Bobcat 731 Skid Steer with Wisconsin Gas Engine: Troubleshooting Stalling Issues

[MikePhua]

Introduction
The Bobcat 731, a compact skid steer loader introduced in the late 1970s, was originally equipped with a two-cylinder air-cooled diesel engine. However, some units, like the one discussed here, have been retrofitted with the Wisconsin VH4D four-cylinder gas engine. This engine swap can lead to unique challenges, especially concerning fuel delivery and ignition systems.
Engine Overview
The Wisconsin VH4D is a four-cylinder, air-cooled gas engine known for its durability and simplicity. It's commonly used in various industrial applications, including skid steers. When retrofitting this engine into a Bobcat 731, it's crucial to ensure all components, such as the fuel system and ignition, are correctly integrated to maintain optimal performance.
Common Symptoms and Initial Observations
Owners have reported that the Bobcat 731 with the Wisconsin engine starts with some effort and runs initially but stalls unexpectedly, particularly as the engine warms up. In some cases, the engine may restart intermittently, leading to operational challenges. For instance, one operator noted that during a late-night task, the machine required over 100 restarts to complete the job, indicating a persistent issue that worsened with engine temperature.
Potential Causes of Stalling
Several factors can contribute to the stalling of the Wisconsin VH4D engine in the Bobcat 731:

• Fuel Delivery Problems: Issues such as clogged fuel lines, a faulty mechanical fuel pump, or a dirty fuel filter can restrict fuel flow, leading to engine stalling.
  
• Ignition System Failures: A malfunctioning ignition system, including faulty spark plugs or a failing ignition coil, can cause the engine to stall.
  
• Vapor Lock: Inadequate fuel return lines or vapor return systems can lead to vapor lock, especially when the engine heats up, disrupting fuel flow and causing stalling.
  

Diagnostic Steps and Solutions
To address the stalling issue, consider the following diagnostic steps:

1. Inspect the Fuel System: Check for any blockages or restrictions in the fuel lines. Ensure the fuel filter is clean and the mechanical fuel pump is functioning correctly. In some cases, bypassing the mechanical pump temporarily can help identify if it's the source of the problem.
   
2. Examine the Ignition System: Test the spark plugs for wear and replace them if necessary. Ensure the ignition coil is providing a strong spark.
   
3. Check for Vapor Lock: Inspect the fuel return lines and vapor return systems for any blockages or restrictions. Ensure these systems are functioning correctly to prevent vapor lock.
   
4. Monitor Engine Temperature: Observe the engine's behavior as it warms up. If stalling occurs primarily when the engine reaches operating temperature, it may indicate issues related to fuel vaporization or ignition timing.
   

Additional Considerations
When retrofitting a Wisconsin VH4D engine into a Bobcat 731, ensure that all components are compatible and correctly installed. Consult the engine's service manual for specific maintenance procedures and specifications. Regular maintenance, including timely oil changes and air filter replacements, can also help prevent potential issues.
Conclusion
Addressing the stalling issue in the Bobcat 731 with the Wisconsin VH4D engine requires a systematic approach to diagnose and resolve potential fuel delivery and ignition system problems. By carefully inspecting and maintaining the engine's components, operators can restore reliable performance to this vintage skid steer loader.