Thanks again for all of the replies, I will let you know how it goes. ...starting with the clutch!

 

What fan clutch did you buy?
Did you see the tech section and the fan clutch replacement with the alternate part number?

 

Why is everybody suggesting fan clutch? This clearly shows it is working fine....

My first bet would be radiator. Has it ever been flushed? I have seen it happen first hand on three trucks in the past year, one of them was mine... Take the truck to a radiator shop and have it flow tested.

I would replace the t-stat anyway because they are so cheap.

 

X2 of the fan clutch sounds ok.

re: started to heat up while I was driving. Normally it takes about 30 - 45 minutes to get hot and if I turn the heat on it will drop the temp back down to normal.

If by "get hot" you mean that EVERY time you drive 30 - 45 minutes the temp rises above the mid-point of the gauge (9:00) then you do indeed have cooling problems. When turning on the heater, you are not increasing water flow (in a 97 D1 and later there is no heater control valve, heater core is always circulating). You are cooling the hot coolant slightly. If this is happening while you are driving 45 mph or so, the radiator is not doing the job it is designed to do, either plugged with mud and leaves in the fins, and/or sludge up inside with who knows what and calcium from non-distilled water use. Also, if it takes a long time to reach 8:00 - 9:00 on the gauge, thermostat could be stuck open, or slid down when being installed and left a crescent shaped opening between top of stat and the mounting hole, which is covered by the gasket and does not leak. If you take out radiator top plug and shine flash light inside you can see edges of some of the tubes, if any white that is calcium. Most bang for the buck is to remove your self, take to small indy shop that works on tractors and construction equipment (Rover MSRP does not enter the equation), and for under $100 you can have it hot acid flushed, the side tank removed, core rodded out, tank re-soldered.

I had mis-mounted stat, bad fan clutch (free wheeled), AC fans wired to run backwards, belt on wrong, leaky wobbly water pump, and radiator that would not flush out with reverse water flow, white vinegar, commercial flush, and removing and soaking with muriatic acid (all standard DIY fun). But the trip to radiator shop made radiator behave like a new one. It is brass and copper and can take the acid.

Simple check - when warmed up, radiator should be about (10- 20F degrees) the same top to bottom on the fins. If lower part is much cooler, it is clogged and not very much water is passing thru there. Test by hand with engine off.

Fan clutch is an issue at slow speed and idle, unless fan blades installed backwards....

 

The problem is pretty consistent.

I can drive for about 45 minutes and the temp gauge stays normal (below half).
After that time it begins to heat up slowly. I can have it idle, have the AC on, and drive at any speed and it doesn't heat up for the first 45 minutes. Honestly I wouldn't have even seen the problem because I rarely drive more than 20 - 30 minutes at a time. The clutch I bought is a BECK/ARNLEY that I bought for 120.00 from Rockauto. It could be that I got a bad tstat when I replaced the old one and I can try to replace that again since they are pretty cheep. There is no mud, leaves, or anything stuck in there and I don't see white when I look in the radiator.

 

Return that fan clutch and go to advance auto and buy Imperial 215157. You will need to enlarge the bolt holes on the fan but it's only $50 and cools better than the OE rover clutch.

 

But as others have pointed out, if you can drive the truck at a constant trot down the road, say 35 - 40 mph, and heat starts to creep up, it is not the fan. You can drive truck (in an emergency) with no fan if you keep moving and only stop for a quick traffic light. The air flow thru radiator is considerable - just try holding a large pizza box out the window at 50 mph...

You really need to look at the radiator.

The thermostat has the job of controlling how cold the engine runs. When it gets cold, it turns off the flow of coolant so engine can warm back up. Otherwise you would never have heat in the winter, and engine would run badly at 20 F outside. The radiator and the water pump work together to control how hot the engine gets once thermostat reaches wide open temperature (nameplate plus 15 - 20 F in the US). Water flows through radiator, radiator needs to be large enough that water spends a little time in there to cool off. Put a big radiator from a big block V8 on a pickup with a small six cylinder and you won't over heat. You also won't have enough heat in winter. Go the other way, use a too small radiator, or take the one you have and reduce the capacity by filling half the tubes with calcium and dexcool sludge, and you just can't keep up with the heat from the engine.

I think you have documented the behavior so that it is unlikely to be the thermostat sticking. That would happen in a few minutes after start up. It is unlikely to be the fan, since it happens while moving as well as while at idle.

Important point - "hot" and "over heat" and "too warm" mean different things to different eyeballs. The guage is slow to respond, and "averages" readings over a wide range. If your gauge normally stays around 9:00, when it moves to 9:30, things may be warmer than you think. A scanner or Ultra Gauge lets you complete work, retest, and stop test before you get hot enough to make gauge move a great deal.

Pix of related items, radiator is not a D1, but shows the idea of sludge blocking tubes, Ultra Gauge on the world's champion mpg Disco, high flow water pump from BP Utah.

 

I agree with Sav it is likely the radiators slightly restricted flow..

I would however change the fan clutch as preventitive maintainance because it will be the next to go if it has never been replaced.

 

Check the fins on the radiator. I have a 96 and 98 and on both the fins were rotted badly. So badly I could take my finger, with almost no pressure, run it across the fins and they would all just fold over or flake off.
Get the radiator re-cored with an extra row. It will be cheaper than a new one and the original is better quality than the ProLine.

 

There's a trick I haven't thought of, I'm going to check this with my temp gun and get some numbers on mine.

In a properly operating radiator would you expect to see uniform temps top to bottom?
And the opposite would be true of a poorly flowing radiator? A temperature difference top to bottom?

I don't know the history of my rad and I'm going to get some temps off it asap now that you mentioned this simple method.
Cheers!

Also wanted to add that I just ordered an Ultragauge and it's on the way to me. They're around $70 right now on sale.