Thread: Discovery 3.9 viscous coupling

Just thought I would share my experience with running high engine temperatures on the V8 3.9 during this past summer.
When switching off engine after running up the mountain (Dandenong Ranges Victoria) where i live the AC electric fans would continue to run for a short period of time.
This was a recent condition as last summer no issues.
The temperate gauge was always indicating in the middle of the range however when connecting Rover gauge the temp was in the high 90 deg range. Average speed 65kph.

Original radiator re cored and thermostat replaced in late 2024.
Checked viscous coupling which roars at cold and then tested when hot with the cardboard to stall fan technique which it seemed was powering the fan ok.
Also heated the bi metal coil on the front of the viscous coupling and observed it actuated the internal valve. Not oil leaks on the coupling housing'

Decide to purchase a new viscous coupling as no other options where jumping out.

All I can say is what a difference the new viscous coupling has made.
No high temperatures when pulling up long hills in above 35 deg conditions.
When the engine gets to approximately 75 deg the fan cuts in hard which you can hear roaring which also saps a bit of horse power.

I have for may years read numerous posts about overheating issues with the rover V8 and just would like to emphasize the importance of the viscous coupling.

Originally Posted by Richard Kaye

How do you test the viscous coupling?

A few methods and a couple of test you can do.

If you have a heat gun, any heat source, like a blow torch(but be careful with flames), hair dryer, etc .. like right said in his OP, heat up the bimetallic spring on the viscous hub, and watch it move. That is the valve to allow the silicon inside the hub to circulate.

When cold, the hub should have some resistance, which after some rotations should free up again. When you heat the hub again after confirming 'freewheeling' it should lockup again .. lockup being resistance, not locking solid.

I use a vice to check vc hubs, fan still attached. Grip the nut into the vice and rotate by hand.

I have one dud fan off a TD5(you can't remove the fan). It has good resistance when cold, then frees up after some spinning by hand. But no matter how much heat I then apply, it won't build resistance again .. ie. dud.
So on a car, it could feel like it's working, but definitely not sucking air when driving in hotter conditions.

Some folks test when on the unit is on the car, get the engine up to temp and then use various methods like a rolled up paper, cardboard or whatever to slow the fan and feel it's resistance.