315 Cat Engine Flutter — What It Is and What Might Be Causing It

[MikePhua]

Symptom Description
A 315 excavator owner reports that their Cat engine “flutters” — especially at higher RPMs — like a fine miss. They've already replaced the injectors, but the issue persists once the machine warms up and the hydraulics are used.  The problem impacts performance and is “just annoying” rather than completely disabling the machine.

Common Causes Discussed by Experienced Technicians

1. Fuel System Contamination
   • Dirty fuel or water in the tank can cause erratic engine behavior.
     
   • A clogged inlet screen in the transfer pump or a dirty water separator can starve the engine of clean fuel, leading to misfires or fluttering.
     
   • Sediment in the fuel tank should be drained regularly; one user admitted to only draining it seasonally (“spring and fall”), which may allow buildup.
     
2. Worn Fuel Pump (Governor Section)
   • On older 315 models (e.g., A or B series), wear in the governor portion of the injection pump is a known issue.
     
   • The suggested fix: remove and send the pump to a specialist (“injector shop”) for a rebuild.
     
3. Grounding or Electrical Issues
   • Poor or broken ground connections can cause erratic engine behavior and auto-idle problems in Cat excavators.
     
   • A dirty or improperly seated engine speed (RPM) sensor may result in faulty speed control and contribute to “flutter.”
     
4. Pilot & Hydraulic Load Influence
   • Some operators note the flutter or miss only appears under load or when hydraulics are being used, suggesting a fuel delivery / pump demand issue rather than a pure ignition or injector fault.
     
   • If hydraulic demand increases suddenly, the fuel system must keep up; any delay or drop in fuel flow can cause transient misfires.
     

Recommended Diagnostic Steps

• Fuel Filter & Water Separator Inspection
  Start with the basics: replace the fuel filter, empty and clean the water separator, and ensure the primary suction screen is clean.
  
• Fuel Tank Drain
  Drain out any water/sediment from the fuel tank. This is vital, especially for machines that sit or run in dusty, wet environments.
  
• Inspect the Fuel Pump
  Remove the injection pump and check the governor section for internal wear — this may require sending it to a specialist.
  
• Check Grounds & Wiring
  Verify engine ground wiring — poor ground can lead to unpredictable control issues.
  
• Engine Speed Sensor
  Clean or reseat the RPM sensor. Check its clearance and signal to controller; poor signal may destabilize engine speed under load.
  

Possible Solutions

• Rebuild or replace the injection pump if wear is confirmed.
  
• Improve fuel system filtration or install a finer secondary filter to catch contaminants.
  
• Repair or reinforce ground wiring, especially around engine and control modules.
  
• Replace or clean the RPM sensor to restore stable engine speed.
  
• Set up a more frequent maintenance schedule for draining the tank and cleaning fuel components.
  

Conclusion
Engine flutter in a Cat 315 is often not caused by something exotic — many times it's a fuel system issue, poor grounding, or a worn injection pump governor. Before chasing high‑cost solutions, start with the basics: clean and inspect the fuel system, verify ground connections, and consider a pump rebuild if needed. These checks can often eliminate the flutter and restore smooth performance.