Hi Everyone

I'm new to the Forum. I have a 1988 XJS v12 in Pakistan, probably the only one in the country.

The issue that I have has been there since I bought the car over 8 years ago and I think I need some expert advise now. The basic problem is that the temperature of the car rises with speed and will then slowly comes back down as one slows down. So at a constant speed of 70 mph the temperature will rise to 95C (203F). Then when I go to 85mph it will rise to 102C (215F). Over this speed the car temperature continues to rise! However the minute I start to slow down, the temperature starts to fall again. Also the temperature rises really fast! It rises within 20-30 seconds and then falls back down over minutes. Also even if the car has to climb even a mild rise, the temperature again rises quickly even at a slow speed. So basically the temperature rises under load.

So the ambient temperature is really high in Pakistan after April, over 35C( 95F) and sometimes nearer 40C(104F). During the winter months the car's temperature will stay around the 90C (194F) until speeds over 100mph, at which point it starts to rise again. This is partly because of the very efficient cooling system that I have developed over the years to try to tackle the problem. However even this cooling system cannot handle the temperature at high speed or high ambient temperature. The car does not lose any coolant. I now don't feel that the problem is with the cooling system but if not the coolant system then what? What could cause the temperature to rise so quickly under load?

So the following is work I've had done on the car:-

i. A custom made radiator which works better than a very expensive aluminum radiator I had originally installed bought from the UK.
ii. Brand new thermostats.
iii. I have a large electric Mercedes fan to replace the Visco fan that is really powerful. At standstill it can bring the temperature down with ambient temperatures over 40C(104F)
iv. The coolant system has been pressure checked for leaks.
v. Replaced the vacuum advance on the distributor.

Apart from the above I have a sports exhaust on the car that I bought from XJ6 Engineering.

Does anyone have any suggestions or has anyone experienced anything similar? What could cause such a rapid temperature rise?

Thanks in advance.

Warm regards

 

Welcome to the forum. It's pretty cool to hear that there's an XJ-S in Pakistan.

How are you measuring temperature?

Such rapid fluctuations in temperature based on vehicle speed and engine RPM would have me suspecting an electrical issue if you're using the in dash gauge. Maybe the speedometer or tachometer signal is bleeding through the printed circuit to the other instruments on the back of your instrument cluster? The instruments can also behave strangely when a strong ground connection is not present, for this reason I ran an auxiliary ground strap to my instruments.

It sounds like you've invested a lot into your cooling system and addressed many of the common weak points. Knowing where and how you're measuring these temperatures might help others come up with possible solutions. Since you're in a pretty extreme climate especially for a British made car, it might be prudent to install an auxiliary temperature gauge on each cylinder bank; just an idea.

 

Which thermostats did you use? I ask because thermostats are critical on the V12, and often wrong! Most have the "foot" that extends to block off the bypass and when hot will only reach to about 38mm. You need to be in the range of 42-43mm long when hot to ensure that the bypass is actually blocked off and the all the coolant is actually going through the radiator. Most thermostats move about 8mm when fully hot, so you are looking for a flange to foot distance of 34-35mm when cold.

I have measured the OEM Waxstat thermostats and they do not extend far enough to close the bypass, they were about 5mm short. Of all the ones I have been able to find in North America, they do not have the jiggle pin to bleed any air, so you will need to drill a small (1.5-2mm) hole to let air bleed. I used a Gates 33188S, but I have no idea if that is available to you.

I have a 1994 6,0 and with the proper length thermostats and everything else as factory the temperature stays in the thermostat range.

 

Hi there

Thanks for the reply. I also at one time thought that maybe it was a bad instrument reading. However I checked the gauges and also the ground. Not satisfied by this I also installed an independent temperature gauge, with the temperature sensor installed next to the thermostat housing on the water rail. I even then installed a third temperature gauge temporarily on the other water rail and all the gauges read exactly the same.

 

Now this is interesting. Let me check this and get back to you. Thanks a lot.

 

OK
Well many congratulations on your work done so far. If you are not losing coolant, then I expect your engine cooling system is just fine and what you are seeing is the inevitable consequence of the ambient conditions and the power being developed!
However, I think you can measure temperatures at a better point in the system: I suggest that you locate an accurate guage sensor in the water pump inlet, near where the factory thermostatic switch is located for the original auxiliary electric fan. The relevant temperature is the exit temp from the radiator to the engine. As speeds and therefore engine work done, rise, the temperature of the coolant exiting the engine will always rise with power developed.
The critical point in the cooling system is how well the radiator and fan system removes this heat from the coolant.
Now, of course the radiator coolant exit temp will rise a bit as power developed increases; the question is whether that temperature rises to unsafe levels.
If this seems a good plan to you, do please let us know the outcome.
One final question, is the water pump in good condition internally?

