Normal Operating Temperature
The reccomended thermostat for the V8 Land Rover engine is 88C (190F),
An awful lot of Disco/Rangie/Defender hot running is due to the lower core tubes being plugged up. Annual flushing is of paramount importance. Land Rover horizontal tube radiators have a VERY small passageway, for maximum contact surface area. Larger cross sectional core tubes, though far less prone to plugging, are less efficient since more water is passing through a tube of approximatelly the same surface area.
Unfortunately, Genuine Land Rover viscous fan clutches only seem to last 3-5 years. The fan clutch is constructed similar to the inside of a torque converter. The bi-matallic spring on the front of the fan clutch reacts to heat passing through the radiator. When it becomes hot, the bimetallic spring contracts, closing a rotary valve cage, causing the silicone fluid to be redirected to a chamber which has paddles, mounted to the rotating hubs (one hub is attached to the mounting stud on the shaft attached to the water pump, the other hub is attached to the body, to which the fan is mounted. As more fluid is directed to this chamber, the risistance of the fluid, between the paddles "couples" the body to the inner hub, increasing the outer hub speed to more closely match that of the inner. As the airflow through the radiator cools, the bimetallic spring "uncolis" and opens the rotary valve, allowing the fluid to go to the outside of the chamber, bypassing the paddles, and the outer hub freewheels. The causes of the fan clutches failure is either that the fluid has leaked out through the valve shaft hole, or that the shaft itself becomes stuck in the bushing, no longer able to freely rotate back and forth. This is because of dust and dirt, passing through the radiator, and getting embedded onto the front of the fan hub. An occassional shot of carb cleaner, and then a spray lithium grease lubricant helps prolong their life.
Another factor not considered in engine temperature is that of the transmission condition. A gearbox that runs hotter than normal because of burned, old fluid, or low fluid, is dumping the excess heat into the radiator. Driving conditions like towing or strong hill pulling tend to raise the transmission temp by as much as 60-100 degrees. This excessively heated transmission fluid is then run through the radiator to cool it, but the temperature differential may be so slight that it's actually rasing the engine temp. A good reason why Defenders have external transmission coolers, as opposed to internal radiator mounted coolers (Left side end tank).
Trapped air posckets cause hot spots in the engine, and do not tend to find their way to the top of the engine when underway. I've heard people expounding on all sorts of methods of burping the air from the system, such as parking on inclines nose up, nose down, and sideways. The simplest method is to remove the radiator cap,place a 6 ich funnel firmly in the hole, fill it with coolant mixture, and let the engine idle, on the level. When the thermostat opens, the air will generally flow up the top hose, and the system level will suddenly drop a few inches. Top it off (engine still running), and close up the cap.
Make sure you're using a low or no phosphate antifreeze to keep aluminum from oxidizing. This is generally what is plugging up the radiator core.
If you find you have a pinhole leak somewhere. GET IT FIXED properly. DO NOT USE ANY, ANY,ANY stop leak products in Land Rover Cooling systems. This product, while very good at sealing a leak, is heavier than water, does not remain in solution, and goes right to the bottom of the radiator. An 8 ounce container (the manufacturer of which recommends you use two containers in a V8) is enough to plug the bottom 6-8 (times three tubes per level) rows of tubes.