Why Does the Atlas Copco XAS 50 Air Compressor Stall at Idle

[MikePhua]

The Atlas Copco XAS 50 and Its Role in Portable Air Systems
Atlas Copco, founded in Sweden in 1873, has long been a global leader in compressed air technology. The XAS 50 is part of its portable air compressor lineup, designed for construction, demolition, and utility work. This model delivers approximately 50 cfm (cubic feet per minute) at 100 psi, powered by a small diesel engine—often a Kubota or Deutz—paired with a rotary screw compressor. Its compact size and reliable output made it a favorite among contractors needing mobile air without sacrificing performance.
By the early 2000s, Atlas Copco had sold tens of thousands of XAS-series units worldwide, with the XAS 50 serving as a mid-range solution for pneumatic tools, jackhammers, and sandblasting rigs.
Symptoms of Idle Stall and Root Causes
A recurring issue with aging XAS 50 units is engine stalling when the compressor reaches its pressure setpoint and drops to idle. Instead of unloading properly, the compressor continues to load the engine, causing it to stall unless the operator manually vents the tank.
This behavior suggests a failure in the unloader system, which is responsible for disengaging the compressor load when full pressure is reached. Key components include:

• Unloader valve: Typically mounted on top of the blower housing, this valve regulates air flow into the screw element.
  
• Diaphragm actuator: Controls the unloader valve based on pressure feedback.
  
• Pressure regulator and control rods: Adjust the timing and response of the unloader system.
  

If the diaphragm tears or the valve sticks, the compressor remains loaded even at idle, overwhelming the engine’s reduced fuel input.
Diagnostic Steps and Field Observations
To isolate the fault:

• Inspect the unloader valve diaphragm for tears or stiffness. A damaged diaphragm cannot respond to pressure changes.
  
• Check for air leaks around the rotor seals or control lines. Leaks can cause false pressure readings and improper unloading.
  
• Verify the ball valve and tank venting system. If venting restores idle stability, the issue lies upstream in the control logic.
  
• Observe the engine throttle response. If the engine drops to idle but the compressor remains engaged, the control system is failing to unload.
  

One technician in Victoria, Australia noted that cracking the ball valve manually allowed the engine to stay at full throttle, confirming that the compressor was not unloading as designed.
Repair Recommendations and Component Upgrades
To restore proper function:

• Replace the diaphragm actuator with a new OEM or aftermarket unit rated for the compressor’s operating pressure.
  
• Clean and lubricate the unloader valve assembly, ensuring smooth movement and seal integrity.
  
• Install a pressure gauge on the control line to monitor unloading behavior in real time.
  
• Upgrade to a digital pressure switch if the original mechanical system is unreliable.
  

In one case, a contractor in Alberta replaced the diaphragm and adjusted the control rod length, resolving the stall issue and restoring normal idle behavior.
Preventive Maintenance and Long-Term Reliability
To avoid future failures:

• Service the unloader valve every 500 operating hours
  
• Replace air filters and inspect control lines quarterly
  
• Use synthetic compressor oil for better thermal stability
  
• Keep the compressor housing clean to prevent debris from jamming moving parts
  

Conclusion
The Atlas Copco XAS 50 is a durable and efficient air compressor, but its control system must function precisely to prevent engine stall at idle. When the unloader valve fails to disengage, the engine cannot maintain idle speed under load. By inspecting the diaphragm, verifying pressure feedback, and maintaining the control components, operators can restore reliable performance and extend the life of this trusted machine.