What is the resistance on the coil yo are using. The coil MUST have a primary resistance of 0.7 ohms maximum. Thos is the coil you shold be using on a single coil setup:
https://www.jagspares.co.uk/product/...6-0/dac4608%23

 

What is the resistance on the coil you are using? The coil MUST have a primary resistance of 0.7 ohms maximum or the amp will fry. This is the coil you shold be using on a single coil setup:
https://www.jagspares.co.uk/product/...6-0/dac4608%23

 

A bad pump is a possibility; but you MUST, as Jagboi suggested, measure the temps on the radiator, both sides, top and bottom. Use one of these:
I still think your radiator is not constructed correctly, by the way!

There is also somehing called the Lutz mod, which does address the cooling circulation issue in the heads. You are correctr that coolant tends to short-circuit through the front water manifolds. This mod consists of reducing the size of the front water manifold exit holes as in this photo. BUT, until you start measuring, you are only guessing. Accurate measuring of temps will show you where the problem is.


Front water manifold with reduced outlets. The smallest hole is the frontmost one. This forces coolant to the rear of the engine and prevents coolant short circuiting preferentially though the front of the head.
.

 

OK,

More thoughts and some repeating.

Elimination is now the name of this game.

I believe the timing needs to set, and drive timing is the only way I do it, any car, not just Jags.

Attached Doco tells you how I do it.

ALSO

That Lucas distributor is well documented as seizing the mechanical advance mechanism, thus stealing 15 Deg of advance at revs, which equates to 15 Deg of RETARD timing, not good, and excessive heat will be developed.
Attachment explaining that also attached.

 

Thanks everyone! You guys have been amazing in your responses and helpfulness. I'll start on the next set of investigations and report back but just wanted you guys to know how much I appreciate this. Having the only XJS Coupe v12 in Pakistan is a lonely business.

Very warm regards

 

Please explain about the size and speed of water flow through your radiator; it is possible that the water is flowing through it too quickly, and also possible, if you have too thick a core, that the air cannot penetrate it well enough to remove heat.
Finally, the airflow through the radiator stack is crucial to cooling. I have an extra slot in my bumper to improve clean airflow to the radiator.
See post number 21 on this thread:
https://www.jaguarforums.com/forum/x...2/#post1859743

 

I rebuilt my 1979 coupe many years ago with a later V12 5.3L HE engine.
The OEM fan was replaced with an electric fan, but the radiator is still the OEM one which has a new core.
The OEM coolant pump was replaced with an electric pump.
This allowed me to cycle the fan on and off manually to check the performance of the cooling system.
Normally the thermostats control the coolant temperature as long as there is a reserve of cooling capacity in the system. Once that reserve is used up the coolant temperature rises if more heat is generated in the engine. With the standard OEM setup the most critical cooling is when the car is idling because the mechanical fan is marginal to draw enough air through the radiator. By cycling the electric fan on and off I calculated the cooling reserve was used up at 45degC ambient. With a stock standard car being driven the fan speed increases and the speed of the car induces more airflow through the radiator. This appeared to give more cooling reserve at typical cruising speed, but driving up gradients or high speed cruise will lower the reserve available.

My 1988 convertible with 5.3L Lucas engine is stock standard. I know from driving here in a hot Australian summer that on a 40degC day the cooling system has reserve for normal cruise but the thermostats will be at max and coolant temperatures start rising if I drive up hills with a significant gradient.
The amount of heat removed from the engine depends on 2 main variables. First the flow of air through the radiator and second the flow of coolant through the engine. The flow of coolant only needs be sufficient to remove heat and lower temperature to stay within thermostat controlling range. The airflow likewise only needs be sufficient to achieve that level of removing heat.
In a well designed system as engine load increases the flow of coolant and air should rise in balance to maximise efficiency of removing heat. If one increases dramatically but the other does not then thermostat control is lost.

I have to say owning an XJS in Pakistan presents some challenges, well done. I would expect even in 40degC ambient the engine should not overheat in normal cruise up to maybe 80mph. The symptoms described so far suggest that not enough air is entering the radiator at normal cruise to stay within thermostat range. How to prove it ?
As already suggested measure temperature at inlet and exit of the radiator coolant hoses. Also in front and behind radiator. Plot the 4 temperatures against engine RPM or road speed as the car goes gradually from idle up to a high cruise speed.
There are a couple of caveats here. The OEM air dam mouldings below the front of the car should be fitted. The engine efficiency must be within spec over the operating range which is a timing and advance/retard issue. Maybe an A/F ratio issue too.

