Let me know how did it go with your problem because my 96 Disco is having the same problem

 

Mine is a 94 with same issue, i have taking every step mentioned here and the service manual, brand new thermostat, brand new water pump, radiator flushed, fan clutch installed correctly, no leaking at all, no milky fluids, new hoses. It just boils when i turn ac on, is driving me crazy big time, i thought of head gasket blown so this morning i dropped a bottle of bluedevil and still same issue, taking care of head gaskets is not an option for me and probably not cost effective since you find a Disco like this for $1200 these days, please help.

 

reverse flush the radiator on the outside!!
The FINS which pass the air are plugged with grease and leaves and fur from horses, dogs and cats.

I am NOT kidding.
Use a garden hose or go to the spray car wash.

 

and... be sure the electric fan(s) are blowing when AC is on (D2 has special settings - D1 whenever AC on); and that they are blowing the right direction (hold a paper towel against grille, not blow it away. My bargain D1 had electric used fans installed, but wired oipposite polarity, so they ran bass ackwards. Fan clutch was blown. At idle, with AC on, the main fan would just about stop. 180-F stat helps.

Rad fins a little delicate for pressure washer.

 

Thank you for the help but i'm hopeless at this point, i tried the pressure cleaning to the radiator with no difference in temp, the viscous fan has the logo facing the front of the car, no leaks, it has the blue devil fluid on it, now happens even without AC on after 15 minutes. At this point i think i have exhaust leaking to the cooling, not enough budget to fix it, not enough time to do it, i'm freaked out, this could be the end of my small mobile business that is doing terrible already.

I used to have a Honda CRV that overheated because of weak electric fan, i took it off and it worked great, of course i couldn't turn ac on while i replaced it, but the fan was obstructing the airflow big time, i will remove one of the fans on the Disco, there is an ac less model that doesn't have more than one fan.

 

1. Don't take off fans. Unplug AC compressor. Then you get the boost cooling of the fans, without the heat load from the AC condenser. If driving on a long trip plug AC back on if truck runs OK at freeway speed without overheating. I just replaced one fan on my 97, it was barely spinning under its own power.

2. Viscous fan should be evaluated when engine hot, and off. Give it a spin and release. Should feel like peanut butter inside, and coast to a stop in under one revolution.

3. Take a paper towel, hold by grille at idle, air flow should suck it to the grille.

4. Review you belt route, it is in the tech area.

5. Blue Devil for block sealer - head gasket repair can use a very small quantity of material to plug a leak. The rest of the mechanic-in-a-jug circulates, settles to the lower rows of the radiator, and decides that needs to be blocked off also. Between sludge and Blue Devil you could have half a radiator now. When warmed up, engine off, you can feel or use an IR thermometer and measure the heat on the radiator fins top to bottom. More than 10F cooler on bottom indicates blockage. If blockage is high enough in radiator, the front of the fan clutch nevers gets warm enough to come back to full power.

6. Since you have a D1 and considerable problems, like plugged radiator, remove thermostat entirely and use a new gasket to seal housing back up. That should drop temps to 140-150 ish. Not good for optimum oil life, engine life, or mpg. May keep you rolling until business improves.

7. Parts stores sell and some even rent block coolant testers. It is a liquid that when it sniffs the coolant detects exhaust gas in coolant and changes color. $60ish tester does maybe 15 tests.

 

You are very helpful with all these instructions, thanks you very much for it, i tried the fans on all the time, it seems to make it worse. I also suspected of electric wiring/sensor giving me a bad reading because it changes dramatically when engine is turning, however it gives me lower reading when the ignition key activates it with engine off, maybe regulator issue but the boiling water sound tells me is overheated. I also noticed ticking noise lately.
I will perform all other tests you told me, i will give my feedback, this might help someone else as well.

 

Boiling water sound tells you its over heating? You mean a waterfall sound in your dash? That just means you have air in the system. Bleed it by holding the expansion tank up and loosening the bleed screw. Do you have any gauge of how hot it is actually getting?

 

I haven't checked the temp with any tool, no idea of real %, but you mentioned bleeding valve, i haven't found any on this car, IS a D1 94, manual says under exhaust manifold, it is stock engine. By the way the heater is bypassed, some say it can cause overheat but others say they had good results.

 

Bypass of the heater removes the water fall sound, and the diagnostic it supplies. It also keeps you from having the heater as a last ditch thing to throw at overheating.
If heater is bypassed the hose needs to make a loop, not be cut off as two stubs.

Radiators of older Discovery and Rangies usually have a fitting on top of the radiator, on the battery side of the truck. You can elevate the right front tire on a ramp or curb or just a steep ditch. With engien cool remove the bung plug, and top off with coolant, run at fast idle to force air up and out.

If your gauge is reading above 1/2 then you are probably really overheating. An IR thermometer can be pointed at the thermostat housing, won't be dead on but will be pretty close.

Radiator can be removed and taken to shop for rod out, they unsolder the side tanks, also flush it with citric acid, runs about $75 near me.

If you have exhaust gas in coolant, hoses can be rock hard, coolant jug can be boiling. There is a chemical test that smells the coolant, and changes color if exhaust gas present. $60 ish, does maybe 15 tests.

You could also have a bad coolant cap, the system has to operate under pressure. A weak cap will usually vent coolant.

If fans on all the time made it worse, check fans for blowing correct direction. Turn key on, AC on, don't crank. Fans should suck a paper towel to the grille on left and right side. Mine had polarity reversed to fans when I bought it and would make theninsg worse when they ran.

People can mount main fan blade reversed if they try hard. Cupped side of blade goes toward engine block.

Serpantine belt can be routed wrong, make water pump run backwards...

There is no coolant drain on the radiator.

You can also have cracked block, head gasket issue, etc. HG pats kit under $200, but heads will likely need machined, another $250.