Troubleshooting the Caterpillar 955E with D315 Engine

[MikePhua]

The 955E and Its Mechanical Legacy
The Caterpillar 955E Traxcavator was introduced in the late 1950s as part of CAT’s evolution of crawler loaders. It featured a robust undercarriage, mechanical transmission, and the D315 diesel engine—a 7.0-liter inline-six known for its torque and simplicity. With an operating weight of around 13 tons and a bucket capacity of 1.5 cubic yards, the 955E was widely used in excavation, land clearing, and material loading.
The D315 engine powered multiple CAT machines including the D4 dozer and early graders. Its mechanical fuel injection and wet sleeve design made it durable but sensitive to lubrication and bearing wear. Thousands of 955E units were sold globally, and many remain in service today, especially in rural fleets and restoration projects.
Terminology Note

• D315: A naturally aspirated inline-six diesel engine with mechanical injection and wet sleeves.
  
• ROPS: Roll-Over Protective Structure, often bolted to the loader frame.
  
• Pre-Cup: A small combustion chamber in indirect injection engines where fuel is initially ignited.
  
• Crankcase Access Panel: A removable plate on the engine block allowing inspection of the lower end.
  

Symptoms and Initial Diagnostic Strategy
When a 955E with a D315 engine fails to start or runs poorly, common symptoms include:

• Engine cranks but does not fire
  
• Low compression or uneven cylinder temperatures
  
• Fuel delivery appears normal but combustion is weak
  
• Audible knocking or metallic sounds from the lower end
  
• Oil pressure drops after warm-up
  

A logical troubleshooting strategy begins with the least invasive checks:

• Verify fuel delivery at injectors
  
• Inspect air intake and exhaust for blockages
  
• Check battery voltage and starter engagement
  
• Use an infrared thermometer to compare exhaust port temperatures
  
• Bleed fuel lines and test injector spray pattern
  

If these checks do not reveal the issue, deeper inspection is warranted.
Bottom-End Inspection and Bearing Failure
The D315 is known to suffer from main bearing wear, especially in machines with irregular maintenance. If the engine spins freely but fails to build compression or oil pressure, a spun bearing is likely. To confirm:

• Remove the crankcase access panel and inspect bearing caps
  
• Look for scoring, discoloration, or metal flakes in the oil
  
• Rotate the crankshaft manually and observe end play
  
• Use a borescope to inspect cylinder walls and piston crowns
  

In one case, a 955E failed after a cold start. The operator suspected fuel issues but later discovered a spun main bearing that had blocked oil flow and caused partial seizure. The engine was removed and rebuilt with new bearings, seals, and a reconditioned crankshaft.
Engine Removal and Loader Disassembly
Removing the D315 from a 955E requires careful planning:

• Detach the ROPS to allow vertical clearance
  
• Chain or remove the bucket arms to access the engine bay
  
• Disconnect fuel lines, throttle linkage, and electrical harness
  
• Remove radiator and fan shroud for front clearance
  
• Use lifting eyes and a spreader bar to extract the engine safely
  

Technicians often forget to remove the bucket control arms, which can obstruct engine removal. In one documented rebuild, the arms were chained back and secured with clevis pins, allowing the engine to be lifted without interference.
Compression Testing and Injector Diagnosis
Compression testing on the D315 requires a specialized adapter due to its pre-cup design. Without proper access, technicians may resort to indirect methods:

• Swap injector pumps between cylinders to isolate faults
  
• Check for fuel at each injector line during cranking
  
• Inspect injector tips for carbon buildup or damage
  
• Retorque head bolts and inspect for coolant seepage
  

If compression is low across multiple cylinders, the issue may lie in the rings or head gasket. Uneven temperatures at the exhaust ports suggest misfiring or poor combustion.
Anecdote from the Field
In 2020, a restoration crew in Montana rebuilt a 955E that had sat idle for 15 years. After replacing the injectors and flushing the fuel tank, the engine still failed to start. A borescope revealed scoring on the #3 cylinder wall and a cracked piston skirt. The engine was pulled, and the rebuild included new liners, pistons, and bearings. The machine returned to service clearing brush and loading gravel with no further issues.
Manufacturer History and Engine Evolution
Caterpillar introduced the D315 in the 1940s as a successor to the D4400. It powered a range of machines until the late 1960s, when it was replaced by the D330 and later the 3304 series. The 955E was succeeded by the 955H and 955K, which introduced torque converters and improved hydraulics.
Sales of the 955E peaked in the early 1960s, with thousands deployed across North America, Europe, and Asia. Its mechanical simplicity and parts availability make it a favorite among restorers and independent contractors.
Recommendations for Technicians and Owners

• Use mechanical gauges to verify oil pressure and compression
  
• Document injector pump positions and fuel line routing
  
• Replace bearings and seals with OEM or precision aftermarket parts
  
• Clean all oil galleries during rebuild to prevent future failures
  
• Keep a service log of torque specs, clearances, and component replacements
  

Conclusion
Troubleshooting the Caterpillar 955E with a D315 engine requires a blend of mechanical intuition and methodical inspection. From fuel delivery to bottom-end integrity, each system must be verified before assuming catastrophic failure. With proper tools, documentation, and patience, even a seized or misfiring D315 can be restored to reliable service. The legacy of the 955E lives on in its rugged design and the ingenuity of those who keep it running.