www.jec.org.uk

Please,
I need to know the normal and maximum running temperature of my 1995 4 Liter XJS. I have purchased a laser thermometer (Only £38.00) while the gauge shows half way between normal and high, the thermometer shows about 84 degrees centigrade when pointed at the thermostat housing. The radeator is like new? the car is only 21000 miles old.
Watts the answer?????
Pete Rushen.
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1995 XJS 4 Liter

In that case the question must be: "What's the SI unit of power?".

Or a joule per second if you want to get technical!

As they are so cheap and easy to change, I would change the thermostat as a first job.

From what I've heard and seen the temp guage needle should remain below the N at the middle of the guage. Anything above N is apparently too hot, according to two Jaguar specialists whom I've consulted on the subject.

The guage on my 3.6 never gets anywhere near N and I had to change the thermostat in the autumn last year to get it to move at all!

The thermostat should open well before 84 degrees (I think mine opens at 72) so as Jon says it's not a bad place to start. I'd also be tempted to flush the radiator out and see if there's crud inside it.

Jon / Simon r

Thanks fellers for your replys, But I still nead to know what tempreture the thermostat housing should be ( 84c ? ) .

I would change the thermostat If I new it was wrong ?

Peter.

Peter,

Wait to see if someone else agrees with me but if your cooling system is operating correctly, your thermostat housing should be just a little hotter than the rating of the thermostat. In my (admittedly so far fairly limited) experience of the XJS 84 degrees is on the hot side, this fact if matched by the fact that your temperature guage is higher than I would expect it to be.

It really depends on what thermostat is fitted to your car - I think the one I fitted to mine last autumn opened at 76 degrees.

Jon / Simon r

Thanks fellers for your replys, But I still nead to know what tempreture the thermostat housing should be ( 84c ? ) .

I would change the thermostat If I new it was wrong ?

Peter.

Hi Peter,

I do not know the specific temperatures for the 4ltr, but I don't imagine they will be very different from those of the V12 (given the V12's sensitivity to high temperatures). I expect your car will be fitted with either an 88 deg C or 82 deg C thermostat. 88 deg is more common in UK to give better heating to the cabin in cold weather. An 88 will begin to open at about 80-84 degrees and be fully open by 92 deg. I would say a laser reading of 84 deg on the outside of the thermostat housing is spot on for an engine that is idling or having been driven normally. Driven with greater 'enthusiasm' I would expect it to creep up a degree or two. I would have no concerns as long as it doesn't exceed 93-95 when driven hard.

Incidentally, I get a more accurate temp reading from my laser thermometer if I point it at the radiator tank where the water feed from the engine enters it. These laser thermometers are calibrated for a specific emissivity. A very reflective surface will give an inaccurate reading.. The black painted surface of the radiator end tank will give best results.

regards,
Glenn.

HI Glen,

Thanks for you informed reply, I have been out with the car again this morning, and driving gently at about 60 to 70 M P H, the gauge shows just on the high side of normal, with a an air temperature of 32 to 35c ( hood up and air con, on). I will try your suggestion of taking a reading on a black surface. Today the reading on the thermometer was 77 to 79c, if I let it idle the reading goes to about 84c. Your observation about the rating of the thermostat rings true as the car was supplied new to Texas.

Best regards,
Peter

Peter,

A couple of comments:

- UK AJ16 4 litre engines have thermostats that open at 82 degrees. I know that your car is originally a US vehicle so don't know if that means it had a differently rated 'stat. Personally I think your readings indicate:

1. that you have an 82 'stat and
2. it's operating correctly

If you let the car start from cold and point your thermometer at the housing then check the reading as the stat opens (you'll feel the hose suddenly fill with water and warm up considerably). I suspect your housing reading will be between 79 & 84 at that point.

Also remember that the minor instruments in very late cars are somewhat less than perfect. Try removing the cluster and checking the soundness of the screw connections on the instruments. You might be surprised at the improvement!

At the risk of tempting fate, I don't think you've got a problem. Remember you're operating in a hot environment. My own car always runs a little bit hotter (according to the gauge) when the weather is hot and the aircon is on.

Looking forwarcto seeing you in the Uk at Littlecote.

Regards,

Paul

HI Paul,

Thanks for your help, Look forward to meating you next month.

Peter.