of the SkyTrak 8042 and Its Powertrain
The JLG SkyTrak 8042 is a widely used telehandler in construction and industrial applications, known for its 8,000-pound lift capacity and 42-foot reach. It’s often powered by a Cummins QSB series diesel engine—a reliable, electronically controlled platform designed for Tier 3 emissions compliance and high torque output. Despite its robust design, some units experience hard-starting issues, particularly after sitting idle, which can be traced to fuel system irregularities.
Symptoms of Fuel Prime Loss
Operators have reported the following behaviors:

• Extended Cranking on First Start: The engine requires prolonged cranking to start, especially after sitting overnight.
  
• Intermittent Fuel Bleed-Down: The inline fuel filter loses prime within minutes of engine shutdown, though not consistently.
  
• Quick Restart When Primed: Once the fuel system is re-primed, the engine starts and runs smoothly.
  
• Occurs Hot or Cold: The issue is not temperature-dependent, suggesting a mechanical or electrical fault rather than thermal expansion or contraction.
  

Understanding the Fuel System Layout
The Cummins QSB engine in the SkyTrak 8042 typically includes:

• Electric Lift Pump: Supplies fuel from the tank to the high-pressure injection system.
  
• Inline Fuel Filter: Captures contaminants before fuel reaches the injectors.
  
• Return Line Circuit: Excess fuel is routed back to the tank or filters to regulate pressure.
  
• Fuel Shutoff Solenoid: Controls fuel flow during engine start and stop.
  

Terminology Explained

• Fuel Prime: The presence of pressurized fuel in the supply lines and filters, necessary for immediate combustion.
  
• Lift Pump: A low-pressure pump that moves fuel from the tank to the engine’s high-pressure system.
  
• Bleed-Down: Loss of fuel pressure or volume in the supply line due to leaks, faulty valves, or siphoning effects.
  

Possible Causes of Fuel Bleed-Down
Several factors may contribute to the loss of fuel prime:

• Faulty Check Valve: If the check valve in the lift pump or return line fails, fuel can flow backward into the tank.
  
• Leaky Fuel Lines or Fittings: Micro-leaks allow air to enter the system, breaking the prime.
  
• Defective Lift Pump: Some electric pumps continue running after shutdown, unintentionally cycling fuel back to the tank.
  
• Improper Filter Installation: A loose or misaligned filter can allow fuel to drain or air to enter.
  
• Fuel Shutoff Solenoid Malfunction: If the solenoid doesn’t fully close, it may allow fuel to backflow.
  

Field Anecdote: Diagnosing a Persistent Prime Issue
In Milford, a technician working on a SkyTrak 8042 noticed that the inline filter would bleed down within two minutes of shutdown. After bypassing the onboard fuel tank and connecting to a clean external source, the issue persisted. He observed that the electric lift pump continued to run briefly after engine shutdown, returning fuel to the tank and causing prime loss. The solution involved replacing the pump with a model featuring an integrated check valve and installing a manual shutoff valve in the return line.
Diagnostic Steps and Recommendations
To resolve fuel prime issues, follow these steps:

1. Inspect Fuel Lines for Leaks
   Use a vacuum tester or pressurize the system to detect micro-leaks at fittings and hoses.
   
2. Test Lift Pump Functionality
   Observe whether the pump continues running after shutdown. Replace with a pump that shuts off immediately and includes a check valve.
   
3. Check Filter Seals and Orientation
   Ensure the filter is installed correctly and that all seals are intact. Replace if in doubt.
   
4. Install Inline Check Valve
   Add a one-way valve between the lift pump and filter to prevent backflow.
   
5. Monitor Return Line Behavior
   If fuel is returning excessively after shutdown, consider installing a manual shutoff or pressure-regulated valve.
   

Preventive Measures and Long-Term Solutions

• Use OEM-Grade Components: Aftermarket pumps and filters may lack proper sealing or valve mechanisms.
  
• Regular Maintenance: Replace filters and inspect lines every 250–500 hours of operation.
  
• Upgrade to Smart Pump Systems: Some modern lift pumps include electronic shutoff logic and integrated diagnostics.
  

Conclusion: Restoring Reliable Starts
Fuel prime loss in the JLG SkyTrak 8042 is a solvable issue with careful diagnosis and targeted upgrades. Whether caused by a faulty pump, leaky fittings, or valve failure, addressing the root cause ensures consistent engine starts and optimal performance. For fleet managers and operators, understanding the fuel system’s behavior is key to maintaining uptime and avoiding costly delays—especially when the machine is needed most.