At those speeds, it sounds like your radiator is blocked.

Be cautious about removing anything from a cooling system if its not working as it should do.

In addition to the other checks already mentioned I'd consider the condition of the coolant, too much water in glycol based cooling systems can cause overheating. Are you losing a lot of coolant?

The rad cap may fail which reduces the pressure in the cooling system and fools the temp sender into thinking the engine is boiling.

Another thing can be seeds/vegetation/mud blocking the fins of the radiator - not easy to spot with plastic shrouds around the rads.

Most viscous fans have a manual locking tab which can be pushed/pulled/bent to lock the fan on, if the fan is at fault.

Thanks for all your messages:

The thermostat is out; we have a long drive down to the Mauritanian border so if we still have a overheating problem I will re diagnose before the dessert proper:

Thanks again:

Ps: What is the best tyre pressure for sand: the recommended pressure for off road - info found on the inside of the door - is 1.75 front 2.1 rear: Ive been told i should go 1 in front 1.5 at the back:

Thanks

Overheating on the freeway with a speed above 100km/h doesn't sound as a drivers issue (as long he/she finds fifth gear and is riding in 2wd).

But I agree, when riding in sand the driver is a big factor in heating the engine, as stated, keep the revs as low as you can/dare without stalling. Especially with the 2H engine (HJ60) which has it's best torque with low revs (latest/newer engines different story!). 20/30km/h in third gear in soft sand is no exception, but keep your momentum tho and be prepared to shift into lower gears quickly. Ride in 2wd, lock your freewheeling hubs and only engage 4wd in tricky situations or when you are bogged.
Always keep an eye on your temp-gauge, if temperature is getting to high, turn your heating on!, stop as soon possible, turn the nose in the wind, open bonnet, keep the engine running (making some revs may help getting the engine on normal temperature quicker). When riding long patches of soft sand in hot conditions with strong wind from behind it can be difficult to find the right mixture of riding-speed, revs and gears to keep the engine cool (not enough airflow through the radiator, making more revs to let the viscous fan make an airflow, but more revs will heat the engine more, etc.), that's when a aditional electric fan could be handy.

To get back on topic:
With overheating you mean temp-gauge going to red or coolant bubbling out of expansion bottle? If only the latter change cap on radiator.

First check; are there any airbubbles in the system? check if the heater works (also the small heater under the front passenger-seat found in North-European models) to clear the system of airbubbles.

To me it sounds like your radiator is shot, bummer.
Inspect your radiator thoroughly, try to find any leaks or traces of leakage (can be very/very small) when the engine is warm touch the radiator if you feel any cold spots the channels are corrodated, any leaks can be welded easily in Morocco/Africa but a rusted radiator has to get replaced (totally or just a new cooling block) more difficult to find and probably expensive.
Next, inspect all the hoses for any traces of leakage (brownish or whiteish traces of chalk) and check/adjust the hoseclamps. When the pressior is getting high (hot) any leak in the system will disrupt the flow of coolant and will cause overheating same goes for airbubbles, after cooling down does the system goes vacuum (the hoses go flat)? If so, you have a small leak in one off the hoses (which closes itself when cooling down caussing vacuum) or the small hose from cap to expansion-bottle is blocked.
Check the resistant of the viscous fan (as stated earlier).

You can check the thermostat, although easier said than done, by putting it into boiling water with a thermometer checking if it opens/closes at the right temperature. Notice there are two types for this engine, rated 88 or 90something degrees (etched on the rim). If it's working correctly it should open just a tiny bit at the rated temperature (rather difficult to determine but doable).
Athough removing the thermostat is common practice in Africa. In fact it is a standard procedure by most mechanics at the first sign of overheating an engine and you can get away with this with most brands and engine types.
But with the 2H engine I was strongly advised not to take out the thermostat! Although it seems to work pretty good.
It has something to do with the specific design, the thermostat has two 'valves', a big one regulating how much coolant is going to the radiator, and a small one at the bottom which regulates how much coolant is going thru the waterpump (if I recall the explanation right). Opening the big one (big loop) blocks the other one and viceversa removing the thermostat will open the big one (as desired) but will not block the small one. A trick of the trade by a befriended mechanic is putting a few spacers on the axle of the thermostat to force it to stay open. Confronted with a faulty thermostat (working at about 100 degrees) I adepted this advice by drilling a few small holes in the big valve (was a bit afraid of loosing the spacers in the pipe-system), works brilliantly!
Athough this turns out to be a perfect bush-mechanic solution (as temporaly removing the thermostat is too) I will replace the thermostat as soon as possible.