 

Hi there

Thanks for the info. I'll try to attach a gauge to the radiator coolant exit temp. However I have a question. As the speed rises surely the air following though the radiator should be more than balance out the increase in rise of power developed?? At 100mph the amount of air flowing through the radiator must be huge!! I also modified the hood with vents from an XKR to allow the air to flow out easily.

This is the bit that has me puzzled?? After a high speed run with engine temperature over 220F, at idle in a very high ambient temperatures, even with the AC running, the cooling system can bring the engine temperature down to 195F. If it can do this at idle, doesn't this mean that the water pump must be working efficiently?

Thank again.

 

Yes, it does. It also indicates that your cooling system is working very well. For all my testing on temps, I used one of these: cheap, accurate, reliable. Just tape the sender on the outside of the pump inlet:
https://www.ebay.co.uk/itm/131936503...pid=2184181326

If you buy extra long wires, you can read it in the cockpit.

TWR raced these cars in Europe in very much lower ambient temps, and cooling was always a problem, remote water top-up systems were installed and all sorts of other stuff too. Also, OIL temps are crucial to cooling, it might be worthwhile installing a larger oil cooling radiator with a big fan in it as well. Oil does at least 30% of engine cooling.

 

For example:
https://www.holley.com/products/driv...ssion_coolers/
or
https://www.ebay.com/itm/13210449310...cAAOSw~yRgEYhc

This could either replace the OEM cooler (if you buy a substantially larger one, or be mounted horizontally behind the bumper in series with it.
I take it that your spoiler and undertray all all in place?

 

OK, my suggestions. If I have over looked something, stuff happens at my age.

1) What temp stats are installed. I suggest 82C for your market, same as here. The open length is CRITICAL, as mentioned.
2) The pressure cap on the header tank is holding correct pressure. One of mine had similar issues, and the 13lb NEW cap was actually blowing off at 4lb.
3) At those ambient temps, the A./C is ON, and that sees the radiator getting blasted with HOT air from the A/C condensor. Temps like that, and higher, down here are common in summer, minus the humidity mostly, but speeds in those conditions are kept waaaay down, ANY car, not just Jags.
4) The timing is retarded, and being Lucas Ignition,the distributor may be seized, and the vac capsule is possibly toast. I have PDF's for all that if you need help.

 

All the sensible answers have already been given. I'm left with offering-up something a bit weird and unlikely: I wonder if the lower radiator hose is collapsing?

As your road speed increases so does the engine RPM and water pump RPM. Perhaps enough suction is developed to partially collapse the hose? Obviously this would restrict coolant flow.

It used to be, on many cars, that lower radiator hoses had a stiff spiral spring installed for the specific purpose of preventing this problem. I never seen one on a Jag, nor have I heard of this being a problem on Jag V12s. But you could surely add one if the mood struck.

Having said that I'll also say that I'm inclined to agree with Greg that you're experiencing an expected condition: engine heat increasing in accordance with the amount of work being done and higher ambient temps. In your case...you're not alone, by the way....there simply isn't enough cooling ability to keep the temp lower. Personally I found nothing shocking in what you described. Only when you mentioned "over 220F" did I begin to squirm in my chair a bit.

Cheers
DD

 

Ok so a little update.

I checked the thermostats and they do open far enough to close the by-pass. The thermostats are 88C, but not sure why this would make a difference?? If the thermostat open at 88C then assuming the cooling system is working, the temperature should not really exceed 92C. Here I do have one question that might or might not help. I have the electric fan on a thermo switch on the water rail just before the thermostat, which comes on at 94C and goes off at 82C. Now I would have thought that as the temperature reduces when the fan comes on, once the temperature falls to 88C then the thermostats should close and the temperature should stay static around this temperature. However the temperature decreases down to 82C where the fan turns off, then the temperature starts to rise again. Why does the engine temperature drop down to 82 when the thermostats should not let this happen??

As recommended I installed a temperature sensor in place of the thermo switch on the water pump inlet, and connected it up to an temperature gauge. The temperatures were exactly the same as that seen on the gauge connected to the temperatures on the water rail. I was expecting the temperatures to be lower as this is the point the water would be coming in from the the radiator? Something doesn't seem right!!! Even with this gauge on the inlet of the water pump, the temperature rises with acceleration and rises really fast, within 20-30 seconds the temperature goes from 90C to 100C. Then the minute I start to slow down the temperature starts to fall immediately!

Lastly I connected the gauge to an independent earth line from the car's boot, to make sure it wasn't an issue with the gauge rising with the rpm due to a bad earth. Again no difference.

 

Have you checked the fan clutch? Some symptoms of a bad clutch are like you describe.

 

hi there

Actually I completely replaced the clutch fan with an electric setup.

 

OK, 3 coffee's down, more to come.

The fact the temp rises with acceleration gets back to the 1960's, and my mentor drummed into me relentlessly, \:

HP = HEAT. The more power you develop, the more heat the cooling system needs to get rid of.
Back of the power, as in slowing down, coasting down a hill, etc. the temp drops, no power being created.

