Hi
I have asked this question before but I didn't get an answer
I have Iveco trekker 440 with a cursor 13 engine.
I use this truck in Libya desert where atmosphere temperature rise up to 47C` in summer time
The coolant water thermostat size is 84C` and the engine temperature rise up to about 94C`untel the coolant fan start, I think it is too high.
What is the best thermostat temperature for that weather?
Thermostats available in our marker (70C`, 74C`, 79C`)

I don't think the thermostat is the problem--I would leave it alone. Instead, I would focus on the Engine Coolant Temperature (ECT) Switch, not the thermostat. The ECT Switch controls the cooling fan, the thermostat doesn't. The ECT Switch may screw into the thermostat housing--check it out.

Check/replace the Engine Coolant Temperature (ECT) Switch, not the ECT Sensor. To test the ECT Switch, remove the electrical connector. Using a multi-meter, set to ohms, check for continuity across the terminals of the switch with the engine cold. The switch should not have continuity, while the coolant is below 189 degrees F (88 degrees C). Start the engine and allow it to reach normal operating temperature. Stop the engine and check for continuity again. The ECT Switch should show continuity, when the coolant temperature reaches 196 to 203 degrees F, and above. If the switch fails to show continuity above this temperature, replace it.

Use a 50/50 mixture of the proper antifreeze and distilled water. Be sure not to use tap or well water, due to the minerals in the water. Well water can be particularly bad. The minerals in the water and the antifreeze will form phosphate scale, which will block off passages in the radiator and heater core. Phosphate scale will also form on the ECT Switch and ECT Sensor, which will adversely affect performance. Also, I would hose off dirt and sand from the front and back sides of the radiator.

Since you are operating a turbo diesel in the desert, only use the proper full synthetic heavy duty diesel engine oil. Never operate a turbo on conventional oil. The higher coefficient of heat associated with synthetics will help the engine and turbo run cooler, perform better, and last much longer.