New viscous fan clutch

Hello all,

Having some issues overheating in warm traffic conditions I took the XJC back to the garage (Jaguar specialist).

He diagnosed a shot viscous coupling as the fan spun too freely so proceeded to replace the Torquatrol unit. While he was at it he replaced the thermostat with a 78c one.

Sure enough the engine runs a little cooler now, even in traffic, temp did not budge (now a bit below the centre of the gauge).

Problem solved? Later on I tried turning the fan by hand when cold and was surprised that it didn’t spin much at all. My understanding from other posts is that it should spin by a turn or so when cold and very little when hot. When giving some revs I can hear the whoosh of the fan now, although this is relative to before when perhaps it was too slow.

So I am a bit puzzled - this is a brand new Torquatrol, does this sound right? Is it badly adjusted somehow?Maybe it is just stiff when new?

Appreciate any comments. I don’t really want to make a hole in the bonnet.



XJC 4.2 (1975), Canadian spec

The basic fan clutch involves a viscous stuff that allows the fan to turn slower than the engine, but works pretty much irrespective of temperature. The improved version, which sometimes has a bimetal coil on the front, reacts to temperature by redirecting some of that viscous drive fluid away from the impeller when cold, reducing the drive a bit. However, I think it’s only effective when the engine is running. When stationary, the viscous fluid leaks back into the impeller anyway.

To elaborate a bit on what Kirby was talking about…

There are two type of clutch commonly used on the older cars. Thermal and non-thermal. The non-thermal is designed to be firmly engaged until xxx-engine rpm, at which point slippage occurs. The thermal type (identified by the bi-metal coil Kirby mentioned) is design to firmly engage when xxx-temperature is reached. Below that temperature, it allows slippage.

Jaguar has used both types. I believe the non-thermal was most common in the 70s models. If yours has a single center bolt rather then the little coil, it’s non-thermal…and what you’re feeling is normal.


Don’t worry, Nick,

I’ve got the same viscous coupling on two BL cars of the same period: the clutch allows the fans to be operated by hand, but no matter how you drive them, they will stop within about 30 degrees.



75 XJ6L 4.2 auto (UK spec)

Thank you all.

160 mile round trip this weekend in cool and generally wet conditions, motorway, country and traffic.

Per the temp gauge it now runs a little cool at times, dipping occasionally below the « N » of Normal. But this generally did not last long. I wonder though if fuel consumption suffers as a result of lower operating temp, I think I got about 17mpg which is on the low side. I was not convinced a cooler thermostat was required.

I cannot see any spring mechanism so guess it is now the non thermal type, i will check. As I rarely exceed 3000rpm I wonder if this is really optimal… isn’t it over-doing the airflow for no benefit? But still, it’s good not to have to worry about overheating.

On another subject i noticed the bolt regulating tension of the belt driving the aircon compressor was sheared!

Hoping this is not too difficult to replace.


Nick Hill
XJC 4.2 (1975)

As a rule, I never recommend thermostats colder than 180F, for two reasons: 1) There is NO benefit whatsoever; one has to wonder about the thought processes of those who think there would be. And 2) because you risk the chance that the EFI system will stay in warm-up mode permanently and never get into fully-warm mode.

There was a recent thread on the V12 forum about how many incorrect thermostats are available. The overall length of the thermostat from mounting flange to the bypass disc is critical, and most of them are wrong. I think the 4.2 utilizes the same or similar thermostat. If I were you, I’d go ahead and order a 180F thermostat and replace the one your mechanic installed.

Thanks. This is twin carbs of course but i think I will do as you suggest. Presumably will not make overheating any more likely…


where are you located? A 78°C stat seems very odd to me. I put in an 88°C stat and made sure by testing before that it operated correctly - no sense in too low operating temperatures - and indeed the first Waxstat delivered opened way too early.

Good luck


75 XJ6L 4.2 auto (UK spec)

Paris here.

Confirmed single screw type clutch.

I fully second Kirbert, Nick…

Cooling is done by the radiator - the sole task of the thermostat is to keep the engine warm by controlling circulation.

Crudely; the hotter an engine runs the more efficient it is - but for obvious reasons, like coolant boiling point, the ‘best’ compromise is some 88C coolant them set by thermostat. And the EFI management system is designed with this in mind - and to some extent also for carbs…

However, this is coolant temps at the exit from the cylinders. Local temps in the block is higher and with sluggish coolant flow as crud may build up over time; local temps may cause boiling with high thermostat settings - and boiling fluid leads to coolant ejection, which cause further overheating in a cascade effect. So lower thermostats may be advisable in some situations based on observations.

To increase the boiling point; system is pressurized to 15 psi - which raises the boiling point to some 120C. This is the ‘red’ line of the temp gauge - and while the engine itself is ‘safe’ as such; the coolant is not.

During warm-up the temp gradually rises, then ‘pauses’ as it reaches thermostat opening temps. Ideally it stops there - but it is not unusual, on the xk, to rise under load; more petrol is burnt. And, of course, with long runs down hill, nothing prevents the temp to drop below thermostat temp…

The trick is to observe the temp variations to assess the cooling capacity. The danger signal is that the temp just keeps rising - at which stage some driver action is warranted. And if system is depressurized; the coolant boils a 100C…:slight_smile:

But a 78C thermostat is seldom warranted - and pointless with a reasonable cooling system…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Thanks everyone!

XJC 4.2 (1976 - not sure why I wrote 1975 previously, must have been thinking not of my first XJC…)


I just did the calculation - if your fuel mileage related to imperial gallons your consumption equates to 16.6 litres/100km. Relating to US gallons you’d be a lot better. My car does 15 under ideal circumstances, but has no air con. It seems you’re not that far off altogether. I’d fix that belt tensioner soon and try to find out what made that bolt for the belt tensioner break - even with some corrosion it’s not that fragile;-)

Good luck with the t-stat though - it’s a nice job, if you clean everything before, have good light and some Caramba at the through-bolts. Before starting to pull things apart you might wish to test the new t-stat in a cooking pot …


75 XJ6L 4.2 auto (UK spec)