Bleeding an Air‑Locked Fuel System on a Terex PT‑60

[MikePhua]

Background on the Terex PT‑60
The Terex PT‑60 is a mid-size compact track loader designed for versatility on construction sites, landscaping jobs, and utility work. It features a Perkins diesel engine (~57 hp per its spec sheet)  and a fuel system that includes a water separator and a primary filter. In machines with air-conditioning (“with AC”), physical access to areas around the engine compartment may be more restricted, which can complicate fuel maintenance tasks like bleeding.

Symptoms of an Air‑Locked Fuel System
An air‑locked fuel system manifests in several common ways:

• Rough running or poor engine performance after replacing the fuel filter.
  
• Visible air bubbles in the fuel return line or water-separator bowl.
  
• Difficulty reaching or operating the standard 10 mm bleed screw due to nearby components (e.g., AC compressor).
  
• Engine sputtering, rough idle, or loss of power after filter replacement or after the tank has run low.
  

Common Causes of Air Lock
Based on shared experiences and experts’ advice:

• A loose delivery‑valve (DV) holder on the injection pump can allow air into the high‑pressure system.
  
• Improperly tightened or installed fuel filter connections may leak air.
  
• A bleed screw that's difficult to access (e.g., blocked by the AC compressor) isn’t being used correctly.
  
• Weak or failing lift pump may not be capable of fully priming the system under load.
  

Recommended Bleeding Procedure

1. Inspect the Delivery‑Valve (DV) Holder
   • Make sure the DV holder nipple is fully tightened (recommended torque ~30‑35 ft‑lb per experienced pump rebuilders).
     
   • Avoid tightening the two‑bolt flange itself — only the nipple that secures the delivery valve. A loose DV holder can make a multi‑cylinder engine run like it's missing a cylinder.
     
2. Prime the Fuel System
   • Use the manual primer (or lift pump) to draw fuel through the filter and into the injection system.
     
   • According to Terex's service manual, you should pump the primer until fuel flows from the bleed screw without any air bubbles, then tighten the bleed screw.
     
   • If the bleed screw is hard to reach due to the AC compressor, you may instead crack a low-pressure fuel line (such as the injector return or pre‑pump line) to let trapped air escape while priming. Several technicians suggest loosening a fuel line fitting on the injection pump, then re-tightening once fuel flows steadily.
     
3. Run the Engine and Observe
   • After priming and tightening, start the engine and let it idle for a minute or two.
     
   • Watch for smooth operation; if roughness persists, there may still be residual air or another issue (like a bad DV holder or a lift pump failure).
     
   • To verify, you can redirect a fuel return line into a clean jar — if you see continuous bubbles, air is still being drawn into the system.
     
4. Additional Checks
   • Confirm the water separator (usually located on the left side of the engine) is drained properly, as trapped water can contribute to air turbulence or foaming.
     
   • Verify all fuel filter housing connections (inlet, outlet, return) are tight and leak‑free.
     

Technical Terms Explained

• Delivery‑Valve (DV) Holder: A part of the injection pump that holds the delivery valve. If its connection is loose, pressurized fuel may blow back, or air may be drawn in.
  
• Manual Primer (Lift Pump): A hand-operated pump that pulls fuel from the tank through the filter and into the injection pump.
  
• Bleed Screw: A small valve used to release trapped air from the fuel system during priming.
  
• Water Separator: A filter in the fuel system that removes water from the diesel before it reaches vital components.
  

Practical Advice & Precautions

• Always tighten fittings to spec. Loose fittings can be worse than none because they subtly draw air without obvious leaks.
  
• Use a clear jar or container when bleeding return lines; watching for bubbles gives real‑time feedback.
  
• Work safely around the engine: wear gloves and eye protection, and be careful with hot parts.
  
• If re‑bleeding doesn't fix the problem, consider that the lift pump may be worn and failing to maintain proper suction.
  

Real‑World Example
A PT‑60 operator reported that after changing the fuel filter, the machine ran terribly. He couldn’t reach the 10 mm bleed screw because of the AC compressor. After discussion with a fuel‑pump expert, he removed the loose delivery‑valve holder on the injection pump, tightened it to the correct torque (30–35 ft-lb), and re‑primed by loosening a return line. Fuel began flowing with no bubbles, he re-tightened, and the engine smoothed out and ran normally again, solving the rough-running condition.

Conclusion
Bleeding an air‑locked fuel system on a Terex PT‑60 can be tricky, especially with limited access due to AC components. The most reliable fix often starts with ensuring the delivery‑valve holder is correctly torqued. From there, manual priming, careful bleeding of return or pre‑pump lines, and observing fuel flow for bubbles can restore clean, steady fuel delivery. Taking the time to do it right will bring back smooth performance and avoid further fuel system issues.