Injector Testing and Fuel System Restoration on the Case 350 Dozer

[MikePhua]

The Case 350 and Its Mechanical Simplicity
The Case 350 is a compact crawler dozer introduced in the 1960s by J.I. Case Company, a Wisconsin-based manufacturer with deep roots in agricultural and construction machinery. Designed for light grading, land clearing, and utility work, the 350 featured a straightforward mechanical layout, making it a favorite among owner-operators and small contractors. Its diesel engine, typically a 3-cylinder naturally aspirated unit, was paired with a mechanical injection pump and pencil-type injectors—components that remain serviceable with basic tools and knowledge.
Despite its age, the Case 350 continues to see use in rural applications and restoration projects. Its simplicity is both a strength and a challenge, especially when diagnosing fuel delivery issues after an engine rebuild.
Terminology Annotation
- Pencil Injector: A compact, direct-injection nozzle commonly used in small diesel engines, characterized by its slender shape and internal spring-loaded valve.
- Pop Tester: A hand-operated hydraulic tool used to test injector spray pattern and opening pressure by simulating fuel delivery.
- Adapter Fitting: A connector that allows the pop tester to interface with specific injector types, often requiring custom threads or modified fuel lines.
- Cracking an Injector Line: Loosening the fuel line at the injector to observe pressure drop and cylinder response, used to identify weak or non-functioning injectors.
Post-Rebuild Performance and Injector Behavior
After a full engine rebuild—including new liners, pistons, rings, and reconditioned cylinder head—a Case 350 may start and run, but exhibit sluggish throttle response or uneven cylinder contribution. In one example, the engine revved slowly from idle to full RPM, and cracking injector lines revealed that cylinder #1 had less impact on engine speed than the others.
This behavior suggests injector imbalance or low opening pressure. Pencil injectors rely on precise spring tension and nozzle integrity to deliver atomized fuel at the correct moment. If one injector opens too early or too late, combustion efficiency drops, leading to poor throttle response and uneven power.
A Story from the Workshop
In New Hampshire, a machinist restoring a Case 350 noticed that his rebuilt engine ran but lacked crisp throttle response. Suspecting injector issues, he purchased a pop tester but found that it lacked the correct adapter for the pencil injectors. After searching for specifications, he opted to modify an old fuel line to fit the tester. Using this setup, he discovered that one injector opened at significantly lower pressure and had a poor spray pattern. Replacing it with a matched unit restored engine balance and improved acceleration.
Adapter Fittings and Testing Solutions
Pencil injectors often use fine threads and unique flare angles, making off-the-shelf adapters rare. Solutions include:

• Modifying an old injector line to fit the pop tester outlet
  
• Using compression fittings with matching thread pitch and flare geometry
  
• Consulting injector manufacturers for test adapter kits
  
• Machining a custom adapter using lathe and thread gauges
  

When modifying fuel lines, ensure that the flare seats properly and that the line can withstand test pressures of 2,000–3,000 psi. Always test in a well-ventilated area and wear eye protection.
Recommendations for Injector Testing and Replacement
To ensure accurate injector performance:

• Test all injectors at room temperature using a calibrated pop tester
  
• Verify opening pressure against OEM spec (typically 2,200–2,500 psi for pencil injectors)
  
• Inspect spray pattern for cone shape and absence of dribble
  
• Replace injectors in matched sets to maintain balance
  
• Use new copper washers and torque to spec during installation
  
• Bleed fuel system thoroughly after injector replacement
  

If injector testing reveals wide pressure variation, consider rebuilding the injection pump as well. A worn pump can affect delivery timing and volume, compounding injector issues.
Preserving Legacy Equipment Through Precision
The Case 350 represents an era of mechanical clarity, where diagnostics relied on feel, sound, and simple tools. Restoring its fuel system requires patience and ingenuity, especially when sourcing adapters and interpreting injector behavior. With careful testing and matched components, even a half-century-old dozer can return to smooth, responsive operation.
Conclusion
Injector testing on the Case 350 demands both mechanical creativity and attention to detail. By fabricating adapter fittings and using a pop tester to evaluate spray patterns and pressures, operators can diagnose subtle performance issues and restore engine balance. In a world of electronic diagnostics, the tactile process of injector testing remains a vital skill for those committed to preserving and operating legacy machines.