Operating temperature of XJS V12

XJ-S
1

Hi all

I have a 1988 XJS V12.

What I really want to know is what should be the operating temperature of the car! I have installed 4 very powerful electric fans front and rear of the radiator and have the 82C thermostats. The radiator is a new aluminium high performance radiator and I have modified to to be single direction. The water pump is also supposed to be the high performance type and is new. The temperature gauge sticks around the bottom of the N under normal gentle driving, however it climbs to the middle of the N and slightly higher when I start to push it. I also installed an independent temp gauge connected to the other bank. When the temp needle is at the bottom of the N the analogue temp gauge shows around 88C. When the needle is in the middle of the N, the analogue gauge shows around 93. When the needle approaches the top of the N the analogue needle is showing 96C.

When I slow down the temperature slowly fall back to 88C-90C.

Is this normal behavior? Most of the posts I have read seem to work the other way around, temperature rising at idle but cooling down after the car starts to move!

I should also admit I have the AJ6 engineering’s ECU, larger throttle bodies and exhaust system installed. Ambient temperature is around 33C.

A friend has an unmodified XJ12 1990 which has a temperature gauge that shows actual temperature. The green part of the temperature gauge runs from 90C to 130C!!! His car was showing 105C under normal driving!!! I’ve attached a picture of the instrument cluster.

XJ12 temp2
XJ12 temp2.jpg829×553 80.6 KB

2

If you have an 82’c thermostat, then the minimum operating temperature is set to 82’c. An efficient radiator/fan combination will then keep the temperature from rising more than 6’c and every time it succeeds in returning water to the engine which results in a temperature below 82’c, the thermostat will simply divert some of the flow staight to the bypass to keep the temperature at a minimum of 82’c.

Essentially, your temperature will cycle between the minimum and ~6’c above that.

Temperatures in your friend’s car have no bearing on your situation. He may have 88’c thermostats fitted and a less well maintained cooling system or a different pressure cap fitted.

kind regards
Marek

1 Like
3

Hi Marek

Thanks for the reply. I guess my question on my friends car is why does the temperature reading on the gauge suggest that temperatures upto 130C are OK???

Also do other owners find that their car raises temperature when then start to push it? Or are the temp gauges generally rock steady?

Thanks

4

Most cars have temp gauges that are calibrated to respond very slowly or to remain solidly in the center/normal/safe zone over a wide range of temperatures to assuage anxiety over temporary increases in coolant temp. For example, my '95 Jag had such a temp gauge and the gauge needle remained solidly fixed at dead center from 174ÂșF to (at least) 205ÂșF. I’m not sure at what point point is was calibrated to begin registering a higher temperature.

On my Honda and Ford pickup the temp gauges are somehow designed to reflect changes very slowly. If the coolant temperature rises the gauge will show the change
eventually. It might take a few minutes.

Older Jags still had the gauges with moving needles :-). . On the older XJSs with barrel gauges the temperature issues are often confounded by unreliable and inaccurate operation of the gauges.

Anyhow


Ideally all cars would have a cooling system that is capable of over-cooling the engine under ALL conditions thus leaving the thermostat to truly control the operating temperature at all times. I don’t believe I’ve ever owned such a car
but I’ve owned about 30 so my memory might be failing. I think every car I’ve owned has shown increased coolant temp
beyond thermostat control range
under some circumstances such as pulling a long grade on a hot day, idling in Las Vegas traffic in August, etc.

Some believe that any increase beyond thermostat control range constitutes overheating. That’s one way to define it. Others define overheating as 'boiling over"
which you’d certainly want to avoid, obviously.

Personally I believe that coolant FLOW to ALL areas of the engine is more important on a V12 than any specific coolant temperature
within reason, of course. If you don’t have full and proper flow, shoot, you can damage the engine with 80ÂșC coolant temperature showing in the gauge.

OTOH, if you’re sure you have good flow, the engine isn’t gonna explode when the needle hits some magic number on the gauge.

I have a Series III V12 like you mentioned and pictured. Under 95% of conditions it runs at 88ÂșC with 88ÂșC thermostats. During a recent heat wave (100ÂșF and higher ambient) the needle would creep up to a bit
just past the center of the gauge. I wasn’t alarmed, personally. My old XJS V12 did the same under similar conditions
as has every car I’ve owned that I can recall

If your pal’s V12 is running 105ÂșC under normal/favorable circumstances then it has a problem for sure. I’m not saying that 105ÂșC, in and of itself, will cause damage. Maybe, maybe not. But at minimum the cooling system should be able keep the temperature within thermostat control range in normal/favorable conditions. If it can’t, there’s a problem.

Just my 2-cents. Opinions will vary.

Cheers
DD

1 Like
5

Doug,

This argument has been repeated far too many times.

