I've got a Hanix mini excavator that was repowered with a Kubota V1505-E by the previous owner. It overheats when working at high idle. At low rpm it will quickly warm up to 71C from cold, then the thermostat opens and it slowly rises to around 82C (180F) and holds around there. When I increase the throttle to about 3/4 or over and start working the machine the temperature will steadily rise until I bring it back to idle. I've seen it get as high as 106C (223F) before I stopped and gave it a break at idle. The temperature comes back down to 82C pretty quickly at idle. I pulled the rad cap and ran it at low idle from cold. There's no movement in the fluid until the temperature reaches 71C, then starts flowing. As soon as it starts to flow these bubbles begin to appear. See video below: Coolant Bubbles It had some pretty high-hour oil in it before I changed it 45hrs ago and it was dark black with no sign of coolant. The current oil also has no sign of moisture. I'm thinking I better pull the head and check for a blown head gasket or a cracked head. Am I on the right track?
If you want another test before pulling it apart, rent a radiator pressure test kit from advance auto, they loan them out for free (you pay for the tool and get your money back whenever you return it). Run the engine, if you're seeing pressures above spec coolant system pressure, good chance it's a bad head or gasket. The kubota d1402 on my excavator exhibited similar problems. The head had cracks and was leaking combustion gasses into the coolant, making the coolant in the overflow bottle dirty. I bought an aftermarket head off ebay for about $600 with gaskets, it's been working well for about a year now.