[xj-s] Atmospheric tank

Hi.
Read an article the other day on atmospheric coolant tank. Mine has been
disconnected since I have had the car and the hose coming from it goes
toward the overflow tank but isn’t long enough.
Not sure what that is all about.
So I put the car on the hoist and removed the panel from behind the Lt front
wheel and wished I hadn’t (actually I’m grateful because if it needs doing I
want it done) The tank cradle was totally rusted and collapsed and some of
the surrounding panels ie sill front and body wall were also rotted …not
a big area but a pain none the less.

I have just finished welding in the replacement panels and am in the process
of fabricating a new cradle. So I would like to know if these tanks are
still considered essential? I will still go ahead with the repairs only
because it’s there and the job’s started but I curious as nobody refers to
them these days.
Cheers Trev

Pknellie
1982 XJ6 Daimler Vanden Plas
1982 very very red XJS V12 coupe (Roxanne)
Mangoplah via Wagga Wagga Australia

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So I would like to know if these
tanks are still considered essential?

Hard to say it’s essential since it was not fitted to the original XJ-S. But life is easier with it. Its purpose is to catch the coolant that is expelled from the closed cooling system as it expands. Then, if the tank is leak free and the hose is air tight, it will be sucked back into the closed system as the engine cools. This makes sure the closed system is completely full of coolant at all times. Coolant will have to be added occasionally due to leaks and evaporation. You can tell when you need to add coolant by removing the cap when the engine is cold and topping up if necessary.

Before I retrofitted my car I was adding coolant all the time. I’d fill it up, drive the car, and it would be low so I’d fill it again. Pretty soon I caught on that I shouldn’t really fill it all the way… maybe only to the bottom of the long filler neck, but that wasn’t always easy to judge.

Rather that jury-rigging a new bracket just call a Jaguar bone yard and buy one.

Ed Sowell
'76 XJ-S coupe, red
http://www.efsowell.us

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In reply to a message from Ed Sowell sent Wed 31 Aug 2016:

I totally agree with Ed. Not essential, but definately a good
thing to have. On my car, I unplugged the windshield washer pump
and used the washer reservoir as an overflow tank, so I can SEE
the coolant level. (Good weather car only!).(See Photos.)
Dave.–
The original message included these comments:

So I would like to know if these
tanks are still considered essential?
Hard to say it’s essential since it was not fitted to the original XJ-S. But life is easier with it. Its purpose is to catch the coolant that is expelled from the closed cooling system as it expands. Then, if the tank is leak free and the hose is air tight, it will be sucked back into the closed system as the engine cools. This makes sure the closed system is completely full of coolant at all times. Coolant will have to be added occasionally due to leaks and evaporation. You can tell when you need to add coolant by removing the cap when the engine is cold and topping up if necessary.


Dave the Limey. 1988 XJS-C. 1960 Chevrolet Apache.
Greenville, Pa., United States
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In reply to a message from DavetheLimey sent Wed 31 Aug 2016:

And on my car, I installed a translucent tubing so that I can
also see if there is coolant.

Photo #1 here
http://www.jag-lovers.org/v.htm?id=1419988589

Please note that if you expel hot coolant from the system and
do not have a provision to have the liquid return where it
belongs upon cooling, you’ll always have air gaps. Not good.

Steve–
The original message included these comments:

I totally agree with Ed. Not essential, but definately a good
thing to have. On my car, I unplugged the windshield washer pump
and used the washer reservoir as an overflow tank, so I can SEE
the coolant level. (Good weather car only!).(See Photos.)


'95 XJS convertible - V12 6.0L
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In reply to a message from sbobev sent Wed 31 Aug 2016:

Definately not needed. The system already has a header/
overflow tank that is more than capable of handling the
600ml thermal expansion as demonstrated by practically all
other jaguar V12 cars other than the XJS HE.–
stephen davis 1976 XJ-S, 1989 XJR-S, Sl SWB XJ12 x 3
gorae, Australia
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In reply to a message from baxtor sent Wed 31 Aug 2016:

Stephen:

I am afraid I am not following. It is not the engine
itself.
Rather, the layout of the components in the engine bay is
what makes the XJS different.

Isn’t the expansion tank the highest point in the system for
the sedan? Isn’t there enough room for the liquid to expand?

I have seen late models sedans only. The engine bay layout
is completely different from the XJS. I can see where the
600ml would go on the sedan, but cannot do so for the XJS
(if the plastic tank in the fender isn’t there).

You have both in the stable, educate me(us), please.
Steve–
The original message included these comments:

Definately not needed. The system already has a header/
overflow tank that is more than capable of handling the
600ml thermal expansion as demonstrated by practically all
other jaguar V12 cars other than the XJS HE.


'95 XJS convertible - V12 6.0L
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In reply to a message from sbobev sent Thu 1 Sep 2016:

Steve, the basic engine and engine bay layout starting way
back with the series I XJ12 has remained the same up to and
including the XJS including the overflow tank mounted
inside the engine bay L/H side front. Other than the HE
none of the other models bothered with the ‘‘atmospheric’’
tank and just made do with the one tank.
Provided the system is filled correctly an engine at
operating temp. Will displace about 600ml from that
overflow tank one time, first time from cold fill, pre HE
models onto the ground under car, HE into yet another tank
(in reality a rust trap). There is a need to keep the
engine coolant system full of coolant and free of air,
there is no need and no advantage to keep the overflow tank
completely full. I do not have a precise capacity of that
tank but l guess at something around 3 litres or so, more
than enough to handle well under 1 litre of of coolant
displacment/replacement.–
stephen davis 1976 XJ-S, 1989 XJR-S, Sl SWB XJ12 x 3
gorae, Australia
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In reply to a message from baxtor sent Thu 1 Sep 2016:

Stephen:
Thanks for the explanation. The quoted section of the
response you gave is what confuses me.
On my car, the overflow tank is lower than the crossover
pipe. Coolant is filled through the latter, resulting in
filling the overflow tank completely.