The thermostat only sets the minimum temperature - it has no control whatsoever over the upper bound that can be reached.

The cooling system always overcools because it is not an active system (save for cutting in the fan). When it does its job of overcooling, the thermostat blocks off the flow to the radiator because the minimum temperature has not been reached.

If the cooling system cannot overcool, then a runaway overheating event has to occur.

Put another way, the thermostat can exert control only when the temperature is below its rated temperature.

What I find worrying in the original poster’s description is that 130’c is reported. If accurate, this implies a very high pressure inside the cooling system or insufficient effective cooling is going on.

kind regards
Marek

6

I should have chosen my words more carefully. By “overcool” I meant keep the temp within thermostat control range.

But being out of thermostat control range doesn’t necessarily produce a runaway overheating event. Unless you consider any temperature higher than control range to be overheating

I agree

That’s not how I interpreted his posting.

He reported seeing
on a friend’s car
 a temp reading of 105ÂșC but was asking, I think, why the green range of the dial extended all the way to 130ÂșC.

It’s a good question as the green suggests “OK” or “Normal”. Apparently Jaguar felt that temp readings that high are no cause for concern. Even though I’m a bit more nonchalant about coolant temperature than many others, I don’t understand the rationale.

I’m reminded of a Road and Track road test of a 1969 E-type. the author mentioned how Jaguar replaced the numeric markings on the gauge face with “cold-normal-hot” range indications
presumably to allay owners’ worries about what constitutes a problem. But apparently the “normal” range was situated mostly on the right side of the gauge face rather than the middle, thus negating the whole concept.

Cheers
DD

7

Gents

Maybe we should try this another way. What could cause the engine to overheat under load. Since even in high ambient temperatures (40C) the car stays at 88C while idling even in traffic.

  1. Faulty head gasket? If so what are the tell tale sign as Oil and coolant both seem clean.

  2. Faulty vacuum advance? This is new and holds vacuum. What other signs are there?

Thanks

8

It’ll overheat if the dissipation of heat is less than the production of heat.

Looking at both factors:-

1/ Heat production will be greatest under load, when the injectors are pulsing for longer than when the engine is not under much load.

2/ Heat dissipation comes via air cooling and water cooling. Water cooling is at a minimum at zero to low rpm and air cooling is at a minimum at low or zero speed.

Accordingly, very slow hillclimbs will be the worst and stop/go traffic will be next. You can shoot yourself in the foot successfully by pulling up too close to the next car in front - the radiator is then blasted with hot exhaust gas rather than getting cooler ambient air. Once the car is stopped [edit:- meaning the car engine], forced coolant circulation ceases and the temperature of water remaining in the heads rises until convection, any syphoning of water and conduction equal the “heat soak” that is going on. An 82’c car can easily see 95’c before the temperature slowly drops away again.

If the radiator/fan combination do not cool enough to make the thermostat want to move to a more closed position, then you are overheating and have lost control of temperature. So long as this is short lived and only a temporary thing, then it isn’t going to cause damage because the temperature doesn’t go suddenly jerk upwards and the thermostat is always lagging in its response to what caused the heat to be generated.

An example of how overheating (but not under load) will manifest itself is often seen at the end of a journey, when the engine with a marginal cooling system is shut down. Although at zero load, heat soak can be a worry simply because heat dissipation is so low.

The “fix” in such a situation is to wait for heat soak to continue, but not long enough to cause damage, and then run the engine for about 45 seconds. What this does is swap the water that was sitting in the radiator with the water sitting in the block and cylinder heads. Whilst the water in the heads got up to 95’c, the water in the radiator has dropped in temperature accordingly as the radiator is insulated from the engine, open to the outside air (which is at a lower temperature than the engine block) and most importantly, it wasn’t being fed hot water as the engine had stopped. Running the engine for longer than 45 seconds is counter productive, as the cold water, after coming into the engine will be warmed enough to eventually force the thermostats to close and block the route to the radiator and thus block the source of cold water.

I don’t think either 1 or 2 will cause overheating if the coolant system can cope with idling in traffic. You’ll need to kill off the cooling by restricting either or both of water and airflow, perhaps via a faulty fan switch or somesuch mechanism.

kind regards
Marek

1 Like
9

For the purposes of this discussion will assume that you consider the 93-96ÂșC you reported earlier to be overheating and want the temps back down into thermostat control range. Which is understandable

One thought is that your array of fans is serving to block airflow. I’m not qualified to comment on the science of it but can’t a spinning fan actually serve as a a blockage under some conditions? I dunno. Others will.

Describe “under load”. Are the conditions simply beyond the capacity of the cooling system to keep temps in thermostat control range?