 

Haydar,

Some more things come to mind, thanks to Greg in teh phone call just now.

1) The chin spoiler/lip under the front edge of teh car is fitted?, and not split and broken?. That item missing is common, as its easy to smash in car parks etc. and with it missing the -ve pressure under the car is seriously compromised, and overheating is reported.

2) Transmission fluid cooling should be removed from the radiator. Most trans fluids run at 90C+, and adding that to coolant to then be cooled by the radiator is a lost cause.
ALL mine were removed and a separate trans cooling installed out the front under the bumper car. When I got to the later Jags I now have, the trans cooler is in fact a separate item, and no longer included in the coolant radiator, mmmm, maybe someone finally woke up.
Snap if my unit on the Red car below.



Looking UP from under the front bumper.

3) Above the transmission, in the tunnel, is a foam insulation pad. About 2" thick as a guess. They are well known to part company with the factory glue and actually sit on top of the transmission, effectively blocking EXIT air from the engine bay. OK, you have bonnet vents, good idea in your climate, but that foam is a killer. I have removed them, too many to count, and used a "modern day" insulation sheet (Speed Shops are a good place), and that has solved many hot running issues for me. Refitting something with the trans in situ is FUN, not, but doable, just make sure you have plenty of COLD beer on hand, and your best sense of humour. My red car, and the S2 V12 were all done with the engine/trans out on the floor, SIMPLE task.

 

Haydar
Please post some close up photos of your engine bay and coolant pipes and radiator area. Including the purge pipes on the top of the radiator cross member. Just in case something important is not there or not right.

 

Haydar
I have a question:
Is the heater still connected to its hot coolant feed on your car?

 

Hi all

Sorry was busy at work and actually needed a little time away from the XJS to keep my sanity

So as suggested above I bought an IR temp detector and performed some tests, ambient temp was very high, around 40C!!! So at idle the temperature on both inlets to the radiator would rise to 92C where the electric fan would kick in. At this time the outlet from the radiator would be around 83C, so a drop of around 9C. The electric fan would quite quickly (5 minutes) reduce the inlet temps to around 83C and the electric fan would then turn off and the cycle would repeat. Now I need to explain that it was quite tricky using the IR temp detector, in that depending on that exact part of the hose I pointed at, there would be significant differences in the readings.

I then revved the car up to 3000 rpm and the temp rose quite quickly to over 100C. The inlet pipes now read around 103C and the outlet pipe of the radiator was reading around 93C, so again a 10C drop in water temp through the radiator. The water rails front to back were reading around 110C, but as they are not that far off from the exhaust despite the head shield.

Now the interesting thing is that after this I let the car idle but it look a long time for the temperature to drop, In fact even after 10 minutes at idle it had barely got down to 90C and seemed reluctant to lower any further. In fact after 15 minutes I poured a little water over the radiator and then the temperature dropped to the point the fan turned off.

So what does this tell us?

1. Is a 9/10 C drop in temp good enough by the radiator? Again the ambient temp was around 40C and the car was stationary, with a powerful electric fan.
2. Could I have a small head gasket leak? The next day I put a pressure gauge on the coolant filler pipe and pressurized the system so see if any coolant would get squeezed into a piston, however the pressure stayed steady without a drop in pressure. Also in the past, compression tests on the cylinders were always consistent.

I bought a new 'improved' water pump for the car a while ago but haven't got round to fitting it yet. So have decided as a next step to install this and see if it makes a difference.

 

This pictures show the Mercedes electric fan that I have installed.





This pictures show the oil cooler and the gearbox cooler. Behind this is the AC radiator.

 

A 10 degree drop certainly seems reasonable. I'm wondering if the radiator is totally full without air bubbles?

On the left side of the radiator cross member there is a hole, and if you look through that you should see a hex head plug. I usually jack up that side of the car to make that point the highest in the cooling system, then remove the plug and add coolant very slowly at the cross over pipe. Slow is key, to let any air rise up and get pushed out the plughole. Once you get coolant out the plughole, then I put the plug back in and run the car. The V12 can be very difficult to bleed all the air out and air in the system will reduce your cooling capacity.

I'm really surprised revving the car to 3000 while stationary would make the temperature go up that much. Can you have an assistant bring the revs up and then inspect the rad hoses to see if any are collapsing under the suction of the water pump?

 

Hey, did you finally managed to solve this, i have exact same symptoms and have been trying to sort it for more than 6 months now without any success. I have exact same readings, same problem, same cycling issue (car can do several cycles efan on/off on idle, but after good run it cant get it down anymore). I have done more or less same work on the coolant system.
my only difference is that my efan switch (200f/185f) is located at the back of the rail and its usually hotter.

