I have been trying to fix my overheating 1998 Disco 1 for sometime and been having no luck. It is MUCH harder of a problem because I live in Sierra Leone (West Africa) and it is very hard to get parts or find mechanics that know what they are doing.
Symptoms:
Sporadic overheating usually at speeds above 35mph. The AC does make the problem worse. I say sporadic because I can go days without any symptoms or It can be as quick as 15 min. It rarely gets hot when cruising slow on dirt roads.

What I've checked:
I have replace the fan clutch with a used one. I flushed the radiator and there was no sign of build-up. I replaced both AC fans and they are currently working. I have flushed and bled the cooling system countless times. I have bypassed the heater core just in case there was air getting trapped. I have since put it back online. I even welded my old fan clutch so it is always blowing full blast just to check that that is not the problem. The belt is tight and routed correctly. Yesterday I replaced both head gaskets. I am stuck! I don't know if I should try to get the heads machined here, but I don't have any more ideas. Please Help!

 

Either there is debris stuck between the radiator and a/c condenser or the radiator is partially clogged.
I am assuming you still have a thermostat in place and it is in known working condition.

 

I have pulled the thermostat out as I don't have a replacement.

 

That may be your problem, without a thermostat the coolant will not be in the radiator long enough to be cooled.
Thats why at higher speeds (faster engine speeds) you are overheating but not at lower engine speeds.

 

Let us think together from your current condition.

1. No thermostat. When I ran mine this way, it would heat to about 135F on a digital scanner, even at 90F outdoor temp, while driving at 50 mph level road. So we know that the thermostat is not causing your temperature spikes. Without one, your miles per gallon will suffer, and there is increased engine wear at lower temps. So OK for diagnostics, but eventually pop one back in. I'm currently running a 180F. Have also run a 160F. Some guys drill a small hole in the flange of the stat (1/8 inch or smaller) that allows any air bubbles to pass by, such a hole or built in jiggle device would go at the 12:00 position when installed.

2. I suffered from trash, leaves, and mud stuck in radiator fins and between the AC condenser and the radiator. Had to pull out the rubber strip that seals them to get at it.

3. With all the work done on the truck, it is sometimes possible to get the fan blades or wiring to electric fans backward. This makes air push out the grille of the truck, and a plastic bag held there won't be sucked toward truck. This air can cancel out one flow or the other, and work at idle, but overheat at some speed. The main rad fan can also be reversed, the proper fit is for the cupped side of the blades to go toward the block. My previous owner had also replaced electic fans, and spliced them backwards, so they ran reverse, with AC on overheat was always certain.

4. Fan belt route is also important, don't want water pump turning the wrong way. The welded clutch may make more load on water pump bearing and create a failure at some point. Here's a pix of a fan and clutch I used from a 2000ish Chevy truck, may not be a lot of these in your area. It is a 6 inch clutch, counter clockwise rotation. Pix shows proper blade orientation, and top belt route. Be sure your coolant cap is tight and in good condition. The 15 psi adds a lot of heat degrees to the boiling point.

5. Radiator flush may not be enough. I had a radiator shop unsolder one of my side tanks, and they use small rods to knock out the calcium build up. Major improvement. This will show up as the radiator being much hotter at the top rows and cooler at the bottom rows. The build up chokes off or restricts the bottom rows, so they appear cooler, my rad shop says they would normally be within 10 - 20 F top to bottom if all is normal. In trying to cure mine, I removed the radiator, laid it flat, tried reverse direction flush, tried willing it with water and "white vinegar", and even tried muriatic acid (brick washing acid). Each made some improvement, and certainly brightened up the copper. But did not complete fix. The rad shop had a hot citric acid they pump thru, and it flushes and dissolves, then they rodded out. Maybe if I had left acid in longer, but was afraid it would eat thru metal, a real issue with alumnum rads. Pix of a D2 radiator to show build up on lower rows.

Since it does not overheat slow, you don't report water rushing sounds in the plumbing under the dash (exhaust gas or air bubbles in coolant), seems like HG are not as big a suspect. There are two temp sensors. The one with a single wire drives just the gauge. The one with a square top drives the ECU. A OBDII scanner can show he digital temp, which changes much faster than the slow moving gauge. You can see that a problem is there before the gauge starts to move off 8:30 - 9:00 position. People have had bad temps sensors or grounded wiring.

How could you add more cooling, with no parts stores on every corner? If you can come by another radiator or transmission cooler or heater core, from a donor vehicle, it could be spliced in to the heater core plumbing, or that big fitting at the 5:00 position of the thermostat hole. That gets hot coolant flow at all times, so it would be additional cooling. While we can buy a new radiator for about $480 - 600 US, that may not be much of an option in your area. The radiator shop I used worked on heavy equipment, like tractors and bulldozers, the D1 radiator was a teacup for them.

 

Forgot to mention - if you unplug the wires going to the AC compressor, you can turn on AC, and the electric fans should come on. This would provide more cooling slow, but at higher speeds not much change. I really think you issues are based in the radiator, which is not an easy item to come by. Now the D1 radiator was also used by some other Rover models, but with different adapters, I had tried to swap one from a Range Rover and the fittings for oil cooler and transmission cooler were slightly different.

 

Savannah:
that big fitting at the 5:00 position of the thermostat hole. That gets hot coolant flow at all times, so it would be additional cooling.

That sounds "cool"

Is that a bolt head or is it part of the thermostat housing casting or whatever?

I am guessing you mean drill it out or what?

 

It is some sort of bolt/plug, and I have seen pix of something thread in there on other sites, maybe an older model. Like a 3.9. See

and more at http://www.conehead.org/Projects/Lan...%20-%20web.htm, very complete. I like the painted plenum, maybe they should be Camel Yellow....

 

Please please please tell me he did not paint the inside of his engine, it looks like he painted the valley.
When that paint comes off it will ruin his engine.

 

Worse than you feared: