P38A P38 2.5 diesel viscous fan rivets.

UnitLandy · 2025-01-15T11:51:18+0000

In the process of putting on new fan belts and noticed the viscous fan coupling has failed. (it's like trying to turn a blade fast in cold tar when the engine is cold. Is that broken?)

I notice the petrol fan viscous coupling uses bolts instead of fixed rivets to the plastic fan itself. Can I use a petrol viscous coupling if I cut the rivets off the diesel fan/VC and just bolt a new coupling to the old fan instead of buying a whole new fan with VC attached. Or is there a reason you need to buy the whole fan with VC for the diesel?

kermit_rr · 2025-01-15T11:58:12+0000

the viscous fan should be hard to turn by hand when the engine is cold. After a few revolutions it should get easier as the fluid moves around

UnitLandy · 2025-01-15T11:59:33+0000

How can you tell if its knackered? I've got it off the engine at the moment.

UnitLandy · 2025-01-15T12:00:47+0000

I thought it works the other way round. When it's hot it it's hard to turn?

UnitLandy · 2025-01-15T12:07:10+0000

kermit_rr said:
the viscous fan should be hard to turn by hand when the engine is cold. After a few revolutions it should get easier as the fluid moves around

Your completely right! Was spinning the nut and it was tough but after a good few turns it's loosened right off. So it isn't knackered! I think that's correct...

MrGorsky · 2025-01-15T12:25:41+0000

You can double check once you've got it all back together by shoving a rolled up news paper right up it's jacksie with the engine running. When cold the fan should stop, when warmed up the fan should shred the newspaper, if you get the opposite to this then the fan isn't working as intended. I'm not sure how safe this is.... either for you or the fan.

I prefer to warm the housing with a hot air gun or hair dryer. It should get much stiffer to turn once warm.

RangeRoller dt · 2025-01-15T12:25:58+0000

If you roll up a newspaper the VC should shred it. The petrol version is a bit dainty for the diesel. Another option is Datateks mod which does away with the viscous altogether. Personally I prefer incremental cooling but it’s good to know your options.

UnitLandy · 2025-01-15T12:44:21+0000

I'll give that a go! Funnily enough it's gone stiff to turn again. I was turning it before and then all of a sudden it loosened right off! But it won't do that again. I've been turning it for a while.

RangeRoller dt · 2025-01-15T12:55:38+0000

There’s cheap patent part replacements but the OE is a fair few quid. More than what I would call reasonable. You can get just the viscous part alone and change it over, but I don’t remember rivets? Perhaps yours is non standard to begin with. As a general rule once you start the engine cold it should roar like a tank when revved, as it warms it should settle down.

kermit_rr · 2025-01-15T12:58:16+0000

yes, leave it to sit cold for 15 mins and it will go stiff to turn as the fluid settles. give it a few turns and it should loosen up until it gets hot, at which point it will stiffen again and be hard to turn

MrGorsky · 2025-01-15T13:00:42+0000

That roar from the fan is my favourite part of starting my car.

UnitLandy · 2025-01-15T13:06:13+0000

Is this from factory or after market?

Grrrrrr · 2025-01-15T13:53:09+0000

UnitLandy said:
In the process of putting on new fan belts and noticed the viscous fan coupling has failed. (it's like trying to turn a blade fast in cold tar when the engine is cold. Is that broken?)

I notice the petrol fan viscous coupling uses bolts instead of fixed rivets to the plastic fan itself. Can I use a petrol viscous coupling if I cut the rivets off the diesel fan/VC and just bolt a new coupling to the old fan instead of buying a whole new fan with VC attached. Or is there a reason you need to buy the whole fan with VC for the diesel?

Petrol turns the opposite way so has opposite thread.

Grrrrrr · 2025-01-15T13:54:08+0000

UnitLandy said:
Is this from factory or after market?
View attachment 333341 View attachment 333342 View attachment 333343

Looks like it is off a bimmer.

UnitLandy · 2025-01-15T13:58:48+0000

Is that BMW? As it is a BMW engine.

brianp38dse · 2025-01-15T14:02:40+0000

They are shear bolts designed for the nut part to come off when its tight you can undo the with a few tools

UnitLandy · 2025-01-15T14:06:20+0000

Will a bit of penetrative oil plus mole grips do or should I drill them?

Datatek · 2025-01-15T16:53:14+0000

The viscous coupling from the petrol cannot be used, it's designed to rotate in the opposite direction.
The fluid goes into a storage compartment when the fan is rotating controlled by the bi-metallic coil on the front. The spindle the coil is attached to can seize. A good dose of Plus Gas will free it off. Off the car, fluid will leak back from storage making the fan hard to turn, how hard will depend on the angle at which the fan is held relative to the port for the fluid.
Personally I cannot abide viscous fans so both my P38's use the aircon fans for cooling, a very simple mod to do. gives much quicker warm up in this weather.

UnitLandy · 2025-01-15T17:10:01+0000

Thanks guys. I think it's actually okay. I'll test it again tomorrow to see if it spins freely after turning it a few times.

Datatek · 2025-01-15T17:40:43+0000

UnitLandy said:
Thanks guys. I think it's actually okay. I'll test it again tomorrow to see if it spins freely after turning it a few times.

If it's on the car, it should still be stiff to turn when cold with the engine off, the coupling requires the centrifugal force of rotation to return the fluid to storage.
As others have said the way to test it is with a rolled up newspaper. The fan should roar at start up until the fluid is returned to storage and then free up, at that point, a tightly rolled newspaper should be capable of stopping the fan. When the engine is hot, the fan should shred the newspaper. Just take care that you do not shred your hand.

P38A P38 2.5 diesel viscous fan rivets.

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