Why Is the CAT 257B Losing Coolant and Not Producing Heat

[MikePhua]

The CAT 257B and Its Cooling System Design
The Caterpillar 257B is a compact track loader introduced in the early 2000s as part of CAT’s Multi Terrain Loader (MTL) series. Designed for landscaping, grading, and light construction, the 257B features a suspended undercarriage, a 61-horsepower CAT 3024C diesel engine, and a closed-loop cooling system. Its cab heating relies on coolant circulation through a heater core, regulated by a mechanical butterfly valve and supported by a pressurized radiator system.
CAT’s MTL series was developed to compete with Bobcat and ASV in the compact loader market. The 257B sold well across North America, with thousands of units deployed in rental fleets and private operations. Its mechanical simplicity and robust frame made it a favorite among operators working in muddy or uneven terrain.
Symptoms of Coolant Loss and Cold Cab
Operators have reported two persistent issues:

• The overflow tank drains within 2–3 hours of use
  
• The cab heater blows cold air regardless of fan speed or temperature setting
  

Initial suspicion often falls on the head gasket, but oil analysis may show no coolant contamination. The engine runs smoothly with no white smoke, suggesting coolant is not entering the combustion chamber.
Heater Core and Valve Inspection
The heater core is located beneath the cab and receives coolant via two hoses—one supply and one return. If the supply hose is hot and the return hose remains cold, this indicates a blockage or valve failure. The CAT 257B uses a mechanical butterfly valve with an O-ring seal to regulate flow. If the O-ring deteriorates, the valve may remain closed or partially obstructed.
Disassembly of the heater box reveals:

• A duct system often clogged with debris or rodent nests
  
• A butterfly valve that may lose sealing integrity
  
• A heater core that can be flushed with a garden hose to confirm flow
  

Operators have found that reversing water flow during flushing can temporarily restore circulation, but valve replacement is often necessary.
Terminology Notes

• Butterfly Valve: A rotating disc inside a pipe that regulates fluid flow, similar to a throttle plate.
  
• Vapor Lock: A condition where trapped air prevents coolant from circulating, often caused by combustion gases entering the cooling system.
  
• Sniffer Test: A diagnostic method using chemical sensors to detect exhaust gases in the radiator or overflow tank.
  
• Radiator Core Leak: A small breach in the radiator that may evaporate coolant before it drips, making detection difficult.
  

Root Cause and Final Diagnosis
After extensive troubleshooting, one technician discovered a leak in the radiator core. By placing cardboard between the fan and radiator, coolant droplets became visible—previously dispersed by airflow. The radiator was repaired, but heat still failed to reach the cab.
Further inspection revealed the thermostat had been removed entirely, likely by a previous owner attempting to bypass a heating issue. Without a thermostat, coolant circulates continuously, preventing the engine from reaching operating temperature and disabling the heater core’s effectiveness.
Installing a new thermostat restored proper temperature regulation. With the radiator sealed and the valve replaced, the cab heater began functioning normally.
Recommendations for Repair and Prevention

• Perform a pressure test on the cooling system to identify leaks
  
• Inspect the radiator core for dust buildup and hidden breaches
  
• Flush the heater core and replace the butterfly valve if flow is inconsistent
  
• Install a thermostat to ensure the engine reaches operating temperature
  
• Use a sniffer tool to rule out combustion gas intrusion
  

Routine maintenance should include checking coolant levels, inspecting hose integrity, and cleaning the radiator fins. Operators working in dusty environments should blow out the radiator weekly to prevent clogging.
Conclusion
Coolant loss and lack of cab heat in the CAT 257B often stem from multiple overlapping issues—radiator leaks, missing thermostats, and valve failures. Accurate diagnosis is essential before replacing major components like the head gasket. With proper inspection and targeted repairs, the 257B can be restored to full functionality, delivering reliable performance and operator comfort in cold conditions.