Ideal running temperature/thermostat

Some of my engines run ar 72-75 deg but one car runs at 95 esp on a hot day with traffic jams. What thermostat might it have? 86 deg…?
Cleaner and better for combustion perhaps, but makes me a bit nervous…
Any input on this?
Change to a 74 or whatever close alternatives we have…?
PS I have an XK 140 which runs at the same hot temperature. (I live in Sweden rather than a very hot climate)

of course the operating temp may or may not have to do with the thermostat…as the condition of the cooling system may be different. That said…there is a lot in the archives on correct thermostats…and how the bellows type works…and how the thermostat must fit so as to not block off bypass or main flow. Many of the thermostats now available are not the bellows type at all. Look for Roger Payne and Dick Maury posts in the archive on thermostats…which are lengthy and detailed. Nick

Excerpt fm Dick Maury Coventry West : One area of great confusion is that if a car is running hot, change to a lower temperature thermostat. The thermostat only controls the minimum operating temperature of the engine. If the car is overheating beyond the rating of the thermostat, changing to a lower temperature will only cause the engine to run cooler when warming up. Once the thermostat is open, it is open. If the running temperature of the engine exceeds the temperature rating of the thermostat, then the cooling system is not adequate to dissipate the heat put out by the engine. This can be caused by a radiator that is partially clogged, debris in the radiator, to high of a antifreeze concentration, inadequate air flow across the radiator and last but not least an engine putting out to much heat because it is not running properly. All of these factors added together will determine the maximum operating temperature of the engine. If you have a car where the temperature goes beyond the thermostat rating, then one of the situations needs to be addressed. Going to a lower temperature thermostat is not the answer. .


and…archives…credit Roger Payne. DO NOT block off the bypass…and thermostats that do block it off should not be used. Without the bypass operational, there is no water flow inside the engine until the thermostat opens. No flow around the cylinders and very little flow to tell the thermostat it is time to open. Very hot at cylinders/head, which is quite bad. A restricted bypass makes it easy to blow core plugs,and worse if you get on it too hard on a cold engine, due to the steam pressure resulting from local boiling in the head having nowhere to go.

Note Bene: It is a very specific thermostat that
fits correctly, operates correctly, and does not block off the bypass:… Be VERY careful the only correct thermostat is a SMITHS or
AC TF 1 72 deg. The original dual-function bellows thermostat is
correct for an XK120. It has a temperature opening of 72 degrees
Centigrade, which is correct for the XK120.
Note that there is a brass sleeve around the perimeter of the
thermostat that moves up and down as the main thermostat valve to
the radiator opens and closes. The brass sleeve acts to open and
close the bypass circuit. Thus, when the main valve to the
radiator is closed the opening to the bypass circuit will be open,
and when the main valve to the radiator is open the opening to the
bypass circuit will be closed.
(Note that many of The bypass type thermostats are available at all of the usual Jaguar supply houses for around $35. The part number is C3731…BUT! they don’t have the ones with
sleeve around the bellows… and this is a critical part of the
original units …Most of the repops which are currently available are wrong.
When COLD, the BELLOWS THERMOSTAT is in the CLOSED position, thus coolant
flow from inlet manifold to radiator is blocked, but the bypass opening
direct to the WATER-PUMP is open, thus all this cold water merely flows
around the engine driven by the water pump, with no flow through radiator.
As the water heats up and approaches the temperature setting of the
THERMOSTAT, it starts to open, thus allowing some flow towards the radiator,
but with the Bellows now starting to close off the bypass opening, thus less
water is flowing direct to water-pump and thus avoiding radiator.
When the coolant reaches the THERMOSTAT operating temperature then the
THERMOSTAT becomes fully open thus maximum water flow through the radiator
and at pressure now as the restriction of forcing water through radiator
build up back pressure, with the Bellows now fully closing the bypass
outlet/hose direct to water-pump, thus no hot water avoids going through the
So the cooling system is now working at its maximum cooling capability, with
100% of heated-by-engine water now being forced at up-to-4 psi pressure
through the radiator, for maximum heat-exchanger loss of heat to the
incoming colder air. Even on a hot day, 40 degrees(Celcius) or 50 degrees
hot air, it still cools a radiator that has 100 degrees C water going through it.
But if it’s a cold day, and we have zero degrees air going through radiator,
then engine-heated water will struggle to maintain the thermostat open
temperature, thus the thermostat will start to close off, to ensure the
engine heated water merely travels via the bypass to the water pump, and not
be excessively cooled through radiator.
All of this is a fully designed system to try and keep engine at optimum
operating temperature, with coolant kept above a minimum and below boiling
point, with the ongong opening and closing of the thermostat a critical
aspect of coolant temperature management.
closes and reopens the bi-pass outlet, is all part of the design.
A non-BELLOWS thermostat only opens/closes the path to the radiator, and
does nothing about closing off the bi-pass, thus maximum water flow will go
via the bypass opening direct to the water pump, with no back-pressure
restriction, and minimal flow will go to radiator as you get back-pressure
from the convoluted/restricted passage of water through the radiator tubes.A non-original type BELLOWS thermostat with
shielding part of the bellows not as was designed, then you will not get
the ‘as-designed’ result.
Using a correct BELLOWS THERMOSTAT is a fundamental part of having an XK
cooling system properly working.
Roger PayneXK140MC OTS; 4.2E OTS; DaimlerSV8Canberra, Australia


