So I think I have an idea what's going on, but I figured I'd throw this to the collective and see what you have to say. '99 DII, stock thermostat and radiator. At the moment, it's about 40 degrees out. At highway speeds, it hangs around 196-198. As soon as it gets below about 35, the temperature starts climbing toward 212 (the temperature the condenser fans turn on at), which is where it seems to want to live.

I'm leaning toward a faulty fan clutch, but if anyone has a suggestion for what could be causing this, I would be grateful.

One other thing that could be related; the heat is fairly mediocre. It's not blowing cold, but it's also not what I would call fantastic. I would say lukewarm.

 

Tstat tstat tstat and as far as heat maybe your heater core is getting plugged up

I would update the tstat (be careful where you buy it )
and even a new rad wouldn’t hurt if yours is old
eBay brand one are good $80

 

I have a stock ‘99 and my temperatures are similar (although mine tops out at 207 in the city). I need to replace the t-stat as well because that still seems to high for me.

Just as an added bit of information, the way the cooling system works is that when the engine is at idle there is not enough pressure generated by the coolant pump to press open the spring in the t-stat so nearly all of the coolant bypasses the radiator and cycles through the heater core. A plugged heater core (or an air pocket, have you bled the system?) would cause temps to climb because the heater core can’t get rid of the heat, which would explain your lukewarm heat output.

The warm weather t-stat not only opens at a lower temp but also doesn’t allow as much radiator bypass at idle speeds (as I understand it).

 

I did bleed the system. I haven't tried flushing the heater core, but that's been added to my list of stuff to do.

Just to be clear, a 180 tstat will be happening at some point, but my primary concern is that even in this weather with the tstat fully open, the system doesnt seem to be able to maintain temp at low speeds without turning on the condenser fan. That's a problem that will be dangerous when it's actually warm out.

This morning, I went out to test the fan, and it turns just as easily when cold as it did immediately after the engine was turned off. So I'm assuming it's pretty well knackered. I'm not assuming that's going to fix everything, but I'm calling it item 1 on my to do list.

 

It does seem like higher temps at lower speeds/RPM's would indicate that the fan clutch may not be engaging when needed. At higher speeds RPM's you are getting airflow due to speed.

 

So I ran another test this evening, and I'm pretty much back to the drawing board. Started the car cold, and you could see the clutch disengage and then reengage around 170. It's possible that it's still not disengaging fully, but it seems to be operating as intended. I've ordered a new expansion tank cap because why not, but I'm a little baffled by this behavior. I'd rather not throw a bunch of money at it just to see what sticks, but there may be a complete cooling system overhaul in my Disco's future. Lord knows it probably needs it.

Side note: the heat works great as the car is warming up and at idle. I may just chalk that up to being unfamiliar with the heating system, but I'll keep an eye on it and keep you posted.

 

The test for the fan clutch is to warm the car up then stop the engine, then try to spin the fan manually, if the fan free wheels or rotates easily the clutch is shot. I don't know that I would depend on watching/listening the rotation of the fan necessarily, they can go bad along a continuum as the silicon leaks out over time. While I definitely understand the desire to not throw money at things the fan clutch is reasonably cheap (aftermarket, Hayden 2991, I think) and you don't have to replace the fan like you do with 03+ Discovery's.

 

What I did was get the engine up to operating temp, then have someone turn the car off and time how long the fan takes to stop. It was about a second and a half, so the fan's definitely firmly coupled. Nevertheless, this thing's got 215,000 miles on it, and nothing about it would indicate that the prior owners replaced anything on it ever, so at some point the cooling system will need a complete overhaul anyway, so you're right, I might as well start with the bits that don't require me to drain coolant when it's 30 degrees out. I've also got a wary eye on the water pump; poor coolant flow could definitely be causing some of these issues.

 

The water pump either works, or the seal leaks and/or the bearing goes bad. The impellar(part that circulates the coolant) is attached to the pulley shaft, so the flow rate isn't going to change and effect cooling.
The only way it could would be if the impellar shaft broke(extremely unlikely), in which case you would have almost no circulation, and overheat within a few minutes of start up.
The waterpump is not your issue.
As said, it sounds like your thermostat, and possibly a gummed up heater core.

A fan and fan clutch is $60, and worth replacing.

 

That's fair. I'm looking into how the thermostat works, and it seems very complicated, but I'll just throw a new one in there and see what happens.

Anyone have any tips on how to take out the thermostat without dumping all the coolant or buying $45 worth of pinch off tools?