Originally Posted by Detroit Disco

Richard, thanks for that summary. It was very helpful.

I just spent most of last year replacing a blown head gasket on my 04 Disco II. I'm now worried that it's running hotter than it should. So i'm going to try and summarize my stats and diagnostics like you did. I'm thinking from the diagnostics below that my 180F Tstat isn't opening at 180F and therefore i should think about a new one. The Tstat is brand new from Atlantic British who are usually pretty reliable so i'm wondering if my diagnosis is correct. I think this is a reasonable start for things to look at when troubleshooting overheating problems but please let me know if missed anything and if anyone has a different recommendation please let me know. Thanks.

Engine Temps:
-Hwy temps in Michigan (70ish deg F) are 208-212F
-After Hwy driving, in traffic temps spike to 220-225F
-Monitoring temps w/ Ultragauge

Cooling System Setup:
-Replaced Tstat during my HG job with a 180F White MotoRad Tstat from Atlantic British. I'm now kicking myself for not checking w/ boiling water before installing.
-Replaced Engine Coolant Temp Sensor when I replaced the HG
-Running Peak Global Lifetime 50/50 premix amber (Disco Mike recommended if I remember correctly) with Purple Ice water wetter
-Reverse flushed the radiator when doing the HG job. Didn't see that much gunk coming out and seemed to flow ok but I've never done that before so I didn't really know what to look for.

Diagnostics:
-Pressurized system to 20 psi for 1/2 hr with no loss of pressure. (Pressure testing kit was about $80 on amazon, well worth it. Anytime i mess with hoses etc.. now I can check for leaks before getting onto the road)
-Opened bleed screw and checked for air pocket. Water was up to the top.
-I can see coolant movement in expansion tank where the throttle body heater return port is on start up (pump is pumping at least something?)
-After running for a minute or two I can stop the fan with a stiff piece of cardboard (i.e. freewheeling). This is expected fan clutch behavior per the Rave manual and wouldn't really explain my problem if the clutch was engaged at startup. I'm glad I did this though so I had a feel for what freewheeling feels like when did this again at a higher temp.
-After idling up to temperature, I can't slow down the fan when I jab at it with my cardboard. Feels therefore like the clutch is engaging properly
-Lower rad hose cool to the touch and reading 82F with IR gun as ultragauge was reading 190F. The lower rad hose didn't start to heat up until engine temp was closer to 200F
-Electric A/C fan came on at 210F with A/C on
-Shut off engine at about 212F
-Checked top of radiator fin temp w/ IR gun and about 6 " lower (that's about the lowest I could get to shooting through the grill openings). No significant difference in temperatures.

Conclusions:
-Fan clutch looks ancient but the fan blades seemed to chew up my little cardboard poker pretty well so i'm assuming not a fan clutch problem
-Water is definitely moving but other than that don't know how well the pump is working
-The radiator actually looks pretty new, I can't imagine its the original. I reverse flushed it when I replaced the head gasket and didn't see much gunk. Lack of a big temp difference from top to bottom makes me think the rad is ok.
-Feels like the Tstat isn't opening as early as it should. Since this is the cheapest and easiest fix i'll probably try this first but i'm a little stumped since I think this brand (MotoRad) and supplier (British Atlantic) are usually reliable.

Originally Posted by Gary Fallon

My Disco 2 has a overheating issue, I started today by bleeding the system, checked spark plugs for signs of a bad head gasket. Spark plugs look great and so does the oil. Took it for a ride 8.3 miles and had great heat, pushed the heat to the defrosters. At 8.3 miles the heat went totally out and my guage needle moved up so I shut it down. I waited 5 minutes andstarted it up, I again had great heat andstarted back to my home, at 6.2 miles the heat went out and it started to overheat. I shut it down. Waited 3 minutes and started it up, great heat and I got her home about 2 miles. Went out and purchased a new thermostat, put the new thermostat in and basically got the same thing. The day was shot and I am in the same place.

Originally Posted by Jason_B

I would get an exhaust gas tester and test that to make sure it’s not head gaskets or cracked block / sleeve issues. Online thermostat won’t help that. A regular thermostat should not be sending the temps to the red. Especially this time of year.