While its generally accepted that the Temperature Gauge Reading in the XJS V12 can vary quite a bit from Car to Car.

Over time you get used to a reading, you begin to feel Comfortable with.

On my Grey XJS the reading never goes near half way, while on one of my other Cars, its almost exactly half way.

Both of these I've come to regard as 'Normal' while any deviation suddenly induces an inexplicable outbreak of Fear and Paranoia, that something is about to go wrong, in a very Big Way!

As overheating the Engine of an XJS V12 is almost like self harming, by slitting your own wrists.

So the Question is does anybody know of any way, of taking a Temperature reading from various parts of the Engine, so we know that the Temperature reading on the

Gauge inside the Car, is providing a 'fairly accurate reading' that we can go by.

And what we should/could do about it, if the Temperature unexpectedly starts to rise.

Many of us have got a 'Laser Thermometer' which is almost as essential as having a Fire Extinguisher inside your XJS. and my Insurance Company is insisting that I have one in the Car.

So either they are psychic or got to hear that 2 years ago, my Car nearly burst into flames. (incident of the stuck Calliper and sudden loss of Brakes)

Just 'Another day at the Office' if you drive an XJS.

Everything renewed now: New Callipers, Brake Pads, Brake Lines and Brake Hoses and even a New Valve Block for the ABS.

So also a very expensive day at the Office!

Hand held 'Laser Thermometers' are very cheap to buy £10 to £15 (UK)

But where should we take our readings from and what sort of Temperature Readings, should we expect to get from various points, either on or connected to the Engine.
Example: The Radiator, Top and Bottom hoses etc.

 

OB,

One of the key temperature metrics of the car is the temperature at which the thermostat opens. I'd always start by taking readings off the 'stat housing and then the top radiator hose.

Paul

 

Yes OB, a handheld laser thermometer is a very handy tool, car guys should have one so they feel comfortable about cars!

for instance ,my most important engine temps are cylinder head, check at front 1st(both heads), then at rear of heads,a straight line of sight to the aluminum.
seems most older V12 get hotter at the back of left side head!

for my car running temp at head are around 180*F, and rear 185*F(180 stats).

so thats where i drilled mounted a Digital readout engine thermo, gage inside car. reads in 2* increments.
factory gage is a novelty, but does tell me of any unusual changes!

you will be shocked when you test the exhaust manifolds or pipes(very hot), but a normal occurance. 300/ 500F load dependent. these manifolds contribute a lot to under hood temps(AKA hot extraction air).

now something nobody can explain to me LOGICLY, is when i test my exhaust pipes under the car, about 2ft from manifold outlets, there is 15/20* F difference each side ,thats at idle(tick over), after time of 10 minutes!

tests at radiator are cooler than engine,(normal), coolant outlets to rad are close ,less than 5*F difference!

also after a hot run , bottom of engine,near sump, oil is much hotter than top of engine, coolant area, suggests a better oil cooler system!
but so far no problems,i'm OK.

and on mine, tests from coolant part of head to quick check to cam covers is quite different ,like 20/30* diff. means to me oil is hotter than coolant.

remember my V12 is 1978, and 20/50W oil factory recommend, new 6L run 10/30W, a lot of guys think no diff. between them, but FORD made some valuable mods to 6L.
the one i like(wish i had) is rear crank oli seal, not changable tho.

YUP get a Laser gage , and have some fun, and expand your knowledge!

 

Hi Paul

That sounds a good place to Start but what I don't know are what Temperatures I should be expecting to find, as I may as well scare myself silly all in one go.

 

Hi Ron

Thanks for that Amazing mine of Information and when it finally stops raining if it ever does, I will take some 'Lazer Temp Readings' and where I took them from.

Then Post up what I have done, with maybe some photo's to use as a guide for anyone else who might want to do a similar check on their Car.

I've had a fair bit to do with boats, where many have had there Exhaust Pipes Insulated and so in view of what you said, was wondering if it may be a good idea to Insulate the Exhaust Pipe on an XJS.

Or at least the pipes that come off the Manifold, as I am now starting to wonder if that may be another way of reducing the build up of heat in the Engine Bay.

 

OB you need to be careful of laser thermometers or IRP's as I'll refer to them. I used to calibrate these for the military and even the Fluke 65F's were notoriously inaccurate if you used the laser pointer as a guide, at 100°C on a blackbody temperature reference I could get up to 15-20°C difference between different units (this is with the laser in the same spot not the overall accuracy between units). Then if you moved the IRP closer or further away on one unit the temperature could very 5-10°C This is due to the way these devices average the temperature measured in the spot. The only one I found that was any good was the Raytek.

IRP's are rated for accuracy at a certain distance from the surface you are measuring. The IRP averages the readings it takes from this spot, which maybe a few cm's in diameter, depending on how far away from the target you have the IRP. For eg if your IRP is calibrated for 300mm with a DTS of 10:1 your spot will be 30mm you need to make sure this spot is not partly over say an exhaust manifold, or you will get a false reading.

The other variance is the surface that you are testing. Heat a highly polished aluminum surface to 100°C and even the Raytek will not read correctly. So measure temperature at a radiator hose, this is the only place I measured my car when doing cooling mods.

So to sum up
An IRP is a good ballpark measure, as long as you know how to use it.
Shop around for one with a smaller DTS this will limit false readings.

Better still use a K type thermocouple and a DVM. You can hose clamp it on the rad hose or fix it somewhere else. This will be far more accurate but you need to let the thermocouple stabilize for a min or so.

