Yanmar SV100 Unpriming Itself Problem

[MikePhua]

Introduction and Background
The Yanmar SV100 is a 10-ton class hydraulic excavator manufactured by Yanmar. It features a powerful 72 hp (≈ 53.7 kW) diesel engine, efficient hydraulic circuits, robust undercarriage, and specs suited for mid-size jobs. 
In recent years, many SV100 owners have reported an issue: after sitting idle overnight or during breaks, the machine will start, die shortly after, and then refuse to stay running or start again without “re-priming.” This means air has somehow been drawn into the fuel system, and the machine gradually loses its prime after stopping. The time span before it fails tends to shrink—after lunch break, mid-day, etc.—making the problem increasingly inconvenient.
Below is a detailed look at potential causes, component definitions (terms), diagnostic steps, and possible solutions.
Important Terminology

• Prime / Priming Pump: The act of filling or re-filling the fuel supply line and fuel filter so that fuel, not air, is present; ensures the fuel system is “primed” and no air locks exist.
  
• Water Separator: A device that removes water from fuel before it enters fuel filters and injection system.
  
• Air Bleeder Screw: A screw or valve used to allow trapped air to escape from fuel filters or water separator.
  
• Vacuum Leak (in fuel system): A leak that allows air to be sucked into the fuel line, usually between tank and engine or in filter connections.
  
• Supply Pump (Lift Pump): A pump that draws fuel from the tank and delivers it to fuel filter / injection pump.
  
• Fuel Filter: Removes particulates from fuel; often includes or works alongside the water separator.
  

Symptoms Observed

• Machine starts after overnight idle, but dies immediately, then won’t restart unless fuel filter (or other part) is “topped up” or manually primed.
  
• Air bubbles seen coming from the bleeder screw on the water separator when trying to start again.
  
• Fuel filter partially empty when the problem is reproduced (immediately after idle).
  
• The time after stopping before problem reoccurs gets shorter with each stop (so time to unprime becomes shorter).
  
• A “tic-tic” sound heard from priming pump (when key is turned ON but engine not cranking).
  

Likely Causes
From the symptoms, a few probable causes emerge:

1. Vacuum or Air Leak in the Fuel Supply Line
   Gaps in seals, loose fittings, cracked hoses, or collapsed hose under suction can draw in air.
   
2. Faulty Priming Pump
   If the priming (lift) pump is worn, has internal leakage, or cannot maintain prime, then air gets introduced or fuel isn’t pulled continuously.
   
3. Clogged or Defective Fuel Filter or Water Separator
   If filters are partially blocked, they may restrict flow, causing vacuum behind them or allowing air to pull past seals.
   
4. Ventilation Problem in Fuel Tank
   If tank vent is blocked, a vacuum can build in the tank as fuel is drawn out, resisting further fuel flow and causing air to be sucked in.
   
5. Loose or Defective Bleeder Seal
   If the air bleeder screw or its seat is not sealing properly, it may allow air ingress when pressure drops.
   
6. Recent Repairs or Maintenance Mistakes
   Any time fuel system components are removed or changed (filter, hoses, pump), improper reassembly, pinched hose, seal misfits, etc., can introduce issues.
   

Diagnostic Steps
To isolate and identify the root cause, proceed with a structured diagnosis:

• Verify tank vent condition: Remove or loosen fuel cap vent, listen to see if restriction; if fuel is being drawn but air cannot enter tank, vacuum in tank can cause flow starvation.
  
• Inspect fuel supply hose: Check for collapse, cracks, soft spots, especially under suction (between pump/tank). Try blowing air backwards into tank via supply line (when safe) to test if hose is clear.
  
• Check priming pump operation: When you turn key to ON (without cranking), listen for expected sounds; the “tic-tic” may indicate pump cycling. Measure pressure and flow from pump if possible.
  
• Bleed the system completely: Use air bleeder on water separator and any upstream filters. Note how long it takes for air bubbles to stop.
  
• Replace or inspect fuel filters and water separators: Even if they are new, inspect for correct assembly and sealing.
  
• Observe behavior over idle periods: See if problem only happens after long idle, temperature changes, etc. Record time until failure after shutdowns.
  

Solutions and Recommended Actions
Based on diagnostics above, here are solutions to try:

• Replace the supply (lift) pump if testing reveals poor suction, inconsistent flow, or audible odd noises during priming.
  
• Replace any hoses that are soft, collapsing under vacuum, cracked, or have improper routing that causes kinks or compression.
  
• Replace or re-service the fuel/water separator and filters; use genuine filters. Ensure that o-rings, gaskets, seals are seated properly.
  
• Ensure fuel tank vent is clean and functioning; replace or clean vented fuel cap if necessary.
  
• Tighten all fittings upstream of filters and pump; ensure bleeder screws are seating properly; use thread sealant if appropriate.
  
• If the machine sits unused for long hours, consider priming or warming procedures before outright startup: a brief crank and check for prime before full ignition.
  

Case Story
An owner in Québec experienced this issue after acquiring their first SV100. After nights of trouble starting and dying, they discovered air bubbles issuing from the water separator’s air bleeder screw. Topping off the fuel filter temporarily solved the start-up, but the problem returned more quickly each time. A forum member suggested inspecting the supply line between the fuel tank and pump, pointing out that collapsed or soft hoses often fail under vacuum, letting in air. Another suggested checking the tank cap vent as a simple test for whether the tank is admitting fresh air. Eventually the owner decided to order a replacement priming pump and planned to check supply hoses and filter components to fully resolve the problem.
Specifications for Context
Here are relevant specifications of the SV100 to understand fuel system demands and tolerances:

• Operating weight: approx. 20,950 lb (≈ 9,500 kg) 
  
• Engine gross power: ~ 73.5 hp at ~ 2,200 rpm 
  
• Hydraulic relief valve pressure: about 3,988 psi (≈ 27.5 MPa) 
  
• Fuel tank capacity: ~ 31.7 gallons (≈ 120 liters) for standard SV100 model. 
  

Knowing fuel tank size, pump demands, and engine consumption informs how quickly a system can lose prime or fail after minor leaks.
Preventative Measures
To prevent recurrence of unpriming issues:

• Regularly inspect fuel hoses, fittings, and filter mounts for air tightness.
  
• Replace fuel cap vents at scheduled intervals or when they become worn or clogged.
  
• Use high-quality filters and purge air carefully after filter changes.
  
• Keep spare priming pumps, hoses, and seals in maintenance inventory for quick repairs.
  
• Maintain records of start behavior (after idle, temperature, etc.) to help identify when failures are predictable and address before downtime.
  

Conclusion
The SV100’s tendency to “unprime” itself arises from small air leaks, either from the supply line, priming pump, tank vent, or filter components. While topping up or priming may provide temporary relief, systematic inspection and replacement of suspect components (pump, hoses, vent, filter, bleeder) is essential. With proper diagnostics, repair, and preventative maintenance, the machine should return to reliable starting even after long idle periods. If problems persist even after all checks, consulting a certified diesel mechanic or Yanmar service authority is advised.