I got my 92 XJS V12 in August so I'm still learning the systems and components.

Can someone give me the theory behind having both an expansion tank and also an atmospheric recovery bottle on this system.

Are not these two components, and their purpose, handled by one single bottle in most other cars?

 

Welcome to the elite! As I understand it, the rather complex cooling plumbing in the XJS all stems from the fact that the header tank is NOT at the highest point in the system. The highest point being the heater valve on the firewall centre. As a result, there has to be an air extraction system (part of which, if you refer to the recent thread entitled "water pump or hose" I have just attempted to explain to another lister, link here: https://www.jaguarforums.com/forum/x...rtible-105728/ )

As a result of the header tank on the inner wing being lower than the water jacket on the engine, there has to be a fill point reasonably high up. This is the cross pipe at the front of the engine with the rad cap on top of the upright spout. As a second result, without an air purge system (see link referred to above), the rad would always have air in the top as there would be no easy path for any trapped air to get out of the system as the header tank is lower than the rad top!

So the thin tube across the rad top extracts any air and it is fed back into the header tank on a continuous basis. because of the restricted space in the XJS V12 engine bay, the header tank itself is not too big, bearing in mind the volume of water in the system and its expansion and contraction. So the header tank is fitted with a so-called 'atmospheric expansion tank' to ensure that the header tank never runs dry - a total disaster on the V12. This extra is fitted into the space at the rear of the LHS front wheel well, BEHIND (and thus completely inaccessible) the inner wing baffle. The idea being that as the coolant expands, rather than just falling onto the road, it expands into the atmospheric tank. And as the engine coolant cools down after the car stops, and the coolant contracts, the header tank sucks coolant back from the atmospheric tank.

So the plan is that the header tank on the inner LHS wing connected to the radiator, bleed pipes etc, is ALWAYS full of coolant. As mentioned, because running out of water totals the V12, this header tank on the inner wing is also fitted with a low water warning light. This light will illuminate when you switch on the igntion and should be looked out for at that time, as it is a test that the circuit is working. If it ever comes on when the car is running, stop and investigate ASAP !

Finally, all this has consequences for refilling the system after (eg) an antifreeze change. The basic way this should be done is:

1. If possible park car on an upward incline, front upward
   
2. Remove cross pipe cap
3. remove header tank cap
4. fill system from cross pipe
5. when header tank starts to overflow, put cap on header tank
6. keep filling up to top of cross pipe and put on that cap when full
   
7. start car with heater on DEFROST and warm up
8. engine still running but not yet hot, undo the brass plug on the LHS rad top (accessible through a hole in the top panel), engine still ticking over
9. top up this hole until it runs out consistently. reinsert plug.
10. drive car around a bit and when cooled off, remove cross pipe cap and top up.
    

[QUOTE=The Wizard of BC;847202]Are not these two components, and their purpose, handled by one single bottle in most other cars?[/QUOTE]

Yes! but the header tank is usually the highest point in the system AND there is enough room for a header tank big enough to handle the expansion and contraction of the coolant, AND have an air space above the coolant when it is hot.

A bit long, but I hope this helps!

Greg

 

Nice write up Greg!

The V12 is not the only one with this extra atmospheric bottle, the AJ6 power cars have it too but perhaps only because the expansion tank is smallish?

No issues with refilling a 6 with coolant though

 

Great article! I have replace a few collant tanks and welded/brazed a few also. I believe Coventry west has the best price/avail. here in USA.

 

Thanks Greg, Your explanation made a lot of sense. One small thing threw me for a moment. You use the term "Header Tank" for the part that the manuals call the "Expansion Tank". Once I figured out that we were using different terms for the same thing it all made sense.

This is not my first Jag. Before they all had pretty standard system layouts though.

Next question. I know this system does not have a system drain plug so what is the lowest part of the system that you know of?

 

Towards the bottom of the RHS of the radiator, just below the lower gearbox oil cooler connection; there should be a tap, or a bung. On some models this can be activated by a rod and lever from the top of the rad.

Otherwise, it is pull off the bottom hose, which is a pain, but worth doing as the bottom hose needs changing regularly to be secure.

Greg

 

I've used a thin piece of copper tubing + plastic tubing + small hand pump to drain the system. I took out the air bleed banjo bolt on top of the radiator, dropped in the copper tube (cut to appropriate length and 90 deg bend at top) put on the plastic tube and pumped it out. To me it seems like it got the rad pretty empty, probably as good as using the drain tap would if your radiator has one (mine didn't). Way less mess than pulling the bottom hose off with the system full and dumping several gallons on the floor.

I think it was 1/4 inch copper iirc.

 

Thanks, Greg, for a very clear well written explanation of how that all works - I wouldn't say long, but just right!