On a Discovery 1 with 171K, I found the following multiple heat problems. PO had ignored some of them. Disco would run normal temp at highway speed, or at any speed on cold days, but began to overheat when in city traffic on a warm (72 degrees) day.

1. Fan shield was missing when I bought the car. Picked one up at the salvage yard. The distictive bump seems to supply extra air flow from the main fan to cool the alternator. On a warm day the alternator was very hot, when I replaced the shield the temp seemed cooler, you could actually keep your hand on the alternator. Since I have gone through two alternators in two months, this might have been part of that problem.

2. Space between radiator and air conditioner condensor is surrounded by removable rubber slide gaskets. My space was 35% full of leaves, bugs, etc. Good to check this from time to time, silicone spray makes the rubber gaskets slide back on easily.

3. Viscous fan was suspect. Did show some stiffness when turning cold, but I had no reference point as the truck was new to me. Inspection showed heavy dark grease on radiator side of fan clutch, so it had been leaking for a while, with rotating speeds forcing out more and more grease. Replaced fan and clutch. New unit was louder, much more air flow. When cold, enough air flows to spin the plastic high temperature fans in front of the condensor.

4. Serpantine belt was threaded wrong by PO. This was the first item I changed when I bought the car, and probably impacted fan speed. On a Disco 1 with air conditioning, belt does not form an arc across all the upper pulleys.

Moral - The possibility of multiple problems increases in direct proportion to the damage that can result.

 

Sounds like you bought a heavy duty fan clutch, that will lower your MPG as well as your engine temp in the winter.
Glad you got it all figured out though.

 

My original post said "new", however, the unit was a fan and clutch removed from a Disco 1 at the boneyard with 215K. Normally I would only use a key part like this from salvage in a pinch. But this unit had a paper Land Rover parts sticker on it, pretty clean and legible. So I guessed that the salvage unit had replaced the fan at some point, and I would think that stickers would not survive long, between heat, oil spray, etc. I bought it to test as part of the diagnosis.

The salvage unit appears physically identical to the unit removed.

How would I distinguish a heavy duty from normal unit?

A decrease in MPG is not good, I am already at 15 MPG - maybe 16 if going down hill. My F-250 5.8 liter gets a whooping 12 MPG (0-60 in 3.2 gallons), and my Mercedes 300TE does 23 - 25 (and out leaks the Land Rover by volume, but can't match the quantity of leak points).

 

Go over to the tech section in the DI forum and read up on the Chevy fan clutch replacement.