Diagnosing Overheating in the CAT D6B Dozer with the D333A Engine

[MikePhua]

The CAT D6B and Its Mechanical Heritage
The Caterpillar D6B dozer, powered by the D333A engine, represents a generation of machines built for rugged reliability and mechanical simplicity. Produced during the late 1960s and early 1970s, the D6B was widely used in land clearing, grading, and forestry. Its belt-driven water pump, mechanical fuel system, and open cooling architecture made it a favorite among operators who preferred hands-on maintenance over electronic diagnostics.
Caterpillar’s D333A engine is a naturally aspirated inline-six diesel known for its torque and longevity. Unlike later models with gear-driven water pumps and electronic control modules, the D333A relies entirely on mechanical systems—making it both durable and vulnerable to age-related wear.
Initial Symptoms and Field Observations
After sitting idle for over a year, a D6B was restarted for property work. Within 45 minutes of operation, the temperature gauge climbed to 220–230°F, and coolant was observed leaking from the radiator cap area, dripping onto the hood and ground. The machine had previously undergone head work and a thermostat replacement, and had held coolant levels consistently until this incident.
Key observations:

• Coolant visible on hood and front grill
  
• No contamination found in engine oil or breather
  
• Leak intensified during high-RPM brush pushing
  
• Radiator required two gallons of coolant refill
  

Terminology:

• PRV (Pressure Relief Valve): A valve that regulates pressure in the cooling system, preventing over-pressurization
  
• Radiator cap: A spring-loaded cap that maintains system pressure and allows overflow when exceeded
  
• Belt-driven water pump: A pump powered by a belt from the crankshaft, circulating coolant through the engine and radiator
  

Cooling System Pressure and Leak Detection
The first diagnostic step is to clean the coolant residue and perform a pressure test on the radiator. External leaks are often caused by degraded hoses, loose clamps, or failed seals. In this case, the radiator cap and PRV were identified as potential culprits. The PRV was found to be dry-rotted and sticking, allowing coolant to escape prematurely.
Recommended steps:

• Clean radiator and surrounding areas with water and compressed air
  
• Install a cooling system pressure tester and monitor for leaks
  
• Inspect radiator cap seal and PRV for wear or sticking
  
• Replace PRV with updated part number 5S1218, noting compatibility with existing cover
  

One technician noted that the fan on the D6B pushes air forward, which explains coolant being blown onto the grill and hood. This airflow pattern can mask the true origin of leaks, making pressure testing essential.
Mouse Infestation and Airflow Obstruction
After 1.5 years of inactivity, the radiator area may have become a nesting site for rodents. Mouse colonies can obstruct airflow, chew through belts and hoses, and leave corrosive waste that damages components. Removing side panels and inspecting the radiator core is critical.
Inspection checklist:

• Look for nesting material between radiator fins
  
• Check belts for chew marks or fraying
  
• Inspect hoses for punctures or swelling
  
• Clean radiator fins with compressed air or low-pressure water
  

In one case, a mouse nest blocked 40% of the radiator core, causing overheating within 30 minutes of operation. After removal and cleaning, the machine ran at normal temperature.
Water Pump and Belt Condition
The D333A engine uses a belt-driven water pump, which can fail due to bearing wear, seal leakage, or belt slippage. A water pump seal kit was on hand, and replacement was planned. If the pump fails to circulate coolant effectively, overheating will occur even with a functional radiator and thermostat.
Signs of water pump failure:

• Coolant seepage from weep hole
  
• Belt squeal or excessive slack
  
• Overheating under load despite full coolant
  
• No visible flow in radiator neck during operation
  

Replacing the water pump requires removing the fan belt, pulley, and pump housing. It’s advisable to replace the belt and tensioner at the same time to ensure proper operation.
Radiator Cap and PRV Compatibility Issues
The original PRV part number 4F2478 was found to be obsolete. Cross-referencing revealed that 5S1218 is the current replacement, but may require a different cover. The complete assembly, including cover and gasket, is listed as 8H8719. Alternate part numbers such as 2S2036 and 7S4327 may also fit, but compatibility should be verified before purchase.
Parts guidance:

• 4F2478 → superseded by 5S1218
  
• 8H8719 → complete PRV assembly
  
• 7S4327 → includes PRV and cover, may replace 3H3413
  
• 5S1211 → alternate PRV cap for newer configurations
  

Operators should be cautious when purchasing refurbished PRVs, as internal spring tension and sealing surfaces may not meet original specifications.
Preventative Measures and Long-Term Solutions
To prevent future overheating:

• Replace radiator cap and PRV with verified compatible parts
  
• Flush cooling system and inspect for sediment or corrosion
  
• Replace water pump and belt if age or wear is evident
  
• Clean radiator core and inspect airflow path
  
• Monitor temperature gauge during high-RPM operation
  

Some operators install auxiliary temperature sensors or infrared thermometers to monitor engine block and radiator outlet temperatures in real time. This helps detect early signs of overheating before coolant loss becomes visible.
Conclusion
Overheating in the CAT D6B dozer is often a symptom of age-related wear in the cooling system—particularly the radiator cap, PRV, and water pump. With methodical inspection, pressure testing, and part replacement, the issue can be resolved without major engine work. In legacy machines like the D6B, understanding airflow patterns, part compatibility, and mechanical behavior is key to keeping the iron cool and the work moving forward. When the temperature climbs, it’s not just the engine under pressure—it’s the operator’s judgment that keeps the machine alive.