I am new and don't know much (stop laffin) did you check the reservoir? My 96 was doing the same and that ended up the culprit..seems it was cracked along the seam. Just trying to help...

 

Okay, not trying to hijack the thread, but might be relevant to your situation as well. Mine is doing the same thing. Runs steady as a rock for about 30 minutes & then the temp shoots up. Found out the electric fans were shutting off. If I turn the Disco off wait about a minute, they run fine again. No idea why, but they are doing it. Leave teh AC off & even at 101d and it runs fine. Relay maybe??

 

If you are going slow and electric fans off, then that could be part of the problem. If going highway speed then it is more than that. The fan relay is switched on by the fan switch for HVAC on the dash applying a ground. Try wiggling switch. May have to remove some dash plastic and electronic spray clean the switch. On a D1 at your miles may want to consider boil and rod out of radiator, about $70 near me at indy shops. And I'll go ahead and put in a plug for the 180F thermostat, a great $10 upgrade IMHO.

 

I might as well update where I'm at with it, so others can benefit, if possible.

After replacing the thermostat for a 180d unit, cleaning-out the crud between the radiator and the condenser, flushing the cooling system with a detergent flush and replacing with antifreeze / water-wetter / water (per recommendations), it was still getting way too hot, most particularly after a freeway blast and then stop-n-go on the surface streets.

I finally decided I was going to pull the radiator and have it gone-through at my local radiator shop. I figured I'd replace the fan-clutch while I was at it. A scheduling issue meant I couldn't get to it when I wanted to, but since replacing the fan-clutch is only a 20 minute job, I decided to get it done one afternoon.

fixed the problem. NO more overheating whatsoever.

so that was probably it all along, even though it seemed to pass the "should spin / shouldn't spin tests.

I'll still need to pull the radiator some day soon (if I keep this POS) because I've got a little weep on the right-side tank, at the trans-fitting.

 

Plan on the rad work, and you may also be looking at a WP because that 150K one has been married to the weak clutch, now a stronger woman is in the house and all hell will break loose.

 

One thing to consider when dealing with an overheat issue. If running the A/C makes it overheat worse, why did you keep running the A/C while you knew you had a problem?

Were you trying to damage it or just did not think the problem still existed?

 

I'm not clear if you're directing this at me, or at the new poster.... but I'll respond either way.
Maybe because it only overheats when the A/C is running, and so one needs to runt he A/C to determine if the steps one has taken have improved the situation.

 

btdt.

 

I would drive it a while to be sure it is cured and be easy on the A/C for a while.

Lots of times with a "new" one you will encounter a series of problems until you get it all sorted out. Good Luck with the Discovery journey.