Diagnosing Misfire Issues on the CAT 336E Excavator

[MikePhua]

The CAT 336E and Its Engine System
The Caterpillar 336E hydraulic excavator was introduced in the early 2010s as part of Caterpillar’s E-Series lineup, designed to meet Tier 4 Interim emissions standards while improving fuel efficiency and operator comfort. Powered by the CAT C9.3 ACERT engine, the 336E delivers around 303 horsepower and weighs approximately 36 metric tons. It quickly became a staple in heavy civil construction, mining, and large-scale earthmoving projects.
Caterpillar Inc., founded in 1925, has long been a leader in diesel engine innovation. The ACERT technology used in the 336E was developed to reduce NOx emissions while maintaining power and reliability. By 2015, the 336E had sold tens of thousands of units globally, with strong adoption in North America, Europe, and Australia.
Terminology Clarification

• Misfire: A condition where one or more engine cylinders fail to combust properly, leading to power loss and rough operation.
  
• Injector Trim File: A calibration file that matches fuel injector performance to the engine’s ECM (Electronic Control Module).
  
• Cylinder Cutout Test: A diagnostic procedure that disables individual cylinders to identify which one is malfunctioning.
  
• ECM (Electronic Control Module): The onboard computer that controls engine functions, including fuel delivery and timing.
  

Symptoms and Initial Observations
Operators have reported intermittent misfiring on the CAT 336E, accompanied by slight black smoke under load and reduced power. No active fault codes were present, but logged codes indicated erratic operation of injector #5. Fuel filters were replaced without improvement. The machine’s behavior suggested a combustion imbalance, possibly tied to injector performance or electrical connectivity.
Black smoke typically indicates incomplete combustion, often caused by poor atomization, timing issues, or excessive fuel delivery. In this case, the smoke was minimal, pointing toward a single-cylinder fault rather than a systemic issue.
Common Causes of Misfire in the 336E

• Faulty Injector: Internal wear or clogging can cause uneven fuel delivery. Injector #5 was flagged for erratic behavior.
  
• Electrical Connection Issues: Loose or corroded plugs at the valve cover or injector harness can disrupt signal flow.
  
• Trim File Mismatch: Replacing an injector without updating the ECM with the correct trim file can lead to persistent faults.
  
• ECM Fault: Rare but possible, especially if injector replacement does not resolve the issue.
  
• Hydraulic Lockout Solenoid Interference: Modifications or “bush fixes” to the solenoid system may inadvertently affect ECM readings.
  

Diagnostic Strategy and Field Testing
To isolate the misfire:

• Inspect injector harness and plug connections for corrosion or looseness.
  
• Swap the suspect injector with one from a known good cylinder and observe whether the fault code follows.
  
• Perform a cylinder cutout test using CAT ET (Electronic Technician) software to identify the misfiring cylinder.
  
• Verify that the correct trim file is installed for each injector.
  
• If injector replacement fails, consider ECM testing or replacement.
  

In one case in Pennsylvania, a technician spent three days swapping injectors and performing tests, only to discover that the ECM required recalibration. The issue was resolved after a second technician from a different branch intervened and performed a deeper diagnostic sequence. The final repair cost was significantly lower than the initial estimate, highlighting the importance of experience and methodical troubleshooting.
Recommendations for Repair and Prevention

• Always use CAT ET to confirm injector faults before replacement.
  
• Replace injectors with OEM parts and install the correct trim file immediately.
  
• Clean and inspect all electrical connectors during injector service.
  
• Avoid “parts cannon” approaches—replacing components without diagnosis often leads to wasted expense.
  
• If ECM replacement is considered, ensure all other possibilities have been eliminated first.
  

Operator Tips and Safety Considerations

• Monitor engine performance during warm-up and under load. Misfires often become more pronounced under stress.
  
• Avoid operating the machine with known misfire conditions, as this can damage the turbocharger and exhaust system.
  
• Keep a log of fault codes and service actions to assist future diagnostics.
  
• If CAT ET is not available, consider hiring a technician with access rather than guessing.
  

Conclusion
Misfire issues on the CAT 336E excavator can stem from injector faults, electrical problems, or ECM miscommunication. While replacing an injector may seem straightforward, it must be paired with proper calibration and diagnostic testing. The complexity of modern diesel engines demands a disciplined approach—one that prioritizes data, experience, and precision over trial-and-error. With careful attention to detail, the 336E can continue to deliver the performance and reliability that made it one of Caterpillar’s most successful excavators of its generation.