[quote]

  1. Faulty head gasket? If so what are the tell tale sign as Oil and coolant both seem clean[/quote]

IMO the best test is to check the coolant for hydro carbons. You can buy a test kit for this. Or, if your local repair shop has an exhaust gas analyzer you can have them use it to ‘sniff’ the coolant to detect hydrocarbons.

In my experience, FWIW, only a major head gasket leak pollutes the oil. A minor leak, tho, can still allow compression to be pumped into the cooling system

If the vacuum advance is new and not being defeated by the control system, and the centrifugal advance isn’t seized (have you checked?), and the base timing is at spec
that’s probably as good as it gets as far as timing goes.

To spitball a bit, is there any reason to suspect a transmission problem? If the trans is running too hot it can add additional heat into the cooling system via the transmission cooler.

Brakes dragging?

Cheers
DD

10

I have installed a seperate cooler for the transmission and brakes are clear.

For a minor gasket leak, would tightening the head bolts help?

Lastly is this behavior of temp needle rising and falling under load normal? Do other v12 owners have the same experience. What temperature is considered overheating?

Thanks

11

Good

I have my doubts

Again, can you describe the “under load” condition you are referring to? A 10 mile climb up steep a mountain road? My V12 will do that on a cool day and the needle won’t budge. On a hot summer day, it’ll creep up a bit but not continue to climb forever. It’s beyond thermostat control, yes, but not a runaway situation. Some would consider that overheating.

If “under load” simply means blasting down the highway at 75 mph under mild ambient conditions I wouldn’t expect any increase at all, judging from my own experiences. But during our big heat wave that sort of driving resulted in my temp gauge showing a couple needle-widths past 90ÂșC
where its stayed for umpteen miles. I wasn’t worried, personally. But some say I should’ve been.

You previously mentioned 93-96ÂșC. In and of itself I don’t see that temperature as a problem. it’s not like the engine gonna blow apart just because you’ve hit 96ÂșC. But if it gets there with very little provocation it suggests you have a problem, especially considering your special radiator, water pump, 82Âș thermostats, etc.

Rather than ask what constitutes overheating you might ask at what temperature others feel compelled to resign resign themselves to the side of the road.

Cheers
DD

12

If the fans are off, maybe. If the fans are running backwards, definitely! But
if the fans are running the correct direction, they shouldn’t be a problem.

Front spoiler in place? Do you have a front license tag on it?

– Kirbert

13

I’m just a listener here as I’m in the process of installing many mods to my cooling system (including new fans) so I can’t add much except to say that recently I read an article on some problems incurred by running push and pull fans together.
When I find the article I will post it. It dealt with a reduction in air flow rather than an increase due to them being out of sequence or phase (paraphrased)??? Worth a thought. Also it’s a lot of plastic in the way of the radiator! Also Kirbert’s comment re fan/s running backwards either by being wired up backwards or blades reversed are easy mistakes
 been there done that.
Trev

1 Like
14

Another thing, how good are your fans?.. I recently bought a couple of new high performance fans on the internet (e-bay). They were rated as “High Performance” suitable for racing and highly modified cars. Described as 220Watt (for the 16") and 120Watt (for the 10") auxiliary fan. They tested on my bench at 84Watt and 36 Watt respectively (peak power). They went back to the vendor.
Trev

15

I’d put an electric water pump in, as well.

Fazal

1 Like
16

Knock on wood. My lump does great on a pair of e fans, and an electric water pump.

And, yes, the greens on the guage to the right of center are misleading. seems lime orange to red might be more indicative.

Carl

17

The “could be” and “try this” lists can go on forever while missing the original question, which was:

“What I really want to know is what should be the operating temperature of the car!”

Concerns over cooling (and more specifically, dropped valve seats on the V12) are certainly legitimate but in my opinion have reached the point of paranoia.

To get closer to the heart of the matter I think a good question might be


“Does ANY temperature increase beyond thermostat control range, regardless of surrounding conditions, represent a danger that dictates corrective measures?”

My answer, my opinion, is “No”. There are some situations where some increase in temperature, over control range, is no cause for alarm.

As for the OP’s situation I’d still like to know what “under load” means.

Cheers
DD

18

Underload means moderate hard acceleration. High speeds of over 100mph. Extended hill climbs.

19

Yeah, I think most of us experience a climbing temp gauge under those
conditions. Jaguar itself reportedly couldn’t get an XJ12 past some
Japanese acceptance test because it’d overheat on hillclimbs – brand new.

While the temp gauge may rise, there are two things to watch for: 1) Does it
settle out at some point, or does it continue to rise indefinitely? and 2)
When you cease pouring the coals to it, slow down and start motoring
around town at normal speeds, does the temp continue to rise, come back
down slowly, or come back down quickly?

– Kirbert

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The temp gauge goes up fairly quickly but when one slows down comes back slowly.