Diagnosing Intermittent Misfire in a Shibaura N844 Diesel Engin

[MikePhua]

The Shibaura N844 and Its Agricultural Legacy
The Shibaura N844 is a 4-cylinder diesel engine commonly found in compact tractors like the Ford New Holland 1920. Built by Shibaura, a Japanese manufacturer with deep ties to Ford’s compact equipment line, the N844 is known for its mechanical simplicity, fuel efficiency, and long service life. It shares design similarities with the Perkins 404C-22, and has been used in various agricultural and industrial platforms since the 1990s.
With a displacement of 2.2 liters and a naturally aspirated configuration, the N844 delivers smooth torque across a wide RPM range. Its indirect injection system and mechanical fuel pump make it relatively easy to maintain, though age and light-duty cycles can introduce quirks that require careful diagnosis.
Initial Symptoms and Fuel System Checks
A well-maintained N844 may suddenly begin misfiring under load, particularly between 1200 and 2000 RPM. In one case, the engine started and idled smoothly, but developed a rhythmic stumble as it warmed up. The misfire worsened with time and was most noticeable during moderate throttle, while wide-open operation at 2500 RPM remained relatively smooth.
The first step in troubleshooting involved draining the fuel tank, cleaning the sediment bowl, and adding a fuel system cleaner. While this improved idle quality, the misfire persisted. Bleeding the injector lines by loosening the compression nuts yielded no clear indication of a faulty cylinder, suggesting the issue was intermittent or systemic.
Valve Lash and Thermal Expansion Effects
Valve lash—the clearance between the rocker arm and valve stem—is a critical adjustment in diesel engines. As the engine warms, metal expansion can alter valve timing and lift. If lash is too tight, valves may not seat fully, reducing compression and causing misfire. If too loose, valve timing may lag, affecting combustion efficiency.
In this case, valve lash was checked and found to be within specification. However, the symptoms—misfire emerging only after warm-up—still pointed to a thermal issue. It’s possible that the engine had not been run under full load in some time, and carbon buildup or varnish deposits were affecting injector spray patterns or valve seating.
Injector Condition and Carbon Fouling
After removing the injectors, they were found heavily carbonized, with nozzle orifices completely obscured. This level of fouling can severely disrupt atomization, leading to poor combustion and misfire. The injectors were sent for professional cleaning and rebuild.
Upon reinstalling the refurbished injectors and bleeding the fuel lines, the engine still misfired at mid-range RPMs. This ruled out injector blockage as the sole cause and pointed toward deeper timing or fuel delivery issues.
Pump Timing and Shim Adjustment
The Shibaura N844 uses a mechanically timed injection pump mounted with shims beneath its flange. These shims set the pump’s advance angle relative to crankshaft position. Over time, wear in the pump drive gear, mounting flange, or shim stack can alter timing, especially if the engine has experienced vibration or thermal cycling.
Incorrect timing can cause delayed injection, leading to incomplete combustion and misfire. Adjusting pump timing requires precision tools and experience, as even a 1-degree error can affect performance. If timing is suspected, technicians should:

• Measure top dead center using a dial indicator
  
• Remove the pump and inspect shim thickness
  
• Reinstall with corrected shim stack based on timing chart
  
• Verify timing with a spill test or timing light (if applicable)
  

In one similar case, a Perkins 404C-22 exhibited hard starting and misfire due to a blown head gasket near the pushrod cavity. Though the symptoms differed, the underlying issue—loss of combustion integrity—was comparable.
Compression and Blowby Assessment
Compression testing and crankcase blowby inspection are essential when misfire persists after injector and timing checks. In this case, the crankcase vent showed no signs of blowback, and manual rotation of the fan indicated consistent compression across cylinders. While not definitive, these signs suggested that piston rings and valves were sealing adequately.
Blue smoke during misfire episodes may indicate oil intrusion, but in this instance, smoke was minimal and disappeared under load. This supports the theory that combustion timing or fuel delivery—not mechanical wear—was the root cause.
Load Cycling and Engine Reconditioning
To test whether the engine simply needed a hard run, the operator installed a rototiller and worked the tractor for two hours, partially blocking the radiator screen to increase operating temperature. The engine improved slightly, suggesting that carbon deposits were being burned off. However, the misfire remained at mid-range RPMs.
This technique—known as load cycling—is often used to recondition lightly used engines. By forcing full combustion and thermal expansion, it can restore injector spray patterns and valve seating. It’s especially effective in engines that have spent years idling or performing light tasks.
Conclusion
An intermittent misfire in the Shibaura N844 diesel engine can stem from multiple overlapping causes: injector fouling, valve lash deviation, pump timing drift, or carbon buildup from light-duty cycles. While injector replacement may seem like the obvious fix, deeper inspection of timing and combustion dynamics is often required. With methodical testing and a willingness to dig into the mechanical heart of the engine, even the most stubborn misfire can be resolved. In compact tractors like the Ford NH 1920, where reliability is paramount, such diligence ensures the machine continues to serve for years to come.