Was driving the 2004 yesterday on a trip up north where it was 100 degrees all day. Highway driving mostly. Was running the A/C and noticed the temps were ranging between 204 and 210. REALLY high considering it's normal range is 185-194. The area I live in doesn't see temps above 72 most days, so I'm wondering if some cooling problem was masked by it's life spent mostly in low ambient temps.

The entire cooling system is brand new, except for the radiator. The 180 degree thermostat is an OEM unit from LR. No leaks. Both fans work great.

Radiator is original, with 82K miles on it. The rest of the cooling system is only 2K miles old.

Thermostat: It can't be the thermostat, because on the way home at night, it was running between 181 and 188 depending on uphill/downhill, with the A/C on.
Viscous fan: Temps didn't fluctuate much whether it was in-town or on the highway. Also, there's resistance spinning it by hand this morning when it was cold.
Electric fan: Just replaced this last week with an OEM LR unit, works like a champ. Also pulled over and visually verified it was working.
Expansion tank cap: Actually, I suspected this since I had a cheaper Lucky8 one installed, so went to a dealer on the way up who happened to have one in stock and replaced it mid-journey. No difference. (Also, FWIW, the OEM one is way beefier/better built than the L8 ones, and it was only $25).

The only thing I'm left with is radiator cooling efficiency.

My 2002 made the same journey last month, same ambient conditions, about 100 degrees if not a bit more. And it made the entire journey, A/C on and all, at the normal highway temps of 188-194. The only difference between the two is that the 2002 has a new radiator.

Any definitive way of testing the radiator before replacing it?

 

That sounds normal to me given I have a similar system... any ambient temps over 85 increases the coolant temp... As far as I hear the only thing that solves this is the inline mod.

Personally I don't worry about it. 210 for a short while (in the big picture) won't hurt anything.

 

I'd be fully on board with accepting 210 here and there if it weren't for having a point of comparison with the 2002.

 

Here in SETX on 2 flawless D2 cooling systems, 180F OEM style thermostats, 50/50 green coolant, Hayden 2991, 620-112 fan blade, I can see temps from 182-215F. If it was one D2 I might be curious, but I also have 2 D1's and the coolant temp range is very similar. D2's ran 210-225F for years way before OBD2 scantools could monitor temps. I had 2 bone stock 03's back in the day, sat em for hours at the beach HVAC on full blast at idle, or out on the trails, traffic, and I never blew a HG. Both had 159 & 189 with zero HG jobs.

If the engine is in good shape & the cooling system 220-225F = it can take it! I hit 229F in the XD out on the trail & she cooled down with zero issues.

 

Oh dang... OK, this makes me feel a little better then.

Also! Was just in Austin/Kyle area this last week! First time in TX. Boy is it hot down there, and that's just in October!

 

Yep TX is pretty dang hot & SETX is very very humid.

They #1 key to engine temps is when you notice a spike or warmer than normal temp if it climbs or eventually levels off and goes back down. If it climbs = investigate & if you see it go down = it should be fine.

I do pre-flight checks on all my LR's and it just takes 30sec tops. Few years ago I drove 5.5hr to Austin, I stopped at a 7-11 and topped off the fuel tank. While stopped I popped the hood just to check it. The guy across from me was in a 90's Toyota Land Cruiser and he said something like thats why I drive a Toyota!!! I don't have to pop the hood to constantly check it... I just smiled back & cranked up my D2 and pulled out into the intersection. He pulled out maybe 15sec later next to me. The light went green and I took off. I heard a sputtering engine & then I looked back and his flashers were on and he was jumping out and opening the hood. I wanted to turn around and ask if he needed any help since he hardly ever opens the hood, but I just laughed and kept on going.

30sec every morning to pop the hood and spot a possible issue is certainly better vs driving along and having something fail when you least expect it!

 

My truck maintains a 100 degree difference to ambient unless it is below 70 degrees

 

We had some cooler weather down in my area this week and the rusty but trusty XD was running around at 184F. Week before with the lovely humidity = 193F.

The worse vehicle I had with cooling issues was without a doubt my 08 Hummer H3 Alpha with a 5.3L all aluminum V8. Darn radiator was super tiny and it was not uncommon to see 213-215F on a daily basis. I looked into the system and I noticed GM only installed an efan like the D1/D2 setup for the Saudi market. In the USA it was just a viscous fan clutch. I did 3 things to the H3 Alpha, I found a lower temp thermostat, I installed an aux efan that I was able to get to run off of the factory PCM with just a software update to enable the Saudi market efan option and it would trigger on with the HVAC or whenever the engine went above 200F. I then replaced the plastic fan blade with a metal unit from an older 98 Ford Explorer V8 which used the same exact fan clutch, and diameter fan blade. With those changes I went from 213-215F to 195-205F tops no matter what the temp outside during the summer and during the winter it was 184-193F. Before I did all that I had tried a 100% efan setup which was NOT cheap, and I nearly blew that 5.3L up when it reached 236F on the way to San Antonio for a wedding. I removed it and went back to the factory stuff, then I worked on my upgrade by doing some research.

The main thing I see in common with the H3 Alpha and the D1/D2 = the size of the air flow thru the rather undersized radiator. I can drive my D1/D2's at 55-65MPH pretty much all day long at say 193F, but if I increase that speed to say 75-80MPH I've reached the limit of the effective amount of air passing thru the radiator to keep it stable/cool. The temps will slowly rise until the air flow once again catches back up to maintain the cooling effect. This is on 2 D1's and 2 D2's.