It is common to sometimes find a single hole in a stat where the jiggle pin has been drilled/pulled out to allow for a small flow of hot water through the radiator at all times; this has several benefits such as stopping a radiator from flash freezing in VERY cold conditions (Siberian cold!) and also means that true engine temperature water will sit right up again the wax stat.
Now the fact that there have been many holes drilled in it suggests to me the previous owner has had issues with it at least READING too hot, at this point we don't actually know that it was actually getting too hot - it may have been. So they may have been assuming the gauge was correct and altering the cooling system to make the gauge say what it should!
A few questions to help diagnose the fault, is the engine original to that Land Rover? What colour is the little band round the terminal on the temperature sender (mounted just next to the stat housing) it should be black. What colour is the coolant, blue/green/rusty is it transparent or opaque?
If you can open the expansion tank up and it doesn’t boil then we are still within the bounds of non damaging temperatures. The engine would happily sit at 100° C all day long that wouldn’t damage it, to be honest the water galleries in the block up against the cylinders probably has water up at 100°C and higher during normal running anyway. With the correct quantity and type of coolant the boiling point will have been raised and the system is pressurised to further increase the boiling point. So bear all this in mind when doing your diagnostics.
Standard car temperature gauges are designed to scare people, they must assume all drivers are like that blonde that sits in the outside lane on an empty motorway doing 50mph – not that she would notice it mind you. They want you to stop and turn off your engine, when this gauge goes into the red the engine is still at a reasonable operating temperature, it’s just hotter than the manufacturer ever expected it to get. I would want to prove how accurate the gauge is, it could be that there is a mismatch and the gauge is reading hotter than it really is, coupled with that, I would want to get an idea of how well my cooling system appeared to be working.
If you have the viscous fan fitted you may want to take it off for the next test, 32mm spanner, give it a good thump to the RIGHT with a hammer and it should undo, just turn it off the pulley and set aside. You say you gauge goes into the red after two miles? Now drive it 2 miles or just as it goes into the red, get out and see if you can put your hand against the thermostat housing? You could also stick your fingers – or even better a thermometer into the expansion tank and see how hot it is. I would expect to still be able to hold my fingers in the water without burning them unless it was being driven flat out up a hill or something. If it’s not that hot then I would be almost CERTAIN that it’s a gauge issue.
If however it is hot then stick your hand onto the radiator to feel the different temperatures on the rad, just watch out for the belt. Feel the top of the rad, it should be hot, then feel across the top and it will either feel all the same or it may even start to cool, then do the same across the bottom, from the point of entry to the point of exit you should be able to trace a temperature drop (do this quickly after stopping because you have no cooling when your stopped with no fan so it will all start to heat up). If it feels like it’s doing its job then we can, at least for now, assume the rad is OK.
Let us know how you get on.
