[xj] XJ6 runs hotter without Thermostat

Well, don�t flame me, but my Jag actually does run slightly hotter
without a thermostat.

Thanks to lots of people from this digest, I identified my overheating
problem to a faulty thermostat.
In order to get it replaced, I had to drive the car, so I decided to
just remove the old one, go to the dealer, get a new one, drive home and
put it in.
Well, temperatures (using water without antifreeze) were very low
initially, but gradually rose and rose AND ROSE AND ROSE until the cat
boiled over once again (that was during stop and got traffic in the
I parked, opened the bonnet and left the car to cool while a had a cup
of coffee.
When I got back 2 hours later, I refilled som,e water and drove home.
There I put in the new thermostat (82 Deg C, orignal Jaguar-part from a
Jag-dealer at a surprisingly low price), put water in the system and
took a long ride.
This time, the temperature rose quite quickly to a little below 90 Deg C
and didn�t go any higher, not even in the city.
I guess this proves that somehow, due to the different flow or whatever,
the Jag may run hotter without a thermostat.
Having tested it, I flushed the system, put in a 1:1-mixture of
antifreeze and water and tested it a again. Perfect.

What I did notice was that fuel economy was way worse without the
thermostat. It guzzled 22 litres on 125km without the thermostat, while
under normal circumstances it would consume no more than 16 liters on
100km in rush-hour city traffic.
I guess that thermostat does have some right to be where it is.

Just my two cents worth.

See ya,


I guess that thermostat does have some right to be where it is.

I guess even Jag engineers have a reason for doing things…


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You have forgotten that plain water boils at a much lower temperature than water with antifreeze in it.
That’s why your car ran hotter with just water in it Marcus.


Beat me to t.

But, countr intuitive that fuel mileage was worse with a hot engne???


Welcome to the forum John… But you’re reviving and answering a 24 year old post. The original poster probably figured it out by now :slight_smile:

No big deal though, happy to have you!

You fail to mention that much of the water will have simply have taken the short easy route back to the engine via the bypass, rather than the long cooling route via the radiator. This assumes the correct design thermostat for the car is one which has a foot on the bottom to block off the bypass at high temperature.

kind regards


John …

You said …

I don’t know if I can agree with that. The purpose of antfreeze is of course to keep the water from freezing but a 50/50 mixture will also raise the boiling temperature (app. 10degrees F). This along with the higher pressure in the coolant system, Boyle’s law, determines the temperature the coolant will actually boil at.

However adding antifreeze to water does not lower the TEMPERATURE of the mixture, it just raises the boiling point temperature. Otherwise why not just use 100% antifreeze to lower the temperature even further?

It’s on molecular level; the antifreeze prevents the water from solidifying and expand by 10% - which would exert pressure on the surroundings measured in tons rather than psi. Instead the mixture turns to slush with no expansion.

The process is maximized at around 50/50, further strengthening has little effect, is needlessly expensive - and plain glycol is not a good coolant…

The main factor in increasing the boiling point is indeed the coolant pressure, as you say. The 15 psi raises the boiling point to 120C - which is in the ‘safe’ green zone on the gauge.

The point is that beyond the boiling point the steam created expels coolant - and there is a runaway overheating. With lack of pressure, system leaks, this happens at proportional lower temp.

Adding further that the gauge reads coolant temps, and unless submerged, lack of coolant, the gauge will misread. And also, local temps in the block may cause local boiling…

So it pays to be observant on more factors than plain gauge reading…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Frank …

All very good information but the point i was making was that The statement “the car ran hotter with just water” is incorrect. What was being discussed was the operating temperature of the engine coolant. That coolant consisting of just water or a 50/50 mixture of water and antifreeze (glycol based fluid).

Of course the operating temperature (coolant temp) of an engine is the result of the heat produced (engine design, load, fuel, etc.) vs heat dissipation (ambient air temp, radiator efficiency, cooling fan. etc.) simply stated all things being equal …
Screenshot 2023-05-23 at 5.24.29 AM

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All heat is produced by the amount of petrol burnt - adding friction and oil turbulence (all of which depends on engine construction etc, of course). And the operating temp of the engine is regulated by the thermostat (meant to keep the engine hot) - and the radiator (which sole purpose is maximum cooling)…

Certainly, water is the best readily available coolant, and with added corrosion protection in a climate with no frost - is a perfectly viable solution. The boiling point of the coolant (knowing old Jaguars!) is not immaterial, but with pressure as the main factor it’s as good as it gets. Without pressure, which indicates a leak, it’s all a bit dicey - my data indicates that 50/50 raises boiling point some 2 deg C? But neither boiling point of mixture itself is ‘safe’ if cooling capacity itself is impaired…

However, Markus’ (very) old post implied boiling without the thermostat and with plain water, and normal with the thermostat in place - is both a bit odd, as you say…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Just a heads up here - I had on-going overheating problems with my series II XJ6 that eventually were found to be due to a pin-hole in one of the “welch plugs” (those pressed metal discs in the wall of the engine block) that are made of mild steel and corrode from the inside. It may be prudent to replace them regardless.


asking just out of curiosity: “welch plugs” are freeze plugs, right? I’m wondering why a pin-hole in a freeze plug would cause overheating problems. If big enough, coolant loss would eventually lead to a lack of cooling capacity, but that kind of hole wouldn’t go unnoticed, I suppose. A loss of pressure inside the coolant circuit would reduce boiling point, but not cause the temperature to rise. What am I missing?

Thanks and best wishes


75 XJ6L 4.2 auto (UK spec)

Jochen - the pin-hole meant that the cooling system could not pressurise as temperature increased. The coolant escaped in a fine jet that left no or little visual evidence - it was hard to spot. So the coolant level went down and the temperature went up until the gauge was in the red.

It’s sort of arguable if the holes is a necessity of the casting process, Jochen - or deliberate fitted to relieve pressure caused by iced up coolant?

That said, I have seen the result of frozen coolant (not on a Jaguar) - and sure enough; there were the welch plugs, stuck to the ice popped out of the block. And since the block was actually unharmed - they sure worked as freeze plugs…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

The antifreeze would still stop the water from boiling at a lower temperature.

John …

Right you are sir …
Screenshot 2023-05-25 at 2.57.42 PM


The principle of distillation is based on boiling off the elements with the lowest boiling points first - gradually increasing temps for successive elements. Certainly, glycol has a boiling point of 386F - but its prime function of antifreeze.

The main protection from boiling remains coolant system pressure. The boiling point of water makes it ‘safe’, without glycol or pressure, for normal engine operating temps - but Jaguar temperatures are sometimes not normal…:slight_smile:

xj6 85 Sov Europe (UK/NZ)