Well, I can only pass on what I learned when I had to address the cooling system, but feel free to ignore if you like.

First, you may find that many (most?) aluminum radiators claiming to fit the XJ-S don't "drop in", but hopefully you'll get lucky or you're a great fabricator.

Second, aluminum radiators are very popular because they a cheaper, look cool, and weigh less. Cheaper can be relative because if it is a well-designed and true 'drop in' unit, you may find a re-core is actually more cost effective (at least in my case it was). On the XJ-S you can't actually see the radiator once it's in place so looking cool doesn't do much, and given the massive weight of the whole car the weight saving isn't going to do anything.

Perhaps the most compelling reason to get your factory radiator re-cored with a more modern higher efficiency core is that it is better performing than aluminum. There is a misperception that aluminum radiators are more efficient at cooling, when in fact they are the opposite. Brass/cooper radiators have much better thermal transfer than aluminum. Aluminum/plastic radiators are used in modern cars because they are cheaper, not better.

Not sure what specifically you don't like about the coolant recovery system, but if you are going to change it, you might want to find out why it was designed that way in the first place. It wasn't British eccentricity, it was deliberate engineering to solve some difficult issues very specific to the V-12.

Cheers

apologies if it seams like I am ignoring advice I do appropriate all comments that help me better understand this car. And there have been many comments in this thread that have done such. I would also like to say Im not opposed to fab work and I am pretty decent at it and have plenty of time on my hands so I dont necessarily need to lean towards options that are drop in. how much was it to re-core your radiator? I figured any large clean aluminum radiator would be an improvement and any improvement would be enough to rectify any cooling issues the car currently has which is why I was leaning towards just buying a new aluminum radiator. I am still seeking more info on the coolant recovery tank and when I feel confident I can replicate its function I will probably design something to work on the internal of the engine bay just because Im not a fan of it being hidden in the fender.

No worries, I just wanted you to have the benefit of hard lessons learned by others here. The cost of the re-core was about equal to the least expensive aluminum radiator available on eBay, which turned out wasn't a drop in and would have required a modification to the oil cooler attachment and banjo bolt to work. I was able to return it, and did the re-core instead. Re-core might be even cheaper depending on where you live. Locally, businesses like radiator shops have been nearly zoned out of existence so that type of work is probably at least 2x what it would be in other parts of the country.

If you get it re-cored, they can put in a more efficient core than what came from the factory, and it will outperform an aluminum unit. Your cooling issues are a symptom of neglected maintenance, not system design. Even your existing radiator might work effectively, if it is pulled from the car and cleaned out as the very wise Grant Francis suggested above.

One of the secrets to a happy relationship with your Jaguar is to avoid throwing money at problems that can be solved by other means.

There are two expansion tanks in the XJS. The first is the one on the inner fender on the left side that has a pressure cap on it, as it is pressurized. Then there is an atmospheric recovery bottle, in the front fender, behind the headlight and that is vented. It's simply there to catch any coolant that might overflow the pressurized tank, but that isn't usual. If the system is filled properly, the atmospheric tank isn't normally used.

I'm actually very unclear about what you think needs to be redesigned in this system, as it does the job it was intended to and isn't a trouble spot on these cars.

One thing I was looking to do is put a atmospheric recovery bottle in the engine bay as opposed to under the fender. would this not be as simple as running the tube that normally goes under the fender to a new vented tank in the engine bay? I would assume I could leave some fluid in this tank so it pulls from the tank into the pressurized tank. currently when I remove the cap to the pressurized tank (when the engine is cold there is a air vacuum and the time it got too hot it was leaking out of the cap making me think the atmospheric recovery bottle isnt doing its job and or the tube to it is clogged.

I'm surprised you'd have coolant coming out of the expansion tank on the inner fender, as it isn't supposed to be full. Also check the level in the crossover pipe on the engine, that should be about 4" below the filler neck.

My guess is your pressure caps are not sealing properly.

Edit to add: proper place to check the coolant level is at the crossover pipe, not at the expansion tank.

My atmospheric tank is under the bonnet, coz I no got ABS taking up room. It is a 1ltr tank, and since the V12 purges approx 600ml Cold to Normal, WHEN WORKING AS DESIGNED, is OK.

The OE atmospheric is NOT a good location, agreed, and all I have ever taken out for servicing, are approx 1/3 full of mud, so YES< the hose from the engine bay is sitting in mud, and the metal joiner at the hole in the wing is usually plugged as well, soooooo, the pressure caps have NO WHERE to release the coolant, so it is pushed out of the seals, NORMAL on a sad neglected system.

Can I suggest another way to look at the problem?

The tube might indeed be clogged, but that is ignoring the elephant in the engine bay -- "the time it got too hot". The system, as designed, doesn't EVER get too hot. So knowing that, what does that tell you?

