the 12-cylinder engine is a hot-running engine to begin with, so if you've changed thermostats, did you install one that opens at a lower temperature? (that depends on your local climate too, but your thermostat may be stuck-closed, or opening at a higher temp than desired).

does your car have an Auxiliary Electric Fan or two?,(in front of the radiator?), Is it coming ON when the gauge reaches a certain temperature? If not, the auxiliary fan Relay, Fuse, or Thermo-Switch (if applicable) may be bad. I am guessing here because I am not familiar with the particular model, but the 12-cylinder engine since the 1970's has a pair of electric "air-pusher" Fans which came wired from the factory to come ON and stay ON as long as the A/C compressor is ON. In other words, they should run as soon as you turn theclimate control to ON. Compare that to the XJ-6 which has a Thermo-Switch to control the temperature at which the Auxiliary Fan comes ON.

All the above assumes your Cylinder Head Gasket is good, and that you have the required mix and quantity of coolant & water in the system. A product called "Water-Wetter" might help with lowering the temp, you just add it to the coolant/water mix. It is usedin race cars. Whenever topping up the coolant, start with a COLDengine, turn the HeaterON to maximum temperature so thatthe Heater Valve opens when the engine reaches operating temp and allows coolant to fill the Heater system too.

And last: is the Water Pump good? No leaking through the "weep hole" ?

 

Also, under the car there might be a plastic piece that takes air coming in from under the car and makes it hit the engine. If that piece of plastic isn't there, then the air keeps going under the car instead of to the engine! Just my 2 cents. It happened to my dads car, so just food for thought!

 

If you gauge goes all the way to the top your car is way too hot. I read an article that a Jag specialist wrote and he said that if the needle rests on top of the N the car is too hot.
Mine is acting up right now and I am having a new rad put it, as a matter of fact it is an after market rad that runs 40 per cent cooler than the original. I am replacing all the belts, the fan belt, fan clutch. and I just replaced all the coolant hoses. Next I am going to have louvers put in my car hood or have a hood scoop put in. And always carry a fire extinguisher.
Mine is a 1988 XJS hardtop and it only started acting up this summer, it wasn't last year. So please, keep your temp below N and don't drive it til it gets past that. You will harm your engine....read what others are saying.....they tell the truth..

 

Are you talking about the spoiler under neath the bumper?

 

The Jag V12 is a very robust engine, there is really only 1 thing that will kill it HEAT and can be very expensive to fix.

I would do the following
1 Have the cooling system flushed and pressure checked make sure the radiator is functioning correctly and the fins are not damaged.
2 Ensure you have replaced the thermostats with the correct ones and they are operating properly. The thermostats must have jiggle pins to purge air from the system. The Jag manual test procedure for the Thermostat is as follows

Place Thermostat in water with a thermometer on the stove (DO NOT LET THEN TOUCH THE BOTTOM OF THE PAN) and bring the water to the boil observe thermostat opens completely before the water boils probably around 80 deg C or so.

3 A stuck advance can also cause the engine to overheat.
4. A faulty main fan clutch
5 . the Front spoiler must be there as it directs air through the bottom of the radiator.

hope this helps

 

Good advice people, but you did notice that the OP is over 3 years ago?

 

More knowledge/data/info just makes the search function more useful!

 

I had to flush mine about 3 times when I first got it. Is it holding water? Mine was running hot and I found that a large number of the water hoses had dry rotted to to non-use and was loosing coolant. At one point, I couldn't figure out where all the coolant was going, so I just giggled the pipes to see if I could expose a crack (while the engine was running and hot ) Woosh! off came the hose and hot as hell but I found my leak. You may want to try this with a cold engine

Until you can prove to the contrary, when temp or water warning goes off - pay attention or it's adios!