I found a correct thermostat new old stock as the new one supplied by the usuals did not block the bypass when open. See photos below to explain how it works.


Thank you ever so much Nick, Matt and Roger. This explains it so well, and as the radiator and pump (and engine of course) is in top condition, it leaves me with an incorrect thermostat supplied by SNG.
I will open the stat housing and report in a few weeks time.
In the meantime, I must try to find 2 correct thermostats somewhere in the world…

To clarify, if it matters: the car I drove for 420 miles on Monday is a 4.2 Mk10, and the other one with the same 92-96 (or even 100 deg when at stsndstill at redlight) is a 140 with a 3.8 engine (and almost sure, (The car is not in Stockholm)) modern alu radiator.

try “”…in UK. and Broadsport, UK.

question for the forum…does the Mk7 use the same/similar thermostat as the XK120/140? and…with the several (I think 3-4) intake manifolds (C2377, C4953) and elbows of the XK120/140/Mk7…is the same thermostat still used…tho it may insert/be located a bit differently.? Nick

So Peder…it seems from the articles and archive and the bobine article…any of these thermostats can be used, various small op temps, that should not matter much: Smiths: x43570/5, x43655 (80d), x43570/28 (70d), x43570/7, X85025/74 (the number after the slant is the op temp and can be other…such as 70, 72 , 74, 80), in AC it will be TF1 (is a 72) , TF2, TF3, TF4, . In Lucas (rare) LF1 LF2, in the QT (likely more common) it is QT100/70 or /72 or /74 . Remax NT100/, and a thermostat listed for the xk140 winter was x43605/3 (83d…winter) . they do come up from time to time on ebay or a web search. Nick today…9/28/19 there are AC TF4 available on ebay…reasonable…altho it is a winter thermostat and will maintain 86C to 90C…they are the correct type.

For those in this thread, a VERY worthwhile, and based on solid fact, series of thermostat white papers.

yes…Paul…those interested in thermostats (everyone?) should visit the Etype forum thread.(the S1 early E type uses similar bellows sleeve thermostats to the XKs)…the paper referred to has additional info…on what else may work. See also the Bob Knijnenburg article in Jaguar Journal, 2 issues Sept and Nov 2016, the Dick Maury posts, Roger Payne posts. I am just a collector of comments on this topic…not a researcher. My goal is to find out what thermostat works in my 120…put one in, and share that info,. It does seem that if all else in the engine/cooling system is in good shape/efficiency then the operating temp of the engine coolant IS the specified full open temp of the thermostat plus or minus a few degrees. The thermostat is in battle with coolant too cool as it leaves the radiator…and coolant too hot as it leaves the engine, so it cycles…to open for more cooling,…and close, as needed, for more warming. So what does it need to do? it needs to fit, to operate, the housing needs to not be corroded, and to actually close off what it should…and not close off what it should not. Some failed, inop thermostats fail in open position, some fail in closed… The engine engineers want an engine to operate optimally within a certain temperature range for a lot of reasons…thus intake water passages, oil lubricity and temps, fuel-air ratio for combustion and more…thus the thermostat… Nick

I was never lucky enough to have one fail in the open position: EVERY damn one that failed on me, failed closed.

