Hot running episode

Hmmm. That means this thermostat might be handy on the day when something else causes the engine to overheat, as it will open fully and stay there. But if this thermostat itself fails to open, its “fail-safe” feature will accomplish exactly nothing.

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Then ~90° should be okay!

Rightfully so, Nick - any odds nothing awful happened…:slight_smile:

xj6 95 Sov Europe (UK/NZ)


I will keep monitoring as usual. I’m slightly puzzled as to why temp gauge reads around “o” under regular driving (I.e. a bit below the middle) even though the thermostat is 88c. The temp sensor is brand new. That said, it rises to the mid point in heavy traffic and stops there. So maybe I am being a bit literal.

On a side note, I filled with approx one third antifreeze, two thirds water. Per handbook this proves protection to -39c. Yet my mechanic does not agree and would have used neat antifreeze in order to avoid limescale formation. It is true the product is marked « ready to use » but per other threads in this forum and elsewhere, I thought cooling capacity was better with water and the recommended mix provided adequate anti-corrosion and anti-freeze qualities. And I am not buying a watery mix at that price, right?!

Temps can fall to -15c in my neck of the woods although not til deep winter so I have plenty of time to adjust if necessary.



100%anti freeze/inhibitor is not the most efficient heat exchanger, somewhere between 30-50 % should give you sufficient cooling/inhibition cover.

You are best off using good water and antifreeze 50/50, and change it every two years. The engine will probably be full of crud if it wasn’t boiled out in the last decades. What really matters is keeping your head studs healthy, so change it often. Antifreeze neat is pointless. 30% is probably okay.

I‘d be optimistic too! The sensor and gauge are inaccurate but if they stay somewhere reasonably that means it is all good.

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I agree that the antifreeze should be 50/50. Note, however, that they sell antifreeze already mixed 50/50! Which means if you don’t check the label and buy the wrong stuff, you could be mixing it 50/50 again and end up with only 25% antifreeze!


If you do not trust your cars temperature sender/gauge, adhesive temperature stripes are a good way to determine how hot it really gets (maximum).
Just stick them wherever you want to know, take the car for a ride and read the temperature stripe afterwards :+1: a lot of helicopters have them at critical bearing or gearbox locations to figure out the maximum temp.

Best Regards


Straight up is 90C, Nick…

But use the infrared thermometer at the base of the temp sensor; this is where the gauge reads. Any discrepancy between gauge and actual temp can be mentally compensated - it’s not really worth while doing ‘something’ to get a perfect match. The gauge is not very precise by nature - its value is basically to show if the temp is rising or falling…:slight_smile:

And 50/50 is the maximum mixture advice given by all engine manufacturers - and antifreeze producers; any more is just counterproductive. It’s not given that if something is good, more of it is better…

xj6 85 Sov Europe (UK/NZ)

Had a good 60 mile run this morning. A few musings

  • my interior heat started off well but became lukewarm at cruising speed, even on max heat or DEF. Warmed up again when back in town.
  • No loss of coolant so far as i can see (full at headertank and to base of expansion tank, as before)
  • after this good run, at idle, get around 90c at thermostat housing, 110c or so on the lower part of the block, 75c at radiator top hose and 55c at lower hose.
  • ambient temps at the time high single digit Celsius
  • But some traces of coolant down the side of the block on both sides. May not be new though.

Any thoughts gratefully received as ever.



A further, shorter outing in town traffic provided full heating, normal operating temperature, 90c or so at the temp sensor, and no new traces of coolant (although left hand side greasy so hard to be 100% sure).

I suspect they were there previously - indeed probably what prompted the guy who sold me the bumpers (also one of the leading UK specialists on the XK engine) to tell me it needed re-torquing (just done).

Have a thousand-mike trip coming up on Friday so don’t want to take unnecessary risks but don’t see major alarm bells …?

Header tank is not to be used for coolant level checking, Nick - the level should be at the bottom of the expansion tank filler neck. Manual explicitely says: ‘Do not remove the header tank cap’…Coolant levels should always be checked with the engine cold - any drop in levels imply a coolant leak…

Some odd numbers; the water rail/thermostat housing should ideally read thermostat rated temps - as should the top radiator hose at the thermostat house exit. The block temps is sort of academic, but the temp difference between the top and bottom of the radiator represent the cooling efficiency of the radiator - which seems ‘reasonable’.

At this stage I’m not too convinced that your thermostat is working correctly…

The variations in your cabin temps might be caused by heater system malfunctioning, but wrong coolant levels or a malfunctioning thermostat may play a part? What happened to the dash gauge reading while the heater was no performing properly…?

xj6 85 Sov Europe (UK/NZ)

I agree with Frank, your thermostat is not closing properly or even caput/stuck open.

