There are definitely Hitachi owners and mechanics on this site. I hope these guys chime in soon. Occam's Razor: "All things being equal in complex systems the simplest explanation is usually the explanation." I also think that if you eliminate the obvious that which isn't obvious becomes visible. As an example, my mother-in-law called the other day. Her car wouldn't start at the grocery store parking lot. She had a new battery installed just a week earlier. She could enter the car, power up the radio, lights, windows, etc... I was thinking bad solenoid, ignition switch, starter, maybe loose new battery connection...but the simplest explanation? Key FOB battery too low to disable security system so car could start but enough juice to shut off the other car security so that she could enter the car and run windows, radio, lights, etc... I just lucked into this solution. I told her to try her duplicate keys. I wasn't thinking about a low FOB battery...but when the car started I put the logic together. I'm wondering in your situation what is the trouble shooting protocol? The cooling system isn't that complex relatively speaking (although the Hitachi crankcase belt clutch in the video I sent you was new to me): radiator cap that doesn't hold pressure, sensor as ianjoub points out, thermostat or thermostat installed backwards (see this on a car but that was years ago), bad water pump (sounds possible), coolant can't be an issue, fan belt skipping/too loose to circulate coolant, I don't think leaks are an issue. Again, it would be terrific to hear from a Hitachi mechanic on this one. I'll bet on a bad water pump if it's making noise. Good luck, A