I don't use infared, it only reads the surface temp, NOT what the coolant temp actually is. It takes a while for the block/housings/etc to warm up to the actual coolant temps and it's dependent on outside air temp as well as how long it's been running, airflow across the part you're attempting to measure, etc. Just not that accurate. It can get you within about 30 degrees of actual coolant temp and that's about it, based on my testing.
Secondly where did you get this thing from? And whereabouts are you located? I know of one (1140) that had similar issues, looked great. Almost looked new! Had cracked head, trashed center cylinder, just enough compression to run, well kinda, on the center cylinder. #1 and #3 were 500+, #2 was like 280. Just making sure you didn't buy it from him.
So that's where I'd look. First thing, fill the cold system cold engine with coolant and remove the pressure cap from the radiator. Start engine. Do you see some bubbles? If so, you have either a leaking head gasket or cracked head, both of which are somewhat common. No bubbles? Good. You've done everything else except measure actual coolant temp, which you can do with a thermometer stuck down into the filler neck, but the problem is....it's going to be HOT, enough to endanger you. Second option stuff a real temp gauge into the head's sending unit and see how hot it really gets before you condemn either the gauge, sender, or wiring. If it is in fact getting hot and there is no other explanation, pull the head and look at the stem holes-very very common for them to get plugged up, and when they do, there's nothing you can do but clean them out and put a new gasket on it. If you saw rusty looking coolant in the past, chances are good that the steam holes are plugged. They are in between the cylinders, small coolant passages about 3/16" dia in the deck of the block, almost in between each cylinder. If they plug, it will run hot-guaranteed-and many times we start replacing parts until we're blue in the face, and never give second thought to this. While the head is off, I'd highly suggest cleaning the surfaces with carb cleaner and a rag, inspecting for cracks. The carb cleaner tends to dissolve the carbon buildup, and if there is a crack usually the dissolved carbon will fill in the crack as you wipe, and it will show up like a sore thumb plain as day once the surfaces are all clean. If it did have rusty looking coolant in the past, that's a dead giveaway that the coolant was never serviced properly. They say to change/flush it every two years but seems like precisely 0.1% of owners actually do it.
I hate to say it, but I'm going to anyway--Kubota diesels are real sensitve, when overheated, they very frequently crack the head.
