Right... I just put my viscous coupling in the oven! Sounds daft but I wanted to check it locked up when it got hot. However, the hotter it got, the easier it was to turn by hand. To me, this sounds like a bust viscous coupling. Any thoughts?

Thats what I'd expect it to do.

The hotter the fluid gets the LESS viscous it gets.

QED.

Thats what I'd expect it to do.

The hotter the fluid gets the LESS viscous it gets.

QED.

But if it's easier to turn by hand, it's easier for the fan to resist the rotation of the engine. I.e. the hotter it gets, the less the fan rotates. Surely that can't be right.

Thought the viscous coupling was there to level out the airflow over the engine/ radiator at various speeds, i.e. car stationary; fan turns at max speed, car moving; air flow generated by cars movement and viscous coupling allows fan to slow!

I thought as temp increased the slip on the coupling decreased and so it would make the fan turn faster increasing airflow - so when going fast you get a lower temp so the fan spins with less force as opposed to when idling and the temp increases so the fan spins faster as the coupling grips and draws in more air.

 

Are you checking it coz you suspect its bust? Phone Nissan parts up and run it by them and see what they say. Middlehurst 01744 26681 are good. Your local dealer arnt they?

When its cold the viscous coupling oil is thick and does therefore transmit more drive to the fan and when well warmed up it slips properly. When the car over heats however a bi metallic coiled sprin clutch expands and locks the fan up so that it is 100% driven.

 

If you've got it off you should be able to see the spiral spring clutch.

I had one lock up on a 2.8i capri,

sounded great - made a kinda power roar - so i left it hehe

just a bit of useless information.. :-)

hehe my dad had an old mazda a few years ago with the fan clutch locked up all the time, that thing was LOUD :eek:

Ok. The reason I ask this is I still have a problem somewhere with engine cooling. I've rebuilt the engine (for another reason!). I've changed the thermostat and water pump as part of the 60K service. I've had a new radiator as the original burst itself open. I've changed the radiator cap. I've changed every water hose in the engine. But still I've had this overheating problem for ages!

 

It's exactly the same since all the new bits went on. At exactly the same point of my journey into work (give or take a mile), the temperature guage starts to rise. The only way of controlling it is to turn the heater on full.

 

All I can think that is left in the cooling system is the viscous coupling. The electric fan is working. Everything else has been changed. What's left?!

I know its stating the obvious but could it be a faulty/inacurate temp gauge?

Are u sure it is actually overheating?

I know its stating the obvious but could it be a faulty/inacurate temp gauge?

Are u sure it is actually overheating?

I know what you mean but it's definately running way too hot. If you don't turn the heater on you can feel the change in the engine. And eventually, the water boils and pours out the overflow. It's only done this a couple of times a while back as I'm always ready with the heater now!

ok, its puzzleing then aint it!

It cant be serious if the heater fan is enuff to bring temp back down can it?

You've done everything to the cooling system so it has to be operating correctly.

What about the actual fan blade? is it the correct one/size? maybe its not drawing enuff air thru?

ok, its puzzleing then aint it!

It cant be serious if the heater fan is enuff to bring temp back down can it?

You've done everything to the cooling system so it has to be operating correctly.

What about the actual fan blade? is it the correct one/size? maybe its not drawing enuff air thru?

Even replaced the fan mate! But I think you hit the nail on the head stating that it can't be serious if the tiny little "radiator" in the cabin heating system can bring it back under control. If this little thing can achieve what the great big thing under the bonnet can't, there's something very wrong with the great big thing under the bonnet!

It must be the rad mustnt it? not partially blocked inside with polystyrene packing beads? I had loads in mine when it arrived.

The timing effects temp too but u know alot more than me so im sure its spot on!

U had this prob before the new rad was fitted aswell?

Ive gotta mechanic mate who knows everything (well, not about Zeds, but hes put a rover V8 in his triumph 2500 ) lol , ill ask him tomo what he thinks ,

U had this prob before the new rad was fitted aswell?

Yeh, had the problem on the old rad as well. Can't be anything other than the coupling surely? I know fluid flywheels and torque convertors work on the fluid dynamics of the oils inside them as apposed to their viscosity. But surely the coupling should stiffen up as it gets hotter.

Yes mike its the coupling im sure of it. spoke to my mate.

Apparently the fluid is silicon based and the thickness of treacle,

and does get thicker (sorry guys not thinner!) as it gets hotter,

creating the friction,

try spinnin the fan when u get to work tomo , if its easy its fooked.