What you’re saying is not what I observe, am I missing
something. How do you keep yours partially filled?

The Book
http://www.jag-lovers.org/books/xj-s/06-Cooling.html
also emphasizes the need to get rid of the air in the
system.

I thought I had a good understanding of what’s what, but
apparently I need to go back and re-read everything.

Thank you,
Steve–
The original message included these comments:

(in reality a rust trap). There is a need to keep the
engine coolant system full of coolant and free of air,
there is no need and no advantage to keep the overflow tank
completely full. I do not have a precise capacity of that
tank but l guess at something around 3 litres or so, more
than enough to handle well under 1 litre of of coolant
displacment/replacement.


'95 XJS convertible - V12 6.0L
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In reply to a message from sbobev sent Thu 1 Sep 2016:

As you said Steve, filling is done via crossover slowly and
bleed screws and overflow cap replaced as radiator and tank
fill in succession until crossover is also filled. On
engine warm up about 600ml is displaced and lost but
cooling system is still ‘‘full’’ however on cool down coolant
moves from tank to engine resulting in the less than full
tank until next heat cycle. I did not mean to imply a
partial fill of overflow tank was a deliberate act, it is
just a result of the expansion.
Not running an atmospheric makes no difference to the
operation nor the importance of bleeding all air from the
system.–
stephen davis 1976 XJ-S, 1989 XJR-S, Sl SWB XJ12 x 3
gorae, Australia
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In reply to a message from baxtor sent Thu 1 Sep 2016:

I will admit Stephen, I am now completely at lost.

I opened up the cap of my header tank, it is filled to the
top. Same for the crossover pipe.

Also, the expansion tank on my car has the coolant level
sensor.
If what you suggest is happening, and after 2-3 cycles of
warm up cool down, this much coolant is ‘‘lost’’ the warning
light will be illuminated.

Is there such a thing as overfilling the coolant system?

Steve–
The original message included these comments:

fill in succession until crossover is also filled. On
engine warm up about 600ml is displaced and lost but
cooling system is still ‘‘full’’ however on cool down coolant
moves from tank to engine resulting in the less than full
tank until next heat cycle. I did not mean to imply a
partial fill of overflow tank was a deliberate act, it is
just a result of the expansion.
Not running an atmospheric makes no difference to the
operation nor the importance of bleeding all air from the
system.


'95 XJS convertible - V12 6.0L
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In reply to a message from baxtor sent Thu 1 Sep 2016:

BTW, your description differs from Kirby’s. Verbatim from
the ‘‘Book’’

Normally, each time an engine heats up and cools down, the
expansion and contraction draws water back from a recovery
tank through a line into the cooling system. However, the
coolant return line from the pressure cap to the atmospheric
tank behind the left front wheel is quite long. Since the
expansion/contraction of an engine only moves a little water
at a time, it requires several thermal cycles to bleed the
air out of the hose (unless you overheat and blow steam).
Each time you open the pressure cap, you allow the water to
drain into the atmospheric tank and the line to fill with
air. If you keep opening the pressure cap to check the
level, it will never get a chance to work properly.

Steve–
The original message included these comments:

engine warm up about 600ml is displaced and lost but
cooling system is still ‘‘full’’ however on cool down coolant
moves from tank to engine resulting in the less than full
tank until next heat cycle. I did not mean to imply a
partial fill of overflow tank was a deliberate act, it is
just a result of the expansion.
Not running an atmospheric makes no difference to the
operation nor the importance of bleeding all air from the
system.


'95 XJS convertible - V12 6.0L
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In reply to a message from sbobev sent Fri 2 Sep 2016:

Steve, the loss of about 600ml happens only one time, first
time after cooling system refill not every hot cold cycle.–
The original message included these comments:

I opened up the cap of my header tank, it is filled to the
top. Same for the crossover pipe.
Also, the expansion tank on my car has the coolant level
sensor.
If what you suggest is happening, and after 2-3 cycles of
warm up cool down, this much coolant is ‘‘lost’’ the warning
light will be illuminated.


stephen davis 1976 XJ-S, 1989 XJR-S, Sl SWB XJ12 x 3
gorae, Australia
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In reply to a message from Trevor Beckwith sent Wed 31 Aug 2016:

Well Trevor, there you have it, Coolant Recovery 101! You might
take a look at Bernard Embden’s site. He went to a lot of trouble
to install a visible overflow tank on his car.
I like being able to pop the hood, glance at overflow tank, close
hood. Done!
Dave.–
The original message included these comments:

Read an article the other day on atmospheric coolant tank. Mine has been
disconnected since I have had the car and the hose coming from it goes
toward the overflow tank but isn’t long enough.


Dave the Limey. 1988 XJS-C. 1960 Chevrolet Apache.
Greenville, Pa., United States
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In reply to DavetheLimey Sept # 2016

Thanks Dave… I have been reading the posts with interests and been
totally entertained by the range of views.
Have to say that as I’m for ever topping up my XJ6 overflow tank I’m going
to go with the atmospheric tank in the mudguard of the XJS.
Steve’s contribution of using a clear tube to view the coolant level
appealed to me. So now I have a galvanised cradle for the atmospheric tank
and a repaired galvanised compartment for it, and also, a clear 5cm piece of
tube
joining the overflow tank to the end of the rubber hