Diagnosing Fuel Prime Loss in a Gradall with a John Deere PowerTech 4.5 Engine

[MikePhua]

The Gradall Platform and Its Diesel Heart
Gradall excavators are known for their unique telescoping boom and versatility in roadwork, ditching, and slope shaping. Many older models are powered by John Deere’s PowerTech 4.5L diesel engine—a robust inline-four that has served in agricultural, industrial, and construction equipment for decades. With mechanical injection and a cam-driven lift pump, the engine is generally reliable, but like any diesel system, it’s vulnerable to air intrusion and fuel delivery issues.
One recurring problem in aging Gradalls is fuel prime loss after shutdown. The engine runs strong when bled, but after sitting for a few hours, it refuses to start without manually bleeding the system again. This issue can be deceptively complex, often involving multiple components that interact subtly.
Common Causes of Fuel Prime Loss
Fuel systems on mechanical diesels rely on vacuum integrity and check valves to maintain prime. When prime is lost, air enters the system, and the lift pump cannot generate enough suction to draw fuel from the tank. The most common culprits include:

• Cracked or degraded rubber suction hoses
  
• Faulty lift pump check valves
  
• Leaking seals at fuel line connections
  
• Improperly installed water separator o-rings
  
• Air leaks in the return line circuit
  
• Internal leakage in the injection pump
  

In this case, the suction hose had already been replaced, and the tank appeared clean. That ruled out external contamination and hose degradation. Attention then turned to the priming pump and its internal valves.
Lift Pump and Priming Pump Confusion
On many John Deere engines, the hand priming pump is integrated with the lift pump and mounted on the side of the engine, driven by the camshaft. This pump contains internal check valves that prevent fuel from draining back into the tank. If these valves stick open or leak, the system loses prime.
To test this, technicians often:

• Pump the hand primer before starting to see if fuel pressure builds
  
• Disassemble the pump to inspect valve seats and springs
  
• Replace the pump if internal damage is found
  

In one case, replacing the priming pump resolved the issue entirely. The old pump had worn seals and a weak check valve, allowing fuel to drain back when the engine was off.
Crimp Testing and Line Isolation
A useful diagnostic method is the “crimp test,” where hose pinch pliers are used to isolate sections of the fuel system. By pinching the suction line after shutdown and waiting, one can determine whether fuel is draining back through the supply side or the return side.
Steps include:

• Run the engine normally
  
• Shut down and immediately crimp the suction line near the tank
  
• Wait the usual time period for prime loss
  
• Attempt to restart without bleeding
  

If the engine starts, the leak is upstream of the crimp. If it fails, the leak is downstream. This test can be repeated on the return line to further isolate the issue.
In this case, crimping both supply and return lines still resulted in a no-start condition, suggesting the leak was internal—possibly within the injection pump or at the pump’s line seals.
Bleeding Procedures and Air Traps
Bleeding a diesel system typically involves:

• Opening the bleed screw on the fuel filter or water separator
  
• Pumping the hand primer until fuel flows without bubbles
  
• Cracking injector lines and cranking the engine until fuel sprays
  
• Tightening injector lines and starting the engine
  

However, if the injection pump itself contains trapped air, bleeding only at the filter and injectors may not be sufficient. Some pumps have a bleed port near the return line fitting, which can be accessed with difficulty. Replacing the seals at the supply and return fittings on the pump may also help, especially if old seals crumble during disassembly.
Other Possibilities and Final Thoughts
While air intrusion is the most common cause of prime loss, other factors can contribute:

• A leaking injector can allow fuel to drain back through the return circuit
  
• A faulty fuel filter head can introduce air during shutdown
  
• A missing or misaligned o-ring in the water separator can cause intermittent leaks
  

Black smoke on startup, especially during aggressive throttle application, may be normal for older diesels but can also indicate incomplete combustion due to air in the system.
Ultimately, resolving fuel prime loss requires a methodical approach:

• Replace suspect seals and hoses
  
• Test the lift pump and priming pump
  
• Isolate lines with crimp tests
  
• Bleed thoroughly, including at the injection pump if possible
  

With patience and careful diagnostics, even stubborn fuel issues on aging Gradalls can be resolved—restoring reliable starts and keeping these machines working where they belong: in the dirt, not the shop.