There is something preventing the system from working as designed, not that the system wasn't designed properly.

I had coolant coming out of the cap on the pressurized tank not the one inside the fender. good to know on the fill level

ah yes this is exactly what I was planning on doing and thank you for the displacement numbers those are very helpful. Can I ask where you relocated the tank to?

Im very aware that a few systems on this car are not working as they should. The tank relocation isnt meant to solve the heat issue thats another thing to tackle. The relocation is just because I dont like the current location and its not working properly on this car currently.

I didn't find any mud there when I opened it up for service, so I'm curious to what is different in our experience. Travel on unpaved roads?

Following on from Grant's points, this is what I did, inspired by his pioneering work, and the absence of OEM parts to replace the worn out originals. Now the thing is filled from the header tank, the crosspipe is spoutless (as on 6 litre saloon cars), the atmospheric tank is in the engine bay, and it works brilliantly well. No air purging problems, just wonderful; but as Grant says, if you have ABS there may not be room. In my case I needed more like a litre of capacity in the atmospheric tank - something to do with being in the northern hemisphere I understand...

But, big but, as everyone has said above, you need the entire thing to be in top condition - ALL hoses, new thermostats, radiator, etc etc, before the car will cool reliably.

Best way to see if you need a re-core is fitting a shop vac to the inlets, and violently sucking the crap out. I've pulled a cup of shit metal and rubber out the last time I did...thats how I know. Now I run a huge spin on coolant filter. No replacement for a re-core though. No need to alter the stock system Grant is right itll do fine even in the Austrailian summer.

More than likely a well cared for car by the previous owner.s, they are out there for sure.

Unsealed roads, NAH, thats an Aussie icon, and mine was full of mud when I got it home from purchase, when I did the "Grants Major Major Service", BEFORE putting it into the fleet running program, and since that bottle has been in the engine bay, NO particles of anything in it at all..

The pressure tank inside the engine bay on a HE is designed to be FULL, and the excess is pushed TO the atmospheric tank, and then redrawn to the pressure tank as the system cools, and that applies to a 100% working system. I originally fitted a clear hose from the pressure tank TO the atmospheric OE position, and as long as there was Green Liquid in that hose when cold, the system was deemed FULL. Removing the caps constantly to check the levels simply allows air into it again, and the "purge cycle" starts all over.

The HE system requires APPROX 4 purge cycles, WITHOUT leaks, or cap removal, to get to this status, and a Black hose to the atmospheric tank simply was not working for my visual requirements.

During this time, and before relocating the atmospheric tank, as long as there was Green fluid in hat clear hose, the caps stayed in place 100%.

The PreHE, however, has a very different system, and their coolant level is about 4" down the pressure tank spout, and they do not have the fill spout alongside the A/C compressor, coz their pressure tank has a spigot out the bottom, that is connected to the heater return pipe, thus allowing that system to FILL via the bottom hose, and push the air out as it fills, very clever. NO atmospheric tank either.

The Plastic Moreys bottle is my atmospheric tank.

1st snap shows the 2 containers in the very bottom RH corner.

The 2nd is looking straight at them, and the Green liquid in the bottom is the COLD setting, and never moves each time the beast cools down.

Located where the HUGE Jaguar washer bottle was, and the smaller washer behind, is a simple Universal Horizontal Washer Bottle from a generic Auto Parts Store.

https://cimg4.ibsrv.net/gimg/www.jag...357b9fddcb.jpg
https://cimg5.ibsrv.net/gimg/www.jag...eabdbdae99.jpg

I notice the tube feeds into the top of the tank. doesnt that prevent any coolant from re entering the system when it cools?

Nope, why would that be???, you NEED the coolant to return to the pressure tank, and the system, as the engine cools down.

That was just how I did it in the day. There was a hole there already, and I just utilized what I had, reinventing the wheel wastes beer drinking time, There is a drop hose under that fitting, that "just touches" the bottom of the bottle, and is cut at 45deg, simple. SAME design as the hidden Jag atmospheric if you look.

ah I was unaware of the hose that drooped to the bottom of the bottle hence my confusion. i was thinking just have a hose on the bottom of the tank and run it upwards above the fluid level but the way you did it defenently makes sense as well now.

alright, so im doing some part shopping and figured I would post some of the stuff I was looking at getting here first

As far as spark plugs I was looking at NGK BR7EF plugs

distributor cap and plug wires off ebay

Im still looking for the plugs for the injector harness, the plugs for the ignition coils, and a distributor rotor so and advice on a good place to find those would be appreciated.

In addition I was considering replacing the ignition coils as I see ones that look identical for $16 on ebay. Anyone know if there would be any issues doing that? here are the ones im looking at
https://www.ebay.com/itm/New-Ignitio...kAAOSw8fpcwT8v