Check if the fins of the aircon radiator aren't blocked with dust, mud, leaves, smashed birds and feathers, paper or other stuff.

One of the Z's design flaws is that the AC radiator is usually blocking a lot of flow to the radiator and can cause severe overheating problems.

 

-Eric

Yes mike its the coupling im sure of it. spoke to my mate.

Apparently the fluid is silicon based and the thickness of treacle,

and does get thicker (sorry guys not thinner!) as it gets hotter,

creating the friction,

try spinnin the fan when u get to work tomo , if its easy its fooked.

This makes sense to me. Does anyone disagree that the fan should get more difficult to turn the hotter the coupling gets?

http://www.twinturbo.net/net/viewmsg.aspx?forum=technical&msg_id=850343

Trust me the viscous coupled fan has a bi metallic coiled spring clutch. Fact is that when the car is moving at 30+mph you do not even need a fan at all so its nothing to do with that. The fan only locks up when its really hot in traffic in summer otherwise there's usually enough airflow even with it slipping as normal.

 

Does the top hose get hard as the pressure builds?

 

When you re built it did you check for, God forbid, cracks in the heads or block? If its chucking water out it must have gas behind it.

 

Or could the thermostat be fitted wrong way round? That would cause same symptoms.

maybe the heater matrix blocked

willieo it doesnt sound that serious coss the heater fan is enuff to bring it under control.

Mike how about making a temporary small bracket to lock the fan , and prove that it is that. my moneys still on degraded/knackered viscous fluid not thickening under heat.

willieo it doesnt sound that serious coss the heater fan is enuff to bring it under control.

Mike how about making a temporary small bracket to lock the fan , and prove that it is that. my moneys still on degraded/knackered viscous fluid not thickening under heat.

Agreed. If the small additional effort provided by the heater can sort it, there's nothing serious wrong. Good idea about locking up the fan manually though. Never thought of that. Some beefy cable ties may sort that out. I'll give it a go next week.

You have to start by running the engine from cold. Leave it ticking over. Every few minutes feel the top hose. If the cooling circuit is fine the top hose will stay cold for a while then suddenly heat up. That point signals your thermostat opening and hot water getting to the radiator. The top hose should also be hard with pressure.

 

Was your radiator new or 2nd hand because it might be furred up inside with no flow. I can understand running the heater cooling it off in this weather because the outside air temp difference means a lot of cooling effect even via a heater. I bet your radiator is stone cold or has cold patches on it - feel the matrix as far down as poss after switching off. If its truly burning hot all over then i'll be amazed.

 

And sorry Trevz but the viscous doesnt get thicker if it gets hotter it just slips. The viscous coupling is designed to transfer just enough torque to shift enough air at tickover and slip when the engine revs higher unless it gets so hot that the bi metal clutch locks up then it shifts a lot more air and you will hear the noise when you rev it when it does so. It will only happen in this weather if you are stuck in a jam for at least a half hour imho.

 

My bet is still a furred up rad or thermostat stuck shut or in the wrong way round so that the waxstat bulb is on the cold side of the circuit rather than on the hot side. Meaning the only real cooling circuit open to the engine is via the heater core.

 

If you are convinced that the rad is good and there are no head gasket probs or cracks then run it with the thermostat out. It should never even reach the normal on the guage if the rad is good.

 

I've been stuck on the m25 mid summer 28 deg C aircon full on and there was no sign of it even getting too warm so I know the z cooling is good even on the TT.

Rad's brand new. Thermostat, water pump, Smithy's hoses all brand new and fitted by me. Definately the right way round. Car warms up at the right rate and stops bang on when the thermostat opens. It will then stay very stable for a long time, until about half an hour motorway driving or ten mins in a jam.

 

It's been like this for two and a half years now, no better, no worse. Turn the heater on and everything's fine. Leave it on for about five mins to return everything to normal then you can turn it off again.

 

I've just had the engine out and changed all sorts of crap. It still starts to go to hot at exactly the same point on the way to work as it did before I took the engine out.

willieo, the silicon oil used does get thicker as the engine warms up, ive been told by a reliable source :) . Its not the heat that does it tho.

Greater velocity = greater viscosity.

As the rad gets warm, bi metal spring heats up and turns arm in housing which moves the oil plate and lets more oil flow between the grooves, giving stronger connection between wheel and housing.

Apparently reasons for slow fan are,

1 loss of oil

2 bimetal spring fault

3 siezed oil plate