The stats on any car do NOT simply open and stay there, they are constantly moving to regulate temps.

This write up I did years ago covers it.

88C in your market, like mine, is borderline for the system, sadly. Hennce the use of 82C, and no more issues.

I still reckon timing may be at play here, and on teh V12, 1 or 2 degrees is all it takes.

 

When you said that the vacuum advance was replaced, did you test it to make sure it actually works? I ask because once I bought a brand new one in a Jaguar box and the two rotating pieces were crimped together too tightly and it wouldn't turn.

The V12 is very sensitive to the vacuum advance working on the Lucas ignition. I'm assuming you have the Lucas, not Marelli when you talk of the vacuum advance? It might be worth splicing in a vacuum gauge right where the vacuum line joins the distributor and see what you are getting at highway cruise. I once had the dump valve fail and that results in no vacuum advance, plus a reduction in fuel economy of about 25%. Retarded advance will result in more heat generation too.

 

OK, This is significant. If there is NO temperature drop across the radiator, then the radiator is not working. To be sure, have a temperature probe on the radiator inlet AND on the radiator outlet. Please post the actual temperatures that you read.

Next point: you say you had a radiator made locally. This is important: Did the radiator follow the original Jaguar TWO PASS design? Diagrammatically, the water flow through the radiator should be like this:



If your radiator has been made as a single pass, like this, it will NOT work properly (because B bank coolant will short-circuit straight down to the radiator outlet):

Finally, the electric pump thermostatic switch and the engine thermostats, as installed in your car are working against each other. You should definitely install 82 degree thermostats. This will mean both parts of the system are trying to achieve the same thing.

 

Hi all

So the next instalment in the saga:-

1. I checked and actually the thermostats installed are 82C.
2. The custom made radiator uses the original side tanks and this has the blanking plate to make sure the water flows as per the diagram above.
3. I took the car to a local dyno tuner that I know who let me use his dyno. So we hooked up the car, put the o2 sensors into the back of the exhausts. Warmed up the engine to 90C and then let lose in 2nd gear, running right up to the red line. The temperature shot up to 100C. The dyno showed 265bhp!! There was no engine misfire and the AFR ranged between 12 and 14. The minute I took my foot off the accelerator the temperature started to drop and within a minute was below 90 again!!!

OK so I have a hypothesis. Maybe my water pump IS weak. There are two outlets on each side water rail, one for the front of the engine and one from the back. Now if my water pump is weak, maybe only the water from the front of the engine is being sucked out, while the water at the back of the engine is flowing very slowly and hence getting very hot. Now since the water from the front of the engine is circling faster, it is staying cool and all the sensors from the front of the engine are reporting a cooler engine. When I accelerate, the water pump starts to pull more water from the back of the engine, which is very hot, so I see a sudden increase in temperature on the gauges. Could this be the case???

I have one more question related to the ignition amplifier. I'll also start a new thread on this as well. I moved the ignition amplifier away from the top of the engine and into the cabin of the car to stop it from overheating. I bought two new coils from the market that matched the specs of the original coils. Now when I would connect both coils in parallel, as per the wiring diagram, the ignition amplifier gets really hot! Hot enough not to be able to touch it. When I disconnect one of the coils, so the whole system is running only on one coil, the ignition amplifier only gets warm!!! What is the reason for this???

I'm currently using a Mercedes coil from a w140 V8 engine. This seems to be a half way house in that the ignition amplifier gets hot, but not as hot as with the double coil setup.

Thanks again.

 

Water pumps are not normally known to fail, so I'd be hesitant to blame it. Have you checked the V belt tension with a proper gauge and verified that the tension is correct and the belt isn't slipping at higher revs?

Second thought: I used to have a 3.8 E Type and when the temperature outside got above about 22, the coolant temperature would continue to rise when the car was driven and not much would bring it down. I had a new radiator, modern electric etc to no effect. Eventually I decided to clean the block internally. I drained the coolant, flushed it completely with water to ensure all the glycol was gone, and then filled it with vinegar. I started the engine, ran it up to temperature, then let it cool and drained the vinegar. It looked like milk coming out, there was so many minerals from hard water that had deposited inside the block. I drained this mixture into a large pan that went under the entire engine, and I let it sit overnight. In the morning I had a layer 1/2" thick in the bottom. I flushed the radiator and block several times with clean water to make sure all the vinegar was out and refilled with fresh coolant. I never had any trouble with overheating after that.

So depending what your coolant and water is like, is it possible that the engine simply can't get rid of the heat fast enough due to a layer of mineral deposits?

 

Another thought, if you have one of the hand held IR thermometers, is to measure the side tanks temperature from top to bottom, preferably with the fan running. I would normally expect to see about a 10 degree drop from the coolant leaving the engine, to it exiting the radiator.

So suppose you have coolant at 90 going into the radiator at the top, it should be about 80 at the bottom of the tank. Do you have a temperature gradient from top to bottom? If you don't, then your problem is the radiator.