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I've driven my DD6 in 40°C in Houston, Texas, in Florence (Firenze), on the Autostrada A1, in the province of Latina, and in Sardegna. So long as the cooling system was in good condition, it had no problems. Even with the radiator somewhat blocked, it had no problems so long as the car was moving forward at more than fast walking pace. V12 radiators do seem to fill up with dirt (see numerous discussions on this site and elsewhere) and the lack of free space under the saloon bonnet doesn't help air flow. The more irritating (but with little potential for damage) problem I've had is the tendency of the idle speed to drop and the engine to stall in very hot conditions. Even in England, I usually open the idle air for summer and close it for winter.
Please, do not take this the wrong way, as I am not advocating for Craig or other member's positions: I couldn't if I wanted to, as I have no experience and I am mechanically ignorant. I am happy to accept that I may well be acting irrationally, chasing the feel good factor, but since it costs so little, please, bear with my silly pursuit My logic tells me that, if the valve does not travel all the way, then not all the fluid is redirected to the radiator and, therefore, not all the radiator's cooling potential can be exploited. I spent north of €5,000 on this car, it would be silly not to invest another €30 to feel good ...
So, does anyone know of thermostats with a proven minimum valve travel of 42mm and a jiggle pin? If none are known, then I will buy the Gates 33188 and drill a hole.
Tomasso, We don't take it wrongly at all and quite understand your position. (Moreover, I'm an old guy, who spent several years working in an oil company and later as a consultant. As such, I am close to impossible to offend). I also have the low temp thermostats, so I couldn't criticise you anyway! I don't know where they came from as David Marks installed them during a service and MoT.
The only way to be sure of the 42mm with any thermostat is to first measure it, then drop it into some boiling water, watch it expand measure it again. That way, you're sure not only that it is supposed to work properly, but actual does.
Of course, you can go a little further, put a thermometer in the water and see how it opens and closes as a function of temperature.
First, I love the Amalfi Coast and return to Positano as often as life permits.
Second, besides our resident experts here, the other person who might know more than any other regarding the Jaguar V12 is Roger Bywater who "has accumulated 30 years of continuous involvement with the Jaguar V12 engine having worked as a Senior Development Engineer at Jaguar Cars in the 1970s in the Experimental Department on fuel injection, exhaust emission control and engine development." Here is what he has to say regarding thermostats:
Until about 1977 the Jaguar V12 was fitted as standard with 82°C thermostats which then changed to 88°C. This gave a small improvement in fuel consumption on the CAFE Highway Test but there was no significant effect on emissions which at that time were most critical long before the thermostats start to open.
It is a common fallacy that this made the engine more at risk of overheating because the thermostats merely set the lower limit of the normal operating temperature range and have no control of the maximum temperature. An engine that overheats with 88° thermostats will still overheat if they were changed to 74° items.
Using a lower temperature thermostat does not offer any protection against overheating. If the radiator cannot dissipate enough heat the system will run hotter until it does – until it reaches the maximum pressure limit of the system.
As well as increasing wear, running at too low a temperature can cause drivability issues through richness and excess acceleration fuelling. A higher operating temperature also helps to prevent moisture contamination of sump oil so there can be several benefits to engine life as long as the oil grade is suitable.
Many years ago it was normal to run racing engines at not much more than 70°C but that was mainly because of inadequacies of the lubricants of the time. Most modern racing engines are designed to operate at around 90°C for maximum efficiency with respect to fuel vapourisation and lubricant performance. In order to reduce drag from radiators F1 engines and some others have been operated at temperatures as high as 120°C.
So to summarize, a lower temp thermostat will NOT prevent overheating, but might cause other than less desirable issues. Engineering is often about tradeoffs, and there are sound reasons that the higher temp thermostats were spec'd. IF an XJS overheats, it is because something is amiss, it is not because of the opening temperature of the thermostats.
BTW, I struggled with the same thinking of "lower temp is better" and found the whole thermostat issue a bit confusing, so you might find my thread useful:
Tomasso, We don't take it wrongly at all and quite understand your position. (Moreover, I'm an old guy, who spent several years working in an oil company and later as a consultant. As such, I am close to impossible to offend).
Interesting! My firm trades oil & gas, so I understand your comment perfectly
Originally Posted by Peter3442
The only way to be sure of the 42mm with any thermostat is to first measure it, then drop it into some boiling water, watch it expand measure it again. That way, you're sure not only that it is supposed to work properly, but actual does. Of course, you can go a little further, put a thermometer in the water and see how it opens and closes as a function of temperature.
This is exactly what Craig recommended and, I am thinking of buying the DM thermos and test them. Worse case, I lose £30!
Tomasso, First, I love the Amalfi Coast and return to Positano as often as life permits.
My favourite road in the world. Especially in spring on a 998R
Originally Posted by Mac Allan
Second, besides our resident experts here, the other person who might know more than any other regarding the Jaguar V12 is Roger Bywater who
... CUT ... Using a lower temperature thermostat does not offer any protection against overheating. If the radiator cannot dissipate enough heat the system will run hotter until it does – until it reaches the maximum pressure limit of the system.
... CUT ...
