Xjs 6.0 v12 1995 Temperature gauge

(SilkCut) #1

Dear All,

my XJS 6.0 V12 since new, when the engine is in temperature, from spring to summer the gauge indicator hand stops at N’s right end. During hard winter, it stays on N.

During the last w-end, with 33°C of external temperature, after some pull up to 180-200 km/h the gauge indicator has gone beyond the N, but less of 3/4. Cruising at 130km/h, it returned at N’s right end.

The coolant has been changed in April.

The question is: which is the behaviour of Your V12 6.0?

Is it normal? Or shall I have to change thermostats?

The car has 68k km.

The below picture taken from Web it is similar to what happened last w-end at high speed.

(AndyB) #2

Dear Anon,

I’ve never seen my 93 6.0 V12 reach anywhere beyond the end of the N - under normal motorway driving it sits slightly to the left hand edge of the N & as the speeds increase the gauge needle will move further across the N but never beyond it - back off on the speed & the needle moves back to the left hand edge of the N.

When were your thermostats last changed?

Has your radiator ever been removed to clear out the debris between it & the a/c condenser?

Has your radiator ever been replaced/recored - it could possibly do with it after 22 years?



(SilkCut) #3

Dear Andy,

thanks for the prompt reply.

The thermostats are original, radiator is cleared every year with air, radiator still original.

During the change of the coolant, the radiator was washed.

Do You think that with the substituion of the thermostats everything will be fixed?

(John) #4

Radiator cleaned with air and washed without removing the radiator is NOT sufficient. You need to pull the radiator to get the crap that accumulates between the ac/oil cooler and the radiator. Blowing some air from either side won’t cut it.

You bought this car new? Very cool.


(AndyB) #5

Dear Anon,

When you say the radiator is “cleared every year” - do you mean that it is physically removed from the car to clear debris between it & the a/c condenser?

When you say that the radiator was “washed” - do you mean flushed internally or just washed externally with a hose pipe?

Changing the thermostats with the correct Jaguar ones (88 degrees for the UK & complete with jiggle pins for bleeding) has got to be the least expensive option in attempting a fix. Your radiator’s tubes may be be blocked internally or you could need a new water pump if for some reason there is a problem with the impeller on the back of the shaft.



(SilkCut) #6

For “clean every year” I mean with the air compressor with the long tip.

Years ago, the A / C radiator was replaced and everything was clean.

During the coolant substitution, the radiator was flushed internally.

I will try with the thermostats substitution,however the behaviour is stable since 1996.


(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #7

The thermostats are original, radiator is cleared every year with air,
radiator still original.

I’ve harped on this before, but it bears repeating: Air will NOT do it! That
radiator needs to come out to be cleaned. If it’s never been done, do it, it’s
waaaaaaaaaaaaaaaaaaaaaaaaaay overdue. Once you’ve done it, do it
again two years later. After that, decide for yourself how often it needs to be
done, but once every 20 years is certainly not enough.

– Kirbert

(Steve) #8

Never say never Andy. Drive the car above 100 mph and then come to a sudden stop. At low engine rpms, the pump cannot move the coolant fast enough through the radiator and I bet you’ll see the needle move past the ‘N’. Briefly. Then, the aux fan will kick in and if there is no obstruction, the coolant temperature will stabilize

I’ve done some measurements on how hot is hot with my dual thermocouple set up:


and have traced those temperatures to what the needle does for the temp. sender DAC11079 – both OEM and aftermarket


One or two needle-widths past the N is not a problem (temp. will be below 96-97°C) but I do agree with the suggestion to remove the radiator and clean it right. This is what I found when mine came off:


Best regards,

(SilkCut) #9

Thanks Steve, complete and deep reply.

So, this means that my engine normally works at 96-97°C.

The question is: which is the normal operating temperature of the 6.0?

My car since new had this behaviour, for this reason I’ve always been quiet and I did not care.

Official Jaguar Service every 2 years (and 5000km).

Now, speaking with some other owners of 6.0 V12, one has the same behaviour of mine, another one normally stays on N but when there is a climb the gauge goes at two/three needle-widths past the N.

Some Years ago, I changed the A/C radiator and we took the opportunity to clean everything, but the guage behaviour was the same.

I live in Milan, so durign the winter we have 0°-7°C, now 25-30°C.

The car has 68000km, I use Castrol 15W-40, never city, mostly highway.

(Steve) #10

With the 90°C t-stats and the factory sender, “normal” is the needle over the N. Coolant temp. should be around 92 under light load

After a more spirited driving, however, and coming to a stop-and-go traffic, seeing the needle just past the N is to be expected, but only for a SHORT PERIOD of time.

If the coolant temperature is ALWAYS higher, then, there is a problem.


(AndyB) #11

Fair comment Steve. The National speed limit in the UK is 70 mph so being a law abiding citizen I’m never likely to drive the car at 100 mph or above… :joy::stuck_out_tongue_winking_eye:

According to the car’s temperature gauge my 6.0ltr facelift XJR-S coupé used to run cooler than my 6.0ltr facelift XJ-S convertible. I believe that once these cars entered the “Forguar” era, the temperature gauges in particular were ‘damped’ - so the customer saw what they wanted/expected to see rather than reality. I doubt there was much in the way of calibration quality control for these gauges which is the reason for variations across cars whose cooling systems are in good shape having had regular maintenance etc etc.