Wouldn’t the spring pretty much ensure that happening?

Seems to me it would: I’ve heard of them failing open, but not sure how.

Mechanical failure, broken spring or structure holding the spring in place?

I recently completed an engine swap in my 1984 XJ6 Vanden Plas and replaced it’s original 4.2L XK engine with one from my 1987 XJ6 parts car. I used the fully assembled intake manifold/coolant rail/thermostat from my 1984 XJ6 Vanden Plas because it was in much better condition than the one on the parts car. Once I got the donor engine installed and running, I made a few drives around my neighborhood and observed that the coolant temperature stayed about 50C and never rose to the normal 90C position on the gauge. I suspected that the thermostat had failed in the partially open position so I removed it and confirmed that my suspicions were correct. The attached picture shows the failed thermostat sitting on a white table at room temperature so that you can clearly see that it is partially open. I replaced it with a known good one that I tested on our stove to make sure it opened and closed properly, and once I installed it the engine coolant temperature rose to the correct position shortly after the engine was started. I am not sure of the thermostat failure modes, but they can fail in the partially open position as shown in this picture.


so…back to Peder’s first question: …if…say,…an 82 or 86 C spec thermostat is installed…and IF the driving operating temp remains close to that spec, …within a few degrees…then it is operating as it should:…and remember that Jaguar did specify that those 83 to 86C thermostats could be used.(more winter so as to maintain desired op temp ) , thus we can say that if the coolant runs at those temps…all is well and good. If you have fitted an 86C thermostat…guess what the coolant op temp should be? Yup…86C or so. the thermostat would do its job and close if / when the coolant temp was LESS…inorder to heat it back to 86C at which point it would re-open…and so on,. It does provide less margin in traffic, hot days, or a marginal cooling system. But no harm in operation close to the thermostat spec. thus same if a 72C or any other spec. (in a magic world where the rest of the system is clean and efficient) All this depends on a correct thermostat that opens to the radiator, closes to the radiator, opens to the bypass (bypass means not to the radiator but instead back to the engine to warm the coolant) , closes to the bypass. Nick

at this point 10/1/2019…I suggest readers/posts be over to the E Type Forum where the very in depth discussion of all things thermostat continues for pretty much all the Jaguar XK, XKE applications,…so one does not have to look in 3 places. I trust the admin and EType folks are OK with the XKs being included in that comprehensive discussion on their forum., Nick

UPDATE// I am about to post some thermostat info for the XK120…on the XK forum…new topic…thermostat for the XK120: the dialogue on the E Type forum also continues with lotsa info, I will say here…use only 4lb radiator caps on XK120, 140, 150, and Series 1 E type. Normal op temp is in the 65C to 90C range depending on the spec temp of the thermostat installed…whose job it is to regulate the coolant temp to the spec of the thermostat: for example: some are 63C, some 73C, some 82C, some 86C. and other temps in between as well.,

I have been compiling information on thermostats for several months, and have begun an e-book on this subject, publishing about a chapter a month. Rather than the usual group grope, you can join my thread on the e-type list, and you can read my book as it progresses here:


If we can move this discussion to one thread, so that I can keep up with everyone, I’ll eventually circle around and post a chapter on early engines.

Some specific points: we measure engine outlet temperature because we want to know if we’re near boilover. But what really matters is the inlet temperature. Too cold and you get excessive wear and thermal shock. So you want to mind those thermostats. Jaguar usually specified 74 or 75c. as the set point. If you go below that, it’s not good. On early cars, 80c was usually specified as a winter temp, so that the return temp would be warmer. Around spring of 1969, the set point was raised to 82c for all conditions, and I believe it stayed there until end of XK engine production.

Never use a bellows thermostat in a car designed for a Waxstat. The bellows collapses above 7 psi, and Jaguar recommended 4lb caps. A wax thermostat can run at high pressure, and that’s much more relevant to preventing boilover than bypass control. I have yet to be convinced that the SNG thermostat is ‘incorrect’ in any sense, as it should provide a reasonable bypass restriction, as well as allowing higher pressures (if you change the cap.)

1 Like