Komatsu PC75UU-2 Drain Valve and Heater Circuit Troubleshooting

[MikePhua]

The Komatsu PC75UU-2 and Its Urban Engineering Origins
The Komatsu PC75UU-2 is a compact hydraulic excavator designed for urban and confined-space operations. Developed by Komatsu Ltd., a Japanese company founded in 1921 and now one of the world’s largest construction equipment manufacturers, the PC75UU series was engineered to meet the demands of tight job sites in Japan’s dense cities. The “UU” designation refers to its ultra-short tail swing and offset boom configuration, allowing it to work close to walls and in narrow alleys without sacrificing digging depth or reach.
Originally painted in Komatsu’s domestic market purple or gray, many PC75UU-2 units were imported into North America as gray-market machines. These units often featured unique configurations, including under-cab fuel tanks and heater circuits not commonly seen in export models. Sales of the PC75UU series exceeded 10,000 units globally, with strong adoption in Southeast Asia and Eastern Europe.
Identifying the Mystery Valve Beneath the Cab
Operators occasionally discover a small valve beneath the cab of the PC75UU-2, often half-open and leaking fluid. This valve is typically mistaken for a coolant drain or heater shutoff. In reality, it is most often a fuel tank drain valve, designed to purge water and sediment from the bottom of the tank.
Terminology annotation:

• Fuel Bowl Drain Valve: A manual valve located at the lowest point of the fuel tank, used to remove water and contaminants.
  
• Coolant Overflow: Excess coolant expelled from the radiator when pressure exceeds the cap’s rating.
  
• Heater Core Circuit: A loop of coolant routed through the cab to provide heat during cold weather.
  

In one case, an operator noticed fluid pooling beneath the machine and assumed it was coolant. Upon inspection, the valve was found to be partially open, and closing it stopped the leak. However, the coolant level continued to drop, indicating a separate issue.
Heater Core Leaks and Pressure Management
The PC75UU-2 features a cab heater circuit fed by coolant from the engine. A shutoff valve on the engine controls flow to the heater core, but only the supply side is blocked—leaving the return line pressurized. If the hoses to the heater core degrade, they can leak even when the heater is turned off.
To resolve this, some operators bypass the heater circuit entirely by looping a hose from the supply to the return port, effectively removing the heater from the system. This is a common solution in warmer climates or when cab heating is unnecessary.
A technician in British Columbia shared a story where a PC75UU-2 was losing coolant during summer operations. The leak was traced to cracked heater hoses behind the cab panel. After bypassing the heater and replacing the radiator cap, the system stabilized.
Radiator Cap Selection and Pressure Ratings
The radiator cap plays a critical role in maintaining system pressure. If the cap’s spring weakens or its seal fails, coolant may overflow prematurely. The PC75UU-2 typically uses a cap rated between 1.1 and 1.3 BAR (16–19 PSI). Using a cap with too low a rating can cause frequent overflow, especially during extended use.
Operators have found that matching the cap by physical dimensions and pressure rating at local auto parts stores is possible, but not always reliable. OEM caps, while slightly more expensive, offer better fit and longevity.
Recommendations:

• Replace radiator caps every 1,000 operating hours or when overflow symptoms appear
  
• Use caps rated at 1.3 BAR for optimal pressure retention
  
• Inspect overflow hoses and reservoir for cracks or blockages
  

Panel Access and Hidden Leaks
Accessing the heater hoses and coolant lines often requires removing side panels. On the PC75UU-2, the left-side body panel may hold residual coolant if leaks are present behind it. Removing this panel can be challenging due to hidden bolts and tight clearances.
Tips for panel removal:

• Use a magnetic tray to hold bolts and prevent loss
  
• Loosen all visible bolts, then gently pry the panel from the bottom edge
  
• Watch for trapped fluid and wear gloves to avoid burns or chemical exposure
  

In Nova Scotia, an operator discovered coolant trapped behind the panel after removing just a few bolts. The leak was traced to a pinhole in the return hose, which had been rubbing against a bracket.
Preventative Maintenance and Long-Term Solutions
To prevent future leaks and maintain system integrity:

• Replace heater hoses with high-temperature silicone variants
  
• Install shutoff valves on both supply and return lines for full isolation
  
• Use hose clamps with smooth inner surfaces to avoid cutting into rubber
  
• Flush the coolant system annually and inspect for sediment buildup
  

For machines operating in cold climates, consider installing an auxiliary heater with external plumbing to reduce strain on the factory system. In Japan, some PC75UU-2 units were retrofitted with diesel-fired cab heaters to improve winter performance.
Conclusion
The Komatsu PC75UU-2 is a compact yet complex machine, with design features tailored to its original market. The valve beneath the cab is most likely a fuel tank drain, but coolant leaks often stem from the heater circuit and radiator cap. With careful inspection, proper pressure management, and strategic bypassing, operators can restore cooling system integrity and keep the machine running smoothly. Understanding these quirks is essential for maintaining gray-market imports and ensuring safe, efficient operation.