Which solution suits you best, will depent totaly on the situation, in which state your vehicle is, and what you are intent to do with it. Exploring the desert will put much more stress on your cooling-system than going further down south on tarmac, afterall once beyond Mauritania there wouldn't be much more situations you find yourself doing +100 km/h anyway.


regards and good luck

Try the above tire pressure, but realize there is no exact perfect pressure it depends on the vehicl, load, tires etc. Bacically you underinflate the tires so they widen out and give you more surface area, but not to low so that the bead breaks on the tire. I have always gone by looks more than pressure. Basicaly I let it out till it looks right. It should look down on pressure when looking at it, but not flat if that helps. BTW I have never been to the middel east and am told the sand there is powder type that is not encountered here in the US. I have done lots of off road driving but mostly in mud and mountains with some sand along the river areas so conditions there may warrent diffrent results. Just don't keep spinning the wheels once you are stuck and bury it to the alels. You can still get out, it just takes a lot more digging and work.

[quote=NCR]
The thermostat ONLY function is to allow the engine to reach it's working temperature faster [quote]

Sorry, but this is wrong.
The thermostat is also there to provide a slight restriction in the cooling system. This slows down the flow of coolant through the engine and allows the coolant to effectively draw heat out of the head and block.
With no thermostat in the engine, the coolant flows around too fast and doesn't have time to transfer the heat properly. The result: Your car will appear fine for a while, but will eventually lose control and very quickly overheat.
We've experienced the same overheating symptoms as you have, with our turbo diesel Toyota Surf. The mechanic though that it had cracked a head, but it turned out to be as simple as a partially blocked radiator and that the previous owner had removed the thermostat.
If you are concerned about overheating, why not try a thermostat that is rated to open at a lower temperature?
That's my 2 cents anyway...
Dan.

removing thermostat
 

What John says is correct. In case of thermostat trouble it is better not to remove the entire thermostat but only remove the internal moving part and put back the housing. The housing with a reduced internal diameter works as an orifice and limits the coolant flow through the engine!

In case of visco coupling trouble; open it up and put a few screws in or two half old piston rings orso and close it again. With the debree inside it will be mechanically locked and rotate at engine speed.

Cheers,

Noel
www.exploreafrica.web-log.nl

Some good cooling advice on this thread.

One final thing that hasn't been mentioned - electrical problems can give you a false reading on a temp dial. We've had several phantom overheats that have been traced back to loose earths or other such electrical voodoo.

Flicking lights, or other devices, on and off to see if they affect the dial is one way to check this, though it sounds unlikely to be the culprit if the problem is speed-linked.

overheating
 

Richard,

Sounds you are driving a Landy! Poor earting lead on the chassis results in high temperature reading in the gauge on Landies

Cheers

Noel

Don't tell me this is another fault unique to the sturdy cube? I'll stick to the delusion that other cars are equally flaky!

Dont remove the thremostat - its there for a reason
 

Thermostats don't just help the engine heat up they keep the engine at a constant temperature.

Without them every time you tax an engine (up hills, over dunes) it will run warmer and then cool down a bit as you go back down the other side, then warmer again at the next hill and so on...

These constant cycles of heat/cooling (which you can see by a few degrees on the temperature gauge) expand and contract everything, and importantly fatigue the head gasket bond with the head and the engine block. While this wont effect the performance in the short term it will reduce the life of the head gasket in the long term.

it's also worth noting that in any car, the radiator and fan combination are designed to be able to over-cool the engine they go with, and the role of the thermostat is to regulate and control that overcooling effect. Without it your engine will always be running cooler, not just while you are warming the engine.

The bottom line is that Landcruisers are designed to run in hot climates over dunes fully laden with a thermostat. If your thermostat is faulty chuck it and by a new one. It will cost you less than a £5 (and in morocco probably less still) It take about 3 minutes to change it, and top up the water.

Testing it in hot water is practically it's hard to do. Thermostats generally fail cos there was no coolant in the radiator (which prevents rusting on the inside), so they get cacked up with rust and don't close or open any more.

If your engine is still running hot with a new Thermostat, there is another reason, find that other reason and fix that (most likely the viscous coupling, they dry out when they get old and don't couple as much so the radiator doesnt "overcool", or alternatively overloading). Dont bodge it by taking out the thermostat which will just disguise the problem.

As an emergency measure it's fine, and on a landrover sure (cos there's no spiritual beauty in the engineering ethos of a LR engine, which is doomed to premature failure from the minute it leaves the factory). But if you found out your mechanic had done it to your everyday commuter car to hide the fact something else was broken you'd be pretty angry so why do it to your overlanding Landcruiser?

An easy(and safe) way to check if the coupling needs the silicone gel is to let the engine run and someone turns it off. the fan is suppose to stop after another 5 turns or so. If the fan doesn't stop wuickly once you've turned the engine off, then it could be the gel. I had this problem with a Hilux, however it was fine when travelling at speed (the faster the better) because the airflow cools the engine. it only became a problem if the car was idling.

Once that was fixed, it was running fine except when going a bit faster (being faster than 100km). when the engine was heating up a bit and especially on long hill or on hot days. First thing to do is switch off the aircon (if you have one). In my case, the aircon condensor sits in front of the radiator and the blades were bent etc, so the airflow wasn't coming through. Check it out and see if it can be straightend with a gentle nudge. After all that, I also had to replace the radiator because of lots and lots of corrugated roads I travelled on here in Oz. there was a crack in one of the corners.