Bobcat 843 Idle Surge Troubleshooting

[MikePhua]

Introduction
The Bobcat 843, a compact skid-steer loader produced between 1987 and 1991, is renowned for its versatility and durability. Equipped with either an Isuzu or Perkins engine, it remains a popular choice for various applications. However, some operators have reported idle surge issues, where the engine's RPM fluctuates unexpectedly. This article delves into potential causes and solutions for this problem, drawing insights from user experiences and technical analyses.
Understanding the Idle Surge Phenomenon
Idle surge refers to the oscillation of engine RPMs, where the engine revs up and down without input from the throttle. In the case of the Bobcat 843, operators have observed RPM fluctuations ranging from 15 to 45-second cycles, with the engine occasionally returning to idle without throttle adjustment.
Potential Causes of Idle Surge

1. Fuel Delivery Issues
   • Air Leaks in Fuel System: Air entering the fuel lines can disrupt fuel delivery, leading to inconsistent engine performance.
     
   • Clogged Fuel Filters: Even if the fuel filter bowl appears full, a clogged filter can restrict fuel flow, causing idle instability.
     
   • Faulty Fuel Injection Pump: A malfunctioning mechanical fuel injection pump may fail to maintain consistent fuel pressure, resulting in erratic idling.
     
2. Electrical System Malfunctions
   • Defective Throttle Position Sensor (TPS): The TPS monitors throttle position and communicates with the engine control unit. A faulty TPS can send incorrect signals, leading to idle fluctuations.
     
   • Worn Wiring or Connectors: Corroded or loose electrical connections can cause intermittent signals, affecting engine performance.
     
3. Air Intake Problems
   • Dirty Air Filters: Clogged air filters can restrict airflow, causing the engine to run rich (excess fuel) or lean (insufficient fuel), both of which can lead to idle instability.
     
   • Obstructed Intake Manifold: Debris or carbon buildup in the intake manifold can disrupt air-fuel mixture, affecting idle quality.
     
4. Engine Control Unit (ECU) Issues
   • Software Glitches: Occasionally, ECU software may have bugs that cause erratic engine behavior.
     
   • Sensor Calibration Errors: Incorrect calibration of sensors can lead to improper fuel-air mixture adjustments, resulting in idle surges.
     

Diagnostic Steps

1. Visual Inspection
   • Check for visible signs of fuel leaks or air bubbles in the fuel lines.
     
   • Inspect electrical connectors for corrosion or loose connections.
     
   • Examine air filters and intake components for dirt or obstructions.
     
2. Fuel System Testing
   • Use a fuel pressure gauge to check for consistent fuel pressure.
     
   • Test the primer bulb for proper operation and fuel delivery.
     
   • Bleed the fuel system to remove any trapped air.
     
3. Electrical System Testing
   • Test the throttle position sensor for correct voltage output.
     
   • Use a multimeter to check for continuity in wiring and connectors.
     
4. Air Intake Testing
   • Measure airflow to ensure it's within manufacturer specifications.
     
   • Check for vacuum leaks in the intake manifold.
     

Preventive Maintenance Tips

• Regular Fuel System Maintenance: Replace fuel filters at recommended intervals and inspect fuel lines for wear.
  
• Electrical System Care: Clean and tighten electrical connections regularly to prevent signal issues.
  
• Air Intake System Upkeep: Replace air filters as needed and ensure the intake system is free from obstructions.
  
• Routine ECU Diagnostics: Periodically update ECU software and recalibrate sensors to maintain optimal engine performance.
  

Conclusion
Idle surge in the Bobcat 843 can stem from various sources, including fuel delivery problems, electrical system malfunctions, air intake issues, and ECU glitches. By systematically diagnosing these areas and adhering to preventive maintenance practices, operators can restore stable engine idling and prolong the lifespan of their equipment.