 

My car runs around 100-110°C in hot weather and never goes above about 115°C in summer stationary in traffic with AC running, this is the radiator water inlet temperature.

Rad outlet temperature in same conditions maxes at about 95°C.

The important thing is not the outright temperature its the radiators ability to cool the incoming water so the coolant does not boil. NASCAR run engine water temps up to 130°C.

Running an engine too cool causes wear and reduces combustion efficiency.

 

on wheeler dealers (xjsc) they coated the manifold to reduce underbonnet heat. it looked quite expensive but maybe worth a try

 

sorry im an idiot it was the jag xj6-c

 

Who knows it might still work?

Have you got a Youtube link?

 

I think this is right

im not too good at this so if it doesn't come out try yourself

 

I put on the wrong one try thishttps://youtu.be/kOF9omlCpnE

 

Hi Warjon

Many Thanks for taking the trouble with such a detailed reply, which will be so useful to myself and others.

Try not to hold your head in your hands, when I tell you I got my IRP from a local Supermarket Lol. (on a Special Offer for £12)

But as it was made in Germany, it might well be good enough to give to give a fairly accurate reading, for all practical purposes.

So when the ground has dried out enough for me to get the Car out, perhaps we could do a sort of Calibration check on the Top hose, to see how our readings compare.

As we have both got the same Car and of the same year, then if we find that those Temperatures Match, at least that is a step in the right direction and then I can take more readings, from other parts of the Engine, which we can compare.

According to the Dashboard Gauge, She seems to be running ok but really wanted to make sure, that the heat was even throughout all parts of the engine.

But as you know the Internal Gauge, cannot be relied upon to read exactly the same on identical Cars.

 

Cheers Leo

Very Interesting!

 

Hi Leo

Yup, you nailed it both times.

 

as a London taxi driver my head is full of streetnames and general rubbish and at the moment flu as well. my computer skills are crap at the best of timesbut with a head full of cement they are risable. that coating looked quite spaceage and they reckoned around 30% heat saving in the engine bay as I remember. I quite fancy it myself on my 3.6 but with 2 extensions on the house pending I think it will have to wait, especially as I don't think I could afford a divorce aswell. That would push the finances possibly to breaking point

 

OB
Getting back to the original question, my answer is "No". As long, that is, as the temp gauge is working and as you said, you are used to where it normally stays.
FWIW in my view, there is rather a lot of general confusion in these discussions between "Under bonnet air temps" on one hand, and engine coolant overheating on the other. If the coolant starts going up in temp, then the temperature gauge will show it very quickly. Remedy: immediately stop the car!
High under bonnet air temps are not going to wreck your engine; BUT they will take a toll on any wiring, hoses, electronics under there. For me, the reason I have tried to reduce my under bonnet temps is to reduce the heat-deterioration "load" that these items have to bear. Not to preserve the engine.
When I fitted a new water pump years and years ago in England, for some reason one of the fixing bolts kept coming loose and water sprayed out from the pump casting on that arc-shaped piece that takes the water to A bank. The first I knew about it was when I saw the temp gauge start rising alarmingly after a really fast 100mph blast. Stopped the car to look and there the spray of hot coolant was. A refit with some super heavy duty Loctite on the threads fixed the problem. My point is that the absolute temp did not matter, what mattered was that I saw the change from the normal position of the gauge.
Right now, I have electric fans activated by the OEM temp switch on the pump inlet. Because I was beset by too frequent on and offs of the fans, which was stressing the relay, I looked for an OEM style fan switch with an on at 90 off at 85 degree setting. I found one with an on at 90 off at 85, as used on various Fiat cars. I checked with the WoOZ and he said no bother it will be fine. And it is; BUT by the time the inlet temps are high enough to trigger the thing at 90 degrees, the temp gauge is quite a bit above the N. So although the engine is quite safe at this temp and nowhere near a dangerously overheating state, the gauge is pretty responsive to even a 5 degree temp hike. I deduce from this that the standard gauge will give very adequate warning of dangerous overheating.
Greg

 

Good advice Greg.

I think a lot of people are a little overly paranoid about V12 engine temperatures, as long as the car does not boil, ie spit coolant everywhere you will be fine.

I boiled the heck out of my car once on a 40°C day when I went for a drive after doing some work. I forgot to put the fuses for the thermo fans back and she spat almost all her coolant onto the ground at a set of traffic lights. Filled her up after she cooled and continued on my journey.

I do think the Marelli cars have less overheating issues due to better ignition control, no advance mechanism to seize.

 

Massive respect to Taxi drivers, having to do the 'Knowledge' and all that Stuff, I couldn't get to the end of the road, if I didn't have my 'Sat Nav' Lol

 

Hi Greg

You are absolutely right, I 'Was' getting confused about the Water vs The Under bonnet/hood Temp.

I think I'm tuning in on it now, so Thank you for that.

With the benefit of hindsight, I think I was starting to Panic because the reading seemed to be too Cold and as such I wasn't sure if the Water was circulating properly (as in not getting as hot as maybe it should.

Hence the desire to check it with the Laser, which I will do anyway to try and learn a bit more.

I've also got in the habit, of opening the (Bonnet/Hood) up right after a Trip, although the 'Cool Down' Fan works perfectly fine.

So Thanks for your help.

If you Check out the Photo, this is where my Gauge sits All the time, when She is Warmed up. (maybe a little bit higher on a Hot day)