In fact the title is wrong. There's no overheating problem as such as almost all sources can tell. Instead there are lots of possible problems that may occur and lead to overheating. The following data is especially for the V8 but others engines are affected in similar ways.
So let's start and say your LR/rr/Disco overheats. Good. Are you sure? Does it really get warm or is it only the temp gauge that tells you so?
The sender and the gauge have a good tendency to go bad after some time. Yep, they are made by Lucas. If your car has already run for some years it might be a good idea to change those buggers. You can either go with a factory unit again and hope that the new one 1) works and 2) keeps working. Or you can switch to some aftermarket suppliers. I bought an VDO gauge and sender (the sender is in a short brass tube that gets spliced into the upper radiator hose) and have no problems ever since. Deadly precise and graded so you actually see HOW hot the engine is. The gauge fits in place of the genuine gauge in the 90 cluster.
For a test get a good thermometre, open the plastic bolt in the rad and drive the car until it shows hot. Get out and put the thermometre in the radiator. If it really reads hot go to the next chapter.
They have a good tendency to block inside. Take it out and have it rodded out by a competent rad shop. This should only cost around 50-70 US$ and will give your rad a second live. Have at the same time installed a release valve to empty the rad next time. Another good idea is to have 2 fittings installed for a possible later installation of thermo switches. One can activate an electric fan while the other may switch an warning light.
Range Rover rads are even more vulnerable as they have a side-to-side flow while the Defenders have a upper-lower flow.
You can check the rad easily by feeling for cold spots when the engine is warm. Any difference in temperature indicates a weak spot.
There are several different sizes of interchangeable radiators with different cooling abilities. Some have an intercooler built in, others have an oil cooler. Some even have both. From the engineering point of view it's not the best idea to mix tasks. If you want to go for the maximum water cooling get an rad that only cools water. I believe early Turbo Diesels had the biggest ones but I may be wrong on this. Talk to one of the rad specialists for Land Rovers and they will surely tell you what's the best. An oil cooler for engine oil is is normally not needed but can be fitted as an aftermarket unit below the radiator. If you have an automatic box you must use an transmission oil cooler. This one can be mounted in front of the normal rad but it will block off some air and heat up the other rad- not the best idea. But you haven't much choice.
The Intercooler needed for the TDi engine is another story. The factory ones are WAY too small to reduce intake temp to good levels. If you get an aftermarket unit it will lower the air temp by up to 40 degrees Centigrade allowing for 12% more power without other modifications except adjusting mixture.
Those can only be mounted in front of the radiator but heat output on the TDi is much lower than on the V8 and generally no problem at all.
An ugly little piece that hides behind the distributor and has a tendency to stick. If you suspect it junk it out. They aren't expensive and can stick in any position. You can get them in various ratings, the lowest being 74°C. But remember that an engine is more efficient the hotter it runs (but not boiling). You can even get aftermarket thermostats with a buil-in safety. It's a small metal tube that extends when the engine overheats and forces the thermostat to open and stay open. You might to have to shop around a bit to find them and they are almost double as expensive as standard units but worth every Penny. And they must be replaced once the engine overheated as the unit can't close again.
The V8's are sensible to ignition timing. If the timing is about 4-5° off you will get an overheating engine. I've set mine at 8° BTDC at 800 rpm (for an 3.5 EFI).
The aluminium block is very vulnerable to corrosion if the wrong coolant is used. You MUST use coolant designed for use in aluminium engines. Corrosion can partially block the water galleys and make an insulation that prevents heat transfer. Once you have this corrosion you have a problem that can only be resolved by stripping and dismantling the engine. Corrosion can even make holes inside the cylinders, allowing the water to leak inside.
Hey, that's my favorite one. The Rover engine has a special design fault in his heads.
The heads are held on by 14 bolts. The problem lies in the bolts 11-14. When they get tightened they put an load on the upper bolts, raising the heads by a slight amount. New engines are tight but as the engine gets older the heads allow hot gasses to enter the engine,oil and water galleys. This normally occurs only when high pressures= high load is applied on the engine. Those hot gasses do all sort of mess inside:
- They heat up the water. Even a small amount rises coolant temp dramatically. As this occurs only under load it's difficult to diagnose.
- They are responsible for oil leaks. Oil is pressed out by almost any seal as the vents can't take that much. Hard to find.
- They make black sludge as they burn the oil. This sludge prevents the camshaft from being lubricated. Lobes wear off and power drops.
A good indication of this blow-by is that the tin seal plate in the V makes a bulge as it's pushed up by the pressure. You can see this plate from front or rear under the intake manifold.
There are several possibilities to remedy this. You can take out the offending bolts without any drawbacks (that's what I made). The 4.6 engines and heads don't have this row anymore while the rest of the engine is pretty identical. Or you can fit an composite gasket from RPI in place of the standard metal head gasket. This gasket doesn't have the holes for those bolts either. This gaskets seals much better but also drops compression by a small amount.
Look at RPI's page for some good pictures of this.
The water pump is not especially fault-prone. But the viscous coupling is. It can fail in several ways. Usually it locks up completely allowing no free turning of the fan blades. But it can also be the contrary, so it doesn't lock even when hot. This shows as temperature rising when driving slowly or at standstill. A Rover engine is capable of idling for days without ever boiling over. If yours gets hot you might have a viscous coupling problem. Those units are horrendously expensive. I got the information that a lot of cars have the same coupling unit that fits so it might be well worth to make a trip to your local junkyard.
You loose the whole unit with a 32mm flat spanner. But remember: it's left hand tread. On the 90/110 V8 you will also have to loose the plastic shroud. There are several hard to reach bolts as well as one or two inside the shroud. Luckily none on the lower side.
Sometimes you can hear a faulty coupling rattle when you shake it. This is a sure sign that it's dead. But the contrary is not true.