You must wash the engine and check again.
I would do that before your trip, because if they reappear it’s most probably the head gasket.

I always put 50-50, and normally you should use distilled water to avoid lime scaling possibly blocking the radiator.
In theory the ready mix uses distilled water.

Good luck and keep us posted.

I agree the thermostat could be unable to close fully. If it is good now it was either an air bubble in the heater, heater system issues or said thermostat. Maybe the thermostat was somehow sticking open a bit for some time.

I see no major alarm bells. If retorquing didn’t snap anything you are lucky with your head studs. Just watch the temperature gauge, if it rises by 20° and drops back down during driving you have to pull over and add some coolant as soon as the pressure is gone. So if you take some coolant on your trip (5-10 liters to be on the safe side) there isn’t that much that can go wrong.

I just hope the leakage has stopped. If not it’s time for a new gasket; I don’t see it failing catastrophically.

Thanks all. My mechanic is a pragmatic fellow. His response: look, I can replace the thermostat and you can hit the road. But these things are notoriously unreliable. Maybe the new one will stick closed and you will find yourself overheating on a long trip. At least with this one, it might be a bit stuck open, so you are very unlikely to overheat, at worst you will run a bit cool. If that’s the case, we can replace it later.

In other words, better the devil you know.

I’m not too worried about the streaks given I wiped them down yesterday and they did not return after my subsequent drive, and I am not apparently losing any coolant.

David, it’s true that the temp gauge dips a bit on the open road, maybe towards the « n » of normal, down from o / r. But it does not fluctuate more than that. Do you mean that when it dips, I should stop, give it a few minutes to allow pressure to subside, then open the header tank and add coolant, I.e. ignoring all those « never open when hot » signs? (Using a sturdy rag to avoid scalding, obviously).

Aristides I am heading to your neck of the woods. If I make it, I’ll look you up…


That is fine. It will also rise when you accelerate or go uphill and in traffic and after shutdown. It will rise when in drive and drop when in Neutral.
You should be alarmed when it goes up sharply for no reason other than a dip in the road or braking or curves. If you keep an eye on it you will catch it in time. Don’t let that keep you from enjoying the trip, I lost a little coolant early in on my trip to the UK and I made it all the way back and some more.

Don’t open when hot is because it can spray out or suddenly boil over. The coolant heats up by some 10-20° after the engine is turned off. So wait until you can tell the pressure dropping.
I was able to refill it immediately by keeping the engine idling. I was very careful; do not take it as advice, but it kept it circulating well below 100°.

That’s the most important, and very good news indeed.

Since when thermostats, the same exact device used for how many decades in millions of cars, are “notoriously unreliable”?
Yes they do brake sometimes, but your car overheated.

A properly working thermostat should Keep the engine inside a certain range, and very close to it’s nominal rating, i.e. the opening temperature.
I don’t know how the XK engine behaves normally and what is the temperature at the « n » of normal but if its bellow the the thermostat’s rating your thermostat is not doing what is supposed to do.
I don’t see much harm on choosing to stick with the “devil you know” but at least buy a new thermostat and have it with you just in case.

Your car is overcooling, not overheating.

The thermostat is there to maintain engine at a desired temp, Nick - varying temps interfere with proper engine operation; thermostat is not just an optional extra.

If the thermostat has failed; it should be changed, and thermostats usually fails in the same manner; you’re unlikely to be worse off if the replacement fails - and better off with a working thermostat. While changing the thermostat doesn’t need immediate action - I would not even dream of driving for long with a malfunctioning one…:slight_smile:

To expand on the subject; the Jaguar does two things when fully open; it closes the thermostat housing bypass while fully opening the passage to the radiator - all coolant passes through the radiator for cooling. Not fully open, the thermostat allows part of the coolant to recirculate back into the pump and through the block - so part of the coolant is not cooled in the radiator. Normally, in cool weather and light loads, it will just cause the temps to vary more or less. But in hot weather or under heavy load the engine may overheat - and in cold weather; the thermostat will not close, and the engine will run too cold. In any case; engine’s fuel demands vary with temp, so mixture may be less than optimum…

I suggest you at least check the thermostat - and change it if faulty. It may or may not be behind your cabin temp variations - and in cold weather you will have insufficient cabin heat. I’m a bit surprised at your mechanics attitude…

xj6 85 Sov Europe (UK/NZ)

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It is, but the head gasket has leaked to the outside and if those streaks do return the coolant would disappear slowly. On a long trip this goes unnoticed until the temperature needle starts to bounce up and down.

For now all it did was overcool and that’s not so problematic for the engine.

No, they are notoriously reliable but the casting isn’t and sometimes the bolts don’t move at all so I‘d be inclined to hope for the best. The friday trip will gell if it did resolve or not.