Diagnosing Overheating in a Caterpillar D333 Marine Engine with Keel Cooling

[MikePhua]

The D333 and Its Marine Adaptation
The Caterpillar D333 is a robust inline-six diesel engine originally designed for industrial and construction applications. Introduced in the 1960s, it became a staple in dozers, generators, and marine conversions due to its mechanical simplicity and torque-rich performance. In marine setups, the D333 is often paired with a keel cooling system—a closed-loop freshwater circuit that transfers heat to seawater via hull-mounted piping.
Keel cooling eliminates the need for raw water intake, reducing corrosion and clogging risks. However, it introduces unique challenges in circulation, bleeding, and thermal regulation, especially when retrofitted or disturbed during repairs.
Terminology Annotation

• Keel Cooler: A series of metal pipes mounted along the hull that dissipate engine heat into surrounding water without direct seawater contact.
  
• Closed Freshwater System: A sealed coolant loop using antifreeze or treated water, isolated from external sources.
  
• Thermostat Bypass: A condition where coolant flows around the engine without passing through the heat exchanger, often caused by missing or faulty thermostats.
  
• Air Lock: A trapped pocket of air in the cooling system that prevents fluid circulation and leads to overheating.
  

Symptoms and Initial Observations
After repairing a transmission oil cooler leak, the cooling system was refilled and restarted. The engine began to overheat at idle, with the reservoir tank and water pump housing becoming hot, while the pipes leading to the keel cooler remained cold. This indicated a lack of coolant flow through the heat exchanger.
Key symptoms:

• Rapid temperature rise at idle
  
• Hot engine block and pump housing
  
• Cold keel cooler inlet and outlet pipes
  
• No improvement after thermostat removal
  
• Pressure gauge reading 160 psi
  

The unusually high pressure reading raised concerns. Typical marine cooling systems operate below 20 psi. A 160 psi reading suggests either a faulty gauge, steam buildup from boiling coolant, or a blockage causing pressure to spike internally.
Root Causes and Diagnostic Strategy
The most likely causes of overheating in this scenario include:

• Air lock preventing coolant circulation
  
• Thermostat removal causing continuous bypass
  
• Blockage in keel cooler lines or fittings
  
• Water pump failure or impeller damage
  
• Incorrect bleeding procedure
  

Recommended diagnostic steps:

• Reinstall thermostat to restore proper flow routing
  
• Fill the reservoir completely and bleed air from the highest point in the system
  
• Inspect keel cooler lines for obstructions or collapsed hoses
  
• Check water pump impeller and drive belt for wear or slippage
  
• Replace pressure gauge to verify actual system pressure
  

In one fishing vessel retrofit in Nova Scotia, a similar D333 setup experienced overheating after a coolant flush. Technicians discovered that the keel cooler inlet was positioned too high, trapping air and preventing circulation. Repositioning the fill point and adding a bleed valve resolved the issue.
Bleeding and Circulation Techniques
Keel-cooled systems are prone to air locks due to their horizontal layout and elevated piping. Proper bleeding is essential to restore flow.
Steps for effective bleeding:

• Fill the reservoir slowly until coolant reaches overflow
  
• Open bleed ports or loosen fittings at the highest points
  
• Allow coolant to escape until no bubbles are present
  
• Cycle the engine at low RPM to encourage flow
  
• Monitor temperature and pipe warmth during operation
  

If no bleed ports exist, temporary fittings can be installed at the thermostat housing or upper coolant lines. In one tugboat retrofit, adding a manual bleed valve at the top of the engine loop reduced startup overheating incidents by 80%.
Water Pump and Flow Verification
Even with correct bleeding, a failed water pump can prevent circulation. The D333 uses a mechanically driven centrifugal pump, which can suffer from:

• Worn impeller blades
  
• Shaft seal leakage
  
• Bearing failure
  
• Drive gear slippage
  

To verify pump function:

• Check for coolant movement in the reservoir during operation
  
• Measure temperature differential across pump housing
  
• Inspect impeller visually if accessible
  
• Use infrared thermometer to track flow path heat rise
  

In one dredging barge in Louisiana, a D333 pump impeller had corroded to half its original size, reducing flow and causing chronic overheating. Replacement restored full cooling capacity.
Final Thoughts
The Caterpillar D333 remains a reliable marine engine when paired with a properly functioning keel cooling system. Overheating after system disruption often stems from air locks, thermostat misconfiguration, or pump failure. With methodical bleeding, component inspection, and flow verification, the engine can return to stable operation.
In marine cooling, flow is survival. And when the coolant moves clean, the pressure stays low, and the pipes run warm, the engine stays ready for the next voyage.