So to summarize, a lower temp thermostat will NOT prevent overheating, but might cause other than less desirable issues.
Yes, I can see how all this does make sense. I will test the valve travel and revert.
You might also want to get your mechanic to measure the distance on your engine too. I have seen some that are 41mm, the one I measured this week was a pre-HE engine in a 1972 E Type. The distance to measure is the flange where the thermostat sits to the machined port face under the thermostat, where the foot of the thermostat will seat.
The distance marked in red is what you want to measure.
Slightly off topic, and not directly relevant to this thread's exact topic, however it is very interesting, to me at least!
Something nobody has mentioned is the speed with which the coolant circulates through the system. This is critical as I found out last summer.
I have always, always, had an irrational dislike of the radiator to water pump inlet bottom hose on the HE V12. It can rub on the engine, is a sod to fit, and has a horribly convoluted path. The water pump inlet spout points backwards as it has to in order to get room for the hose.
Now my car has electric fans, thus one less drive belt, no fanshroud as in effect electric fans come with their own, if fitted close to the rad as mine are. So there is extra room in front of the engine. Then I saw a V12 E type water pump inlet spout, part number C36960 in this diagram: https://www.martinrobey.com/jaguar/e...ump/late/23376
and it got me thinking. OEM the end of this part has a threaded plug that takes a block heater. This I got welded up and the part bolts straight up to the H.E. water pump. I made up a bottom hose as shown below that ran in front of the engine and into the radiator exit spigot.
IT just fitted and was improved in various was to make it fit better, this being the final version:
In which the fan switch was removed from the join and was replaced by a slightly angled joiner for better fit, sealing, and security, and I was going to weld a boss for the fan switch into the inlet. Job apparently done. Much easier to fit the hose, you can see it, far shorter and better path from the rad to the pump, and much better straighter-angled inlet into the pump from the inlet spout. What could possibly go wrong?
Well testing proved fine until I took her out on the dual carriageway!
I immediately noticed that the faster I went the hotter the engine became, slow down and guage dropped, speed up and up it went!
From this I concluded that these flow "improvements" had resulted in the coolant circulating so much faster through the radiator that it was, (at higher revs and thus more pumping velocity) not in the radiator long enough for adequate cooling. So off this lovingly crafted apparatus came and was replaced by the OEM system!
I still mourn it, and still detest the OEM bottom hose though!
Last edited by Greg in France; 02-24-2024 at 05:44 AM.
After four years on this forum, I'm still amazed at the ingenuity & passion of its members 🤷🏻♂️ ... remarkable, really!
@ Greg in France ... Greg, you cannot stop now! You must now design a pump regulator, controlled by a mechanical or digital switch that reads the vehicle's speed & coolant temperature and responds to them. Then test the system under various loads, adjust the system's parameters for perfect cooling, et voilà ... it's not rocket science 😋 can't be that difficult 🤷🏻♂️
Originally Posted by Jagboi64
You might also want to get your mechanic to measure the distance on your engine too. I have seen some that are 41mm, the one I measured this week was a pre-HE engine in a 1972 E Type. The distance to measure is the flange where the thermostat sits to the machined port face under the thermostat, where the foot of the thermostat will seat.
The distance marked in red is what you want to measure.
I remember the phone discussions, and the too many bottle of Red consumed.
Coolant flow speed is a science, and was explained to me in 1966, by a class mechanic thatactually CARED about his craft.
Coolant had not been invented in OZ yet, so water, AKA Liquid Mud, was used in all cars.
He explained that the thermostat NEVER simply opens and stays there, it is constantly moving. Engine/pump revs flow more water,which is passed through the radiator etc. The THROAT size of the stat is the critical design item, and is in fact based around pump flow and radiator flow etc etc. Radiators need time to cool what is inside them as it passes through, too slow, and the Inlet to the engine is too cold, and the stats work overtime. Water too fast, the Inlet is too hot, the stats open to attempt too cool the engine, and run out of travel, and are at "wide open" and the stat has now lost control of the system, and the engine will overheat.
REMEMBER, By-pass stats were NOT an item in those days. There was a port in the stat base that by-passed water until the stat opened, then that opening was the passage of least resistance, so the water went that way to the radiator. Simple.
Also explained was the fact that car makers, then and now, are after every profit $ they can squeeze. Nothing in the cooling system in this case is there for fun, its there for a reason, NOT always obvious.
On the V12, that "seat" inside that casting can corrode, and the stat disc cannot do what its designed to do.
ALSO
That disc to port seat is NOT a 100% seal, LOOK AT IT, there is no way it can seal 100%, BY DESIGN. BUT that port needs to be closed before the stat reaches fully open.
Those cars with corroded seats ran hotter than wanted. Repairing that seat, and the cars ran cooler, simple again.
I have never followed the "V12 cooling system is borderline adequate" discussions.
RUBBISH I say.
Lack of care over many years, radiator too old, yes, they have a use by date, wrong stats, restricted air flow, IN and OUT of the engine bay, (trans tunnel foam is my #1 go to), timing, pressure cap, fill pipe blank cap, Lower lip spoiler missing, etc etc.
All these need to be 100%, and mine are, and mine ALL run spot on, and our summers are HOT 42C++, and traffic flow is crap at best.
YES I run Efans, and NO smog junk, etc. My cars are all daily Drivers, and MUST DO what I want every day, year round. Never had an issue in too many years.
As I racer I typically only glance at these subjects, but I’m thinking people are missing a fundamental part of cooling. It’s called flow dynamics.
Every time coolant has to make a sharp turn the opportunity for the inducement of air into the coolant exists.
Hence the need for pressurization.
Watch a flowing river sometime. Notice the white foam happen when flowing around sharp obstacles in the river?
Avoid those sharp turns! I eliminate those as much as possible. By having bends rather than intersecting paths.
Using a flat plate I welded an aluminum tube to make the bend that sharp casting has then on top of the cylinder head I again welded to aluminum plate bent tubing to flow into the discharge rail. That all went to a holding tank on the firewall. And out a hose going up to the radiator. Everything was aluminum tubing except for the minimum connections ( done with silicone hose) As a result my engines always ran cool even on the really hot days I was able to get by using a cheap aluminum radiator. ( for street I highly recommend the stock brass one)
I actually had to slow the water pump down ( both to take some drag off the motor) and to keep the sharp turns in the block from airiation.
I have never followed the "V12 cooling system is borderline adequate" discussions.
RUBBISH I say.
.
I agree. And disagree
IMO, fear of dropped valve seats has made many Jag V12 owners very over-sensitized about coolant temperature and gauge readings. This stems from two things....both of which I happen to disagree with.
First, there is a contingent who insist, often loudly, that any increase in coolant temperature beyond thermostat control range constitutes "overheating" and thus means there's a fault which must be identified and corrected. I disagree with that definition, personally. Over the last 20-some years I've read the accounts of those owners spending truckloads of money and effort to achieve this cooling system nirvana. Sometimes successful, sometimes not. They've worried themselves to death over a few degrees of coolant temp which almost certainly has no bearing on the dropped valve seats problem.
Which now brings me to my usual soapbox about coolant flow versus coolant temperature. My belief is that the dropped valve seats are primarily caused by lack of coolant flow to all areas of the cylinder head. IMO, the valve seats don't care if the coolant temp is 82º, 88º, 95º, 100º...whatever. What the valve seats care about is coolant flowing around them to take away heat.
To be honest it pains me that some V12 owners have been robbed of the enjoyment of their cars because they spend every moment watching and wondering if that temp gauge is gonna rise a needle's width or two past the middle of the gauge when, in truth, what they should be worried about is something they can never see: coolant flow thru the cylinder heads.
Unfortunately we don't have x-ray vision. We don't really know if this flow thru the heads is happening and there's no actual assurance that lower coolant temperature guarantees 100% flow. All we can do is keep everything in the cooling system 100% up-to-snuff, very clean, and properly bled.
All of the above is purely "IMO" .
Now, as to V12 cooling systems being "borderline"......
I think this stems from two things.
First, many owners have owned other cars with cooling systems that simply did not require the level of maintenance and concern the the V12 cooling system does. There are other cars out there that do perfectly well with something less than perfect cooling system maintenance. They don't require that everything be 100% up-to-snuff. I've owned some of these cars.
Second, some cars DO have cooling systems that will keep the temp within thermostat control range 24/7/365.....even in Las Vegas in gridlock traffic in August. If that's the standard you insist on then any other system will be considered borderline....or worse
Extremely Well said. If the stock cooling system work so well for most. And less than stock work under all out racing conditions clearly some people worry for nothing.
But is worry about the cooling a symptom of a greater problem? Fear of the unknown?
I remember now, thanks again Doug, my Engine Reconditioning Apprenticeship days, and the MIGHTY Dodge Side Valve 6 cylinder engine.
It has a :water"distribution tube" pushed into the block form behind the water pump. It has slots strategically placed facing upwards, and it was to spray water on the Exhaust Valve seats. AH the memories of removing those suckers.
So, valve seat issues are NOT new, as Doug, and us all know.
Water Dist tube, Dodge.
Here ya go for those that have NO idea what this Old Fossil is talking about. The things you guys have missed.
I tested the David Manners 82°C thermostats [ https://jagspares.co.uk/thermostat-82c-v12-engine-ebc3576-3.html ] they open @ 82°C, the valve moves 8mm, and they have a small hole with a jiggle pin to prevent the hole from getting clogged.
I'll send a couple to my mechanic in Eboli 👍🏻
So, the cooling system is now completely brand new:
water pump
NAR radiator KIT:
Aluminium radiators with separate transmission cooling unit
Shroud
Large Spal fan
Small Spal fan
Actuator switching unit
hoses (every single one)
Clips (every single one)
Coolant system filter caps
Expansion tank has cleaned and rustproofed by anodising (I think, I can't remember how he did it)
02-22-2024, 09:32 AM
My logic tells me that, if the valve does not travel all the way, then not all the fluid is redirected to the radiator and, therefore, not all the radiator's cooling potential can be exploited. I spent north of €5,000 on this car, it would be silly not to invest another €30 to feel good ... 