(PeterCrespin) #12

Autostrada del Sole? Normal behaviour. Any time I drove into Italy from France the Jaguar (or motorcycle) went 30 mph faster all by itself, I swear! The Polizia did not seem to mind provided you were not a total idiot.

Steve is right. It is sad that normal changes in gauge readings caused warranty issues because customers did not understand. Jaguar had to change to ‘idiot’ gauges for oil pressure etc. so they never varied and eventually they dropped two of the gauges altogether on the XJ8.I don’t believe there is anything wrong with your car

(Mark Eaton) #13

It is actually very difficult to measure temperature accurately.

Even quality sensors that can resolve many fractions of a degree are still +/- 1-2° accurate (absolute) and need calibrating.

You could note that a standard automotive NTC type sensors is +/-5% accurate.
Or datasheet here:

This states +/- 5% (which seems to me to be a nice way of saying 10%) accuracy.

So at 92°C it could be reading 87°C or up to 97°C and still be in spec.

I guess that’s why they put the “N” on the gauge and got ride of the real numbers.

In my mind, the trend is more important. If it has always read that, then fine. If it has suddenly changed, then look for a reason.


(PeterCrespin) #14

Exactly. The trend is what it’s there for and the possible/likely causes (cold day/slow driving/hot day/ modest & stable divergence from typical reading etc.).

No harm asking the question and the owner has had the car from new, but there must have been lots of silly complaints to dealers to make Jag go to idiot gauges after 80 years of genuine pressure/temp readouts, not necessarily hyper accurate.

(SilkCut) #15

Yesterday, I tried with 35°C to replicate and staying at a cruising speed of 120 km/h the needle of the gauge stays on the right end of the N, but downshifting to the third gear and reaching 180-200km/h, the needle goes beyond the N, leaving it totally uncovered.

At car wash, however, I discovered something that never happened before: the radiator cap in the central position (with the “coolant” adhesive) cooled out, sprinkled almost everywhere on the engine.

One month ago, I changed all the coolant, most probably the cap has been removed for the second time in 20 years and now the gasket does not work anymore.

Can You confirm that both the radiator caps, (on the left and in center) are of the same type (CCC6707)?

In any case in September I will substitute both the thermostats.

Is it true the story of drilling a 1/8" hole in the upper lid of the thermostats?


(Mark Eaton) #16

This was posted on the Jag Lovers site back in 2010. I don’t know for sure, but I thought both caps were different …

In summary:

A cooling system on ANY car with an overflow tank (atmo catchment
tank) MUST be air tight ‘‘EVERYWHERE’’ right to the overflow tank tube vent tube.

The Atmo tank is not itself pressurized.

BOTH gaskets on the pressure cap on the header tank must be perfect.

The crossover tube must have good gasket(s).
If a pressure cap is used on the crossover tube,which is unnecessary, it must have a GREATER/HIGHER pressure rating than the header tank’s pressure cap…

The tank fittings where the caps twist on must be in good repair and FLAT.

The vent tube on top of the radiator mount MUST be clear.
The BULGE in this tube is a VENTURI and MUST be functional to suck air and deliver it to the header tank

The BANJO bolts MUST allow flow in and out of the tube. Thick copper gaskets may impede the flow.

If all this works as specified above, even the small amounts of air
are expelled while you travel via the VENTURI. When the engine
cools down it WILL refill the airtight inner wheel fender mounted
header tank via suction.

(SilkCut) #17

Finally I decided to do the complete work on the cooling system:

  • radiator disassembled and externally did not present residual dirt
  • open radiator and was only clogged for 20%
  • now I will proceed with the reconstruction of the radiant mass
  • replacement of thermostats
  • replacement of viscous fan coupling
  • cleaning of the various ducts

I hope that the sum of all things (radiator clogged at 20%, viscous joint with old oil, original thermostats) lead me to have the needle of the instrument on the left of the N and on the N when load is applied to the engine.

How is the needle usually in your XJS 6.0?

How many of you have replaced / rebuilt the radiator and the viscous fan coupling?

(Steve) #18

Generally stays within the “N”, more towards the second vertical line.
After a spirited driving on the highway, let’s say going 70-80 MPH for some time, if you get caught in slow or stop-and-go traffic, it may observe that the temperature gauge moves one needle-width past the “N” at which point you should hear the roar from the aux. electric fan – mine is a convertible and the noise is clearly heard.

(SilkCut) #19

News after the maintenance:

  • the radiator was clogged for 20% on the right side, but more than 50% on the left side, in particular in the radiator section reserved for the left bank .The radiator is divided into three sections (1/4 left bank, 1/4 right bank, 2/4 common). It has now been rebuilt, improving radiant capacity but leaving the original side cassettes.

Now, on a 120 km / h cruise the needle is immediately before N (without covering the N) and it takes a long time to get there.

I think the result has been achieved and there is a good safety margin for the torrid summers.

I recommend this work at all, the risk of ruining engine heads or valves is high.