 

My temperature guage starting riding the top of N and when it gets at N I pull it over and let it cool down. So today I took my car to the car wash and pressure washed the fins in the radiator. I used degreaser first, then hot soapy water, let it soak then rinsed it, you should have seen the junk falling under my car. I hosed it good from between the grill, then I got down under the bumper and took the pressure washer to the bottom of the radiator as well. I washed the side that faces the engine as well....what could be gotten at.
The car runs cooler, only ran half way to N instead of running all the way up to N and over the top. The best test will be in rush hour traffic on a hot day and we'll see if it still stays at half way to N.
The crap that came out of there was amazing, because it was not obvious from just viewing the radiator, it was all stuck between the fins, some of the debris looked like small pieces of burned wood, some small rocks, lots of sand, dead bugs on the surface of the fins, and other stuff that could not be identified. My car runs cooler....give it a try yourself, can't hurt it and maybe it will solve your problem, hope this helps, let me know if it has helped.

 

Good work !

I've seen some where the build-up of debris between the radiator and condensor blocked off a full 30% of the air flow.....


Cheers
DD

 

Thanks Doug for the "good work" compliment.
Today was the "test", I ran my car all day and for several hours in hot weather in rush hour traffic where one sits at every light and could walk faster than drive and my car's temperature guage only got up to the bottom of N. the bottom part of the needle was sitting on the bottom of N. Before I washed the rad fins I could only drive my car for 20 minutes before the temp was up to N and shooting over the top where I had to shut it down and open the hood. But one has to make sure they fire up the car and run it for a minute or so then shut it off again to let the coolant circulate so the car's temperature does not shoot up because of heatsink.
I only managed to get some of the rad in the first cleaning and it made such a difference and I am going to clean it again in a couple of days. For anyone doing this make sure you don't use a pressure wash that is too harsh and bend the radiator fins.
With that all said I am waiting for a radiator upgrade from Faster Jags in Dallas Texas. All aluminum and runs 40% cooler than the original rad. I didn't think it made sense to pull the rad, recore and the put it back in because the cost of a brand new upgrade was only about $150 more and would last many more years. I already had all the coolant hoses changed anyway.
But for those of you who don't intend on buying the upgrade as I have ....clean your rad fins, it helped me alot and it might help you as well. Well good luck to all those Jag lovers with hot cars and hot engines. Keep
'er cool ~!!~

 

Is there a website for thie Faster Jags?

 

http://www.fasterjags.com/

 

yes it is fasterjags.com
or email Brian at brian@fasterjags.com
Tell him that Donna in Canada ordered a radiator from him and that I gave you his name and email address if you haven't already got it from the other guy who gave you the address.
I see you got a XJ6, faster jags is a site for the XJS, I don't know if it uses the same radiator that the XJS uses, I got a XJ6 as well-a 1987 model.
Still prefer my XJS though, much different driving experience and a two door which I prefer.
I haven't had a lot of trouble with my XJ6 except the head gasket went on it.
If you want parts call John Koish at Welsh Enterprises in Ohio, he is a great man to deal with and really knows his stuff. I get stuff shipped from him all the time. And along with my rad from Dallas I am getting Welsh Enterprises to send me all the belts, fan belts, fan, fan clutch next week. I have previously ordered spark plug wires, coolant hoses, boot covers, hub carrier and stuff like that from him.
They deal with all makes and models of Jaguar parts new and used. Good luck, if you need more help post another note here and I will do my best to help you again. Donna

 

I have an XJ6 and an XJS convertible with the V12, I just don't have it on my profile yet!

 

I would check the centrifugal advance in your distributor to see if it is seized. They are notorious for freezing up and if they do you will overheat. To check it remove the distributor cap and try to turn the rotor. It should turn easily about 10 degrees and snap back when you release it. If it doesn't then it is seized.

 

have a similar problem it takes around 10 minutes to heat up and goes down at 40 mph and the oil press. goes down as the heat goes up.

Mine does that too make sure to use 2050 motor oil with supplement. But apparently it's common in the big cats. 86 XJS V12