I meant that you might take the thermostat out and running your car without it. Was a bit ambiguous. The thermostat is quite a restriction to flow. I have solved heating problems in two engines by taking the thermostat out.

Because your water temperature runs at a certain temp does the cylinder head? I would doubt that the water temp is equal to the temp of the cast iron. It has to adsorb heat from the metal and probably is slightly lower than than the block/head. Maybe you could point your ir meter at the coolant hose flowing out of the engine or at the coolant sensor. Maybe you have done this.

Are the passage ways inside the block corroded and restricted? Is your engine running inefficiently causing lots of heat.

Lots of good ideas from others as well.

My car with 200 on it ran at 95deg I changed the thermostat (both were labeled as 80deg) and it runs now at 85.

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.