Hi everyone.

I've ordered some thermal labels to stick on bits of the engine (covering the range 88-93-99-104-110℃, which seems like a good place to start), and while I will stick some on the top radiator hoses, does anyone have any suggestions for the best places for the others? They're irreversible, so use-once types. (I also intend to use at least one to see just how hot the engine bay gets after shut-down. While I try to leave the bonnet open after a run, it's not always possible.)

I'm mildly concerned the car might be running a bit warm - in traffic, with the a/c on, after a fast run, the needle moves as far as one needle width to the right of the middle (i.e. there's one needle space between the needle and the central marker). Never more than that. In fact at its max. I could see the needle move right a hair, then move back, then move right, then back, just sort of twitching within the width of a hair or two, presumably as the cooling system did its job. Hopefully....
She always reads a little high in traffic, and drops to below the middle outside stop-start traffic, so it's not totally abnormal (and I have 88 degree thermometers rather than 82 ones, which might affect this), but the weather's getting warmer and I want to be sure the cooling system is completely up to snuff. The radiator, hoses, and thermometers have all been replaced, so I'm just being cautious, because I have a long road trip coming up in a couple of weeks.

Anyway, thanks in advance for any hints. Always appreciated.

 

Contrary to popular belief thermostats have no bearing on the running temperature of most engines. They do however control warmup speed and 88°C thermostats will warm the engine faster and should be fully open by around 95°C.

Engines are designed to run around 90-100°C, if the engine runs too cool then there will be excessive engine wear as most engine wear occurs at startup and warmup.

As long as the temperature gauge does not keep going skyward then the cooling system is working. There are a few reasons that each temperature gauge will read slightly different from car to car, these are.

* Manufacturing tolerance in the temperature sender and gauge mechanism.

* Voltage will effect the reading as the gauges (at least in the pre-facelift) are not supplied with a regulated voltage.

* Poor connections in the wiring between the sender and gauge. This can change with temperature.

As you have addressed the cooling system I would rest easy and go on your trip with confidence that the car will be fine.


BTW... My car temperature gauge runs just above 1/2 way between N and C on the highway, but in traffic on a 40°C day with AC on will go as high as middle to top of N then it does not move any further.

When I was renovating her I broke the spade terminal off the temperature gauge sender, I ordered a new one from my local Jaguar supplier and this sender had the gauge needle run a fair bit higher than usual so I pulled the one off my spare 6.0L and this was different again.

 

That's very reassuring, Warren, thanks. I think your reply should be made a sticky at the top of the forum page, as it's got some very useful information in it.

I have done what I could to address the cooling issue, which is something everyone agrees that the V12 absolutely needs to be up to snuff on. So I do tend to get a little paranoid. Every time it isn't bang on the middle (or under) I start to worry. It would not be much of an exaggeration to say that I check the temp gauge every time I am stopped at the lights (and often while driving...).

I saw the Coolant warning light flash on for a second a couple of days ago, and when I checked the level this morning it was definitely lower than usual, but if it's leaking, it's not leaking onto the floor, at least not obviously. So I topped it up a bit with green ready-mix, and will keep an eye on things.

Right now I've touched up a stone chip, filled in some pinholes in the mufflers, and taken the oppo to polish the exhaust pipes (they look much better), and am now dyeing my driver's seat with Gliptone, so hopefully Lady Mary will appreciate the attention I am lavishing on her and be a good girl for the road trip.... (^_^)

 

I too was paranoid when I first bought my car. But after a few major overheats, one where it spat all its coolant on the ground, my fault as I forgot to put the fan fuses back in before going for a drive on a 45°C day, it was ok until I stopped at traffic lights. I have never had
valve seat issues. I put this down to the Marelli ignition and possibly better valve seat fit. I have not heard of 6.0L's dropping valve seats.

The major issue with the V12 is the rear of the engine runs much hotter than the front, this is a product of the design of the cooling system. Norman Lutz determined that adding restriction to the front water manifold thus forcing more coolant to the rear of the engine went a long way to solving this issue.

I have gone a step a further with the modified water rail system for my 6.7L which does away with the small coolant pipe between the front and rear manifolds. Mine has a 25mm aluminium pipe which increases the flow significantly from the rear manifold.

 

Interesting. Mine's still a 5.3, but it is Marelli. I suppose that's one of the issues with discussing a car as long-lived as the XJS: it wasn't static during its production run, so what might apply to an early 1980s car might not affect a late 1980s car as much, might not be an issue at all on an early facelift, and might be something else entirely on a six-litre....

But still very reassuring, thanks. I have got myself into worrying that the moment it overheats, the engine basically implodes....

 

I've always treated the gauges as indicators, rather than precision instruments. Once you get to know where "normal" is, then you can be aware of any abnormal conditions, rather than going by what the gauge says in terms of a value.

I had a scare with the voltmeter, until I bought a voltmeter that plugs into the cigarette lighter. Now I ignore the number on the voltmeter, but I know that where the needle usually sits is proper voltage, even if it appears low.

If you can see the temp needle moving up and down, that's good as it's the thermostats opening and closing.

Before a long trip I would inspect the large plastic fan if it's original or fairly old. I can't remember if you have replaced it or not. If it's a yellow fan, which really started it 's life as white, you're looking for cracks at the base of the hub. If you see any, replace the fan blade with the black one, part# EBC4553.

