I have a 2003 Disco with 100k miles. When I purchased the vehicle it had a copper T in place of the thermostat (I didn't realize that when buyin the truck). I ended up doing most of Mike's recommended high mileage maintenance. I flushed the coolant, installed a new factory thermostat, checked the radiator fan (viscosity seemed good). I also bought an Ultragauge so I can read temps while driving. The car ran for three weeks with the temps right at 195 degrees once it warmed up (3-4 minutes), it might go up to 203 if I sat ideling for a long time. Now it runs at about the same temp 197 but the second I sit idle (even on a cool day, outside air temp 65 degrees) the temp goes up to 220 in a matter of 2-3 minutes. Feeling the lower radiator hose under the Tstat it's much cooler than the two coming out of the top. Is the T-stat stuck? If so, is there something that might have cause a new one to fail so fast (lasted 1 month)?

 

I've read a number of times where people say these don't always open as they should. They recommend you boil them on the stove to verify what temp they open. I thought that was mostly with the Aftermarket ones, but I guess it could be OEM too. I have had about 5 new thermostat failures over the years, for the record. That's out of about 50 I've installed though.

 

What kind did you get? I would recommend the Low temp stat. Make sure you get e genuine one. A few vendors sell them and we have them for $55 if you need it.

 

If it's just when you sit idle, I'd lean towards a weak fan clutch or restricted radiator.

 

It's a factory stock thermostat. I don't think it's the fan clutch, the temperature starts to change within seconds of idling. With the engine off, the fan has that "peanut butter" viscosity that I've heard on the forum. The fan clutch must have been replaced at some point, it has a manufacture date stamp of 2008.

It seems to me (but I don't have the experience everyone on this board has) that if the two upper hoses are hot (ie 210 degrees hot) and the lower hose is much cooler (easily 40 degrees cooler) that the thermostat is stuck closed. Is that not necessarily true? Drove it to work today and watched the temps on the ultragauge, 195.8 while driving 70 mph (fluctuated 2-3 degrees depending on accelleration). I put the truck in neutral a few times while driving 70 mph for a short period, the temps imediately started to climb, back in gear, RPMs up and temp immediately starts to drop.

 

So the first clutch lasted 5 years, and it's been 5 years since 2008

 

Not sure on the mileage between the two and the second one was manufactured in 2008, may not have been installed sam year.