Well its kinda good to hear others with the same or similar temps, but it also kinda worries me. I want to build this to wheel and overland. I’m in Bakersfield and we routinely have temps in the 110’s. That plus 100 easily has me riding 210. How are you guys in the hot climates like me able to take this thing 20+ miles off-road in the heat and not have an overheating issue? Are you guys with the inline mod running the cap loose or tightened? I remember reading the post about lower boiling temps when pressurized but the In-line mod sticky recommends a loose cap. I know I am stressing the disco with oversized tires and a roof rack, but I haven’t even added bumpers and a winch yet.

This morning on the way to work prior to a hill the ambient was about 80, and the temp was 180-187, while climbing the hil it got up to 208 at the highest and rode around 200-203 in the flat section after that. Speed on the gauge was around 60-65, but with the tires I’m sure its closer to 70-75. You are right though when I slowed it down to a 50-55 the temps slowly dropped.

 

I typically run mine with the cap loose as I have a small external HG coolant leak and I’m trying to avoid putting pressure in the system in so far as I can avoid it. BUT! I learned the hard way that you may not always want to do this. If you truck is running warm (above 200, lets lay) and you shut it off the residual heat in the engine will continue in to your coolant, which isn’t being circulated. If your cap is loose that coolant will boil at 212 and turn to gas and start to push coolant out of the overflow.

This exact thing happened to me recently during a trip- hot day, up an incline to the air BnB, shut off the truck and it puked coolant all over the driveway. If I would have had the cap on I don’t think it would have happened (at the risk of making my leak worse).

 

I'm in the low-desert of AZ and I have been working to keep my temps low for a while. I feel like I've done everything possible. Currently running the inline mod from https://inlinethermostats.com. Really liking this kit.

Prior to that, I had the LR gray soft spring. The inline does run a tiny bit cooler and reacts faster, but in the truly hot head, it doesn't make a difference over the gray one.

When off roading and idling around a lot, I typically use 4 Lo so that when I am moving, the revs are a bit higher and more air gets moved. But, honestly, off roading in 115 degree weather doesn't typically happen for obvious reasons.

Example temps:
Ambient: 113 and dry as a bone
Cruising around town 50mph, light to light, AC on high: 197-204
Stopped at a light for a minute or 2: 201-208
Highway 65+ mph: 205-212 (once saw 215 on a long grade)

Ambient: 80-90 degrees
Cruising around town 50mph, light to light, AC on high: 188-197
Stopped at a light for a minute or 2: 194-200
Highway 65+ mph: 197-202

Ambient: 70 degrees and below
Cruising around town 50mph, light to light, NO AC: 173-183
Stopped at a light for a minute or 2: 188-194
Highway 65+ mph: 188-197

 

That's about as good as it gets for a Disco. Your gray spring must have been a good one. I have had so many, some good, some not good. The consistency of the inline mod is what drove me to permanently.

 

Yep. It's a little more consistent, a little cooler in cooler ambient, and it's a simpler design. All good things.

 

I continue to run my trucks with the caps one turn loose to eliminate the risk of splitting hoses or plastic components. The coolant shouldn't boil until about 225ish but the heat soak is real. One of my trucks has a coolant level such that when full the manifold line can push enough coolant up in to the cap area when running and it dribbles out the overflow tube. Not big coolant loss, but I can smell it on my daily commute and it is irritating to have to top off every couple of weeks. I connected a hose to the overflow tube and ran it through the cowl corner hose and up under the a pillar trim, exiting midway up on the windshield so I could monitor it. The level goes to about the cowl height and never gets higher, no spitting or vapor out of the top of the tube at all.

 

I'm not sure about the loosening of the cap method. I mean, if you have a head gasket problem, yeah, it makes sense. But, these trucks are designed to run at 20psi. The pressure helps lower the boiling point and it seems to cool more efficiently. I had the CARRS 4x4 aluminum reservoir on my Disco. It only has a 16psi cap and it would spew coolant from time to time. Once I put the plastic one back on, no spewing and my temps were more stable. That's just my experience. If I can find a 20psi cap for that reservoir I'll put it back on.

 

I'm in Florida. My temps get up to 208-212 when driving 65+ on the highway. Here's my most recent experience.

Ambient: 90 and high humidity
Cruising around town 50mph, light to light, AC on high: 194-197
Stopped at a light for a minute or 2: 197- 202
Highway 65+ mph: 208-212

 

Pretty much the exact temp results I've had on 4 D2's over the years.

 

The high ambient temps and associated driving temps you have are pretty close to what I am seeing as well. I haven’t really had a chance to drive it much in nice cool wether to see how it acts then. Bakersfield is a hot unyielding hellscape in the summer. That being said it is where I live so I want to go wheeling occasionally when it is 100+ outside and still run my A/C. It is crazy to me that these motors wouldn’t be able to handle an incline in the heat at low speeds without me having a panic attack staring at my Ultra-Guage.