 

Hi there,

Although this is a new login, I'm the originator of this thread. I've could get the forum to reset my password for some reason so I'm using another account for now.

So after literally years of trying to solve this problem, I did actually find a solution.

When the car was at the workshop for some work, the mechanics accidently damaged the engine wiring, which had the effect of damaging my coil and ignition amplifier. It may even have damaged the ECU. Although this sounds terrible, I think this was a blessing in disguise. Rather than replace the damaged parts, I ordered a complete aftermarket ignition system from a company in Norway ( Jaguar V12 EFI kit, Distributor delete - Waste spark - mobeck.com ) which I installed. At the same time, I modified the PCV (also known as the crankcase pressure valve) and installed a catch can, rather than allowing it to feed back into the intake via the air filter.

After to above modifications, the car is completely transformed. The faster I now drive, the cooler the engine runs. Apart from just cooler, the engine performs like a modern car. Gone is the occasional misfire at idle, engine performance is very strong and I think it is returning better fuel economy, though if you run a v12 jaguar, this is not going to be one of your priorities. I can't stress how much difference the ignition system has made, it now starts on the button, even if I leave it for weeks.

So, my theory on why the car was overheating boils down to the following:-
1. There was a major misfire at higher speeds (or under load) that is hard to detect due to the nature of the v12.
2. The engine timing was not being advanced correctly under load.
3. The ECU was not fueling the car properly under load due to age of the electronics, vacuum problems, etc..
4. Under load excessive head gases were being released into the intake, which meant oil was burning in the cylinders causing the temperature to rise.

I think in my case, number 1 was definitely a problem. Before the modifications, if I was driving at freeway/motorway speeds and I lowered my window, I would get a strong smell of petrol. I always thought I had a leak in the fuel tank. I changed the fuel tank and all the fuel lines, but the fuel smell never went away. Ever since I changed the ignition system, I don't smell the fuel anymore!!

Anyway, good luck with your car and hope the above helps.

Warm regards

 

hey, thank you for replying back! Sounds like my issue, I also think I have a misfire which I can’t locate, I can feel it on idle as a slight vibration at the same interval. I’m not sure I want to spend 5k yet, however. Well it’s sounds like I have to go through ignition completely. Ecu you mean Marelli ignition control module. ? Coils new , amplifiers both new, distributor new, cap, rotor. But my wiring is terrible. /

 

Hi there, just be careful you don't end up spending more money trying to get the current system working than it would have cost to get a good aftermarket ignition system. Remember that in the system from Mobeck you get a complete set of injectors, which alone are quite expensive. When I analyze the amount of money I spend before buying the Mobeck system, I wished I had gone for something like this from the beginning. The jaguar's own ignition system also relies heavily on vacuum switches and throttle position sensors to fuel the car correctly, especially under load. Failure in any one of these can cause all sorts of problems, from rough idle to misfires. Also, the system Mobeck gives you the option to delete the ignition distributor, which I think is essential.

Where are you based? Maybe there are cheaper aftermarket ignition options available to you?

Anyway, good luck.

Warm regards

 

oh i feel you, i have already spent around the price of mobeck on redoing my whole cooling/ignition system just to find out that problem is elsewhere. And i still not 100% sure overheating is due to a misfire, im not 100% sure its a misfire either. what can happen is i buy mobeck and it wont solve the issue.
Question, does mobeck eliminates the existing injector/sensor/ignition harness as well? i guess it should replace all that?
Thank you

 

So, the Mobeck system gives you everything you need, which includes the injectors, coils, wiring loom, ecu, ignition leads, o2 sensor, temperature sensor, crank position sensor with bracket, blanking plate for the distributor delete. You end up removing all the vacuum lines, as they are no longer needed. Even the fuel pressure regulators don't need their vacuum connections. Although it sounds expensive, once you factor all these points, it starts to sound more reasonable. Also, Ole, the owner of Mobeck, is a really good and helpful guy.

Look, maybe start by just modifying the PCV. If too much oil is getting past the PCV and into the intake, this oil in the cylinders will considerably increase the temperature.

Unfortunately, there is no cheap solution to your problem. It is now very hard to buy original parts and the ones that are produced are usually cheap Chinese copies that are problematic in themselves. I remember having to buy 3 throttle position potentiometers, that fit under the throttle dizzy, before I found one that worked at least partially worked.

I have recently bought the Mobeck manual conversion kit, which I'm fitting as we speak. Should make things interesting....