Places where I would put the stickers are on the water rail behind each thermostat. Can't really think of many other places that would tell you much. Maybe on the radiator side of the thermostat housing? Then you can tell if the thermostat is open or not by looking at the temperature difference across the housing.

Intake manifolds are cooler, because of the amount of air going through them. Putting stickers on the cam covers will tell you oil temperature, but there isn't much you can do about it.

 

Out of curiosity, which of the 2 filler caps did you add coolant to? The one that goes across the front of the engine, or the one on the left side tank?

 

Warren 100% right. Your car is absolutely fine. What is important to understand is that a small needle movement will indicate about a 1 or 2 degree change, no more.


I have a off at 85 on at 90 thermostatic switch in the water pump inlet that controls the main fan (ie the cooled water temp exiting the rad). This is to prevent excessive starting and stopping of the fan. On my car the needle goes from N (water temp about 83) to 40% of the way to H before the electric fan cuts in (water temp 90). I have measured this accurately with a digital thermometer. So on my car the water is only 5 degrees C hotter than standard when the fan cuts in, even though the needle is quite a bit past N.


As to the temp strips, I would put one on each of the thermostat housings, one on the cam cover at the front, and one on the oil filter. They can be left in place as they will record the highest temp they reach, which is useful. They do not go up and down as the temp varies.
Greg

 

Thanks very much Jagboi and Greg.

"Normal" in my car in traffic has generally been a little to the right of the middle line in traffic, but a little to the left on a clear run. Mind you, I have noticed it being less eager to dip down when moving faster now that the weather is warmer (completely logical, of course). Which I guess just shows how much it's air-cooled, or something.... I did not know how little a change was required to move the needle. That is also somewhat of a load off my mind.

I have replaced the fan too - sorry, forgot to mention that. That was when I got the Wizard aluminium rad in, plus the hoses and thermostats. It's a brand-new black fan.

I had sort of intended to put a sticker on one of the top radiator hoses, as they always seem to be the hottest part - after a decent run they are too hot to touch. The top of the manifold varies between very very warm but still able to rest hand on briefly to definitely too hot for comfort.

With the help of the very useful parts diagrams at Jaguar Classic Parts, I think I have identified the water rails, and where to put two of the stickers. Do these look right?

...and...

(Actually, probably where I labelled "thermostat cover" would be where it should go, right?)

Thanks very much everyone. It seems like every time I look into the engine I learn something new - or rather, every time I look in with the point of finding something, rather than just going "oh... my... god..." (joke. Mostly). Being able to see one in the flesh, out of the car, would be very interesting....

Jagboi: I added the coolant to the central/front one, as per the car's instruction manual. I assume (hope) that's right.... I also remember reading somewhere that both caps should not be removed at the same time.

 

Well, the stickers arrived yesterday, and I stuck them on various places today, which wasn't entirely easy, at least with the front of the water rails - both getting the stickers down there (with the help of some long chopsticks), and finding places large enough for them, even though they're pretty small.

I took the car for a drive today, getting the engine thoroughly warmed up, and then checked temps. Then I left the car while we shopped and had an early dinner, and then drove back and checked temps again after the heat under the bonnet had increased through sitting (as an aside, it can be quite unnerving having the temp needle shoot well past the normal mark after being shut down for only a few minutes - I know it's normal, and it drops right back again, but still a bit unnerving...). The temp needle behaved normally throughout, varying between slightly over and slightly under.

I used six stickers (dropped the first into the bowels, and it's now sitting a bit above the engine mount...).
Two on each top radiator hose. The LH (as seen from the driver's seat) registered 88 degrees C, and no more, while the RH one didn't even register that.

Two at the front of each water rail, which seem to be the consensus "best place". The LH one was hard to attach smoothly, so I'm not sure how thoroughly it sensed the heat, but thoroughly enough, I guess.

LH after run read at least 93 degrees:


RH after run read the same:


One on the capstan thing in the vee to check underhood temps - even after sitting it didn't show any change, so the ambient air under there presumably does not exceed 88 degrees.

Then when I got back, or in other words after the car had been sitting for an hour and then driven for another 90 minutes, I checked again.
LH water rail at front looked as if the 99 degree square was starting to blacken, though not that dark (many of the labels had slightly greyer 99 degree squares anyway).

RH water rail at front was clearly showing temperatures exceeding 99 degrees, though not 104.

LH water rail at rear showed 93 degrees:



So it does seem that the coolant reaches 100 or so degrees at max, but not significantly over. At least I don't think it's significant (I hope it's not...). Warmer than the temps reported by Greg, however (and the needle didn't go more than a hair or two past the middle at most).

 

I was measuring oil temps and cam cover temps.
Your water temps are 100% fine. The highest water temps you record (eg 90/100) occur after the car has stopped, not during a run. When a hot car shuts down, the residual heat in the engine goes into the water jacket, which as it is not circulating any more, heats up a bit. All 100% normal.
Greg

 

I thought (hoped) as much, but it's definitely nice to have your reassurance. When I referred to the temps you recorded, I was referring to when you said this: "I have a off at 85 on at 90 thermostatic switch in the water pump inlet" - from that paragraph I assumed you were measuring water temps, not oil.

She's going to be making a long road trip in a couple of weeks, so I'm just making sure all's well. My father's coming over for a visit, and I don't want us to be stranded at the other end of the country....

 

That all looks normal to me, nothing to worry about.

 

Thanks. Definitely worth checking to be sure.