Just remember the downside of the electric fan mod... You have NO cooling system if your alternator dies... And that fan (with no alternator) will kill your battery faster than a new Ranger Rover will depreciate. Well in about 18 minutes anyway.

 

I installed the inline t-stat mod late last summer, after suffering a critical overheating event (boiled it dry). After I did install the inline mod and installed an Ultragauge, I consistently run right around 177-180 with a 180 t-stat from Extinct as well. Granted, it's been colder winter temps around here for the last several months, but even on days in the 50s it doesn't go about 179-181. Towards the end of last summer when outdoor temps were still pretty warm (upper 80s, low 90s) the truck still ran low 190s.

The truck already had a slight head gasket leak when I bought it last July (thought it was an exhaust leak at the time) which recently turned into a bigger blow-by of exhaust to exterior. Luckily I haven't had any issues with coolant getting into the oil or exhaust getting into the coolant. It's loud enough now, that I'll be pulling my heads this weekend to replace the head gaskets.

 

Not that big of a problem, your truck will run less than 10 minutes without an alternator anyway. Longer post coming on that but basically the fuel pump kills it anyway.

 

Since you had sludge in the coolant, I would replace the radiator. I am running in 100 degrees weather on the freeway below 190 degrees coolant temp: new radiator, 180 degrees thermostat, new fan unit, new coolant. I did it step by step and each made a small difference.

 

so I installed the inline thermostat and it helped a bit but I am still getting 208-212 and occasionally 215 for a second until the aux fan kicks in when driving 75 mph with the AC on in Texas where the air temp is around 80 degrees right now.

without the AC on - the temp stays below 200 (largely 194 for the most part) even at 75 mph on highway or sitting at a light idling.

All of that is honestly is only a few degrees different than what I was seeing before inline.

You are saying that is a problem and I should not be seeing that, correct?

 

Your temps look too high to me, especially with the outside air temp not being too hot. I would say you have an issue like a partially clogged up radiator, or perhaps a viscous fan not doing it’s job, but if those temps are still high as you’re travelling on the highway, I wouldn’t really expect that the fan is the issue.

 

in 2020 the Disco2 had a new water pump, Nissens radiator, new cooling hoses and coolant reservoir. I guess they radiator could be clogged but that seems unlikely given its not that old.

Also the fact that I only have temp problems with the AC running and does not happen each time but most commonly when I get to higher highway speeds - like over 70 mph and if I slow down the temp definitely drops - makes me wonder if there is something related to that causing the issue - just not sure where to start.

Is it normal to continue to have large swings based on driving speed even after the inline mod?

Would a small air leak potentially cause that type of issue? or extra air in the system overall that needs to be purged?

 

Sounds like you may need a new radiator (again) if you had sludge when you changed coolant. I'd change the temperature sensor as well. I have a 180 degree OEM t-stat and run 183-188 cruising at 60 mph.

 

but if it was clogged radiator wouldn’t it have issues cooling when sitting in traffic or at stop lights for long periods of time?

it actually goes down in temp to 188-192 when I get caught in bumper to bumper traffic for example

 

@FtWDisco2 here is my typical

A small update, my original thermostat was cycling slow. So I popped in my spare.

Had a run-up to 100 Mile house in BC and back, 600 miles or so.

Temps averaged 177 to 194, depending on grade and speed, hit 201 on a 12% grade at 60 mph around 85 deg out for a couple of minutes

Except for an 8-mile 8.5 to 9% grade in 90F temp hit a whole 201F. Popped the heat on she dropped to 197 in a couple of minutes once the fan was fully engaged stayed at 196 - 198 and dropped back to 183 once I crested. Not unexpected that is a long climb brutal climb with lots of hard corners and 55mph average speed.

Drive home was largely flat and level early in the morning 171 to 181 for 3 hours, 181 to 194 the last hour.

As to temps dropping at idle if you have any kind of blockage or flow restriction that is expected. Functionally you try to jam more fluid through than the system can take so it "backs up" underload and flows better at lower rpms.
Also did you the bleed hole in the thermostat to the top it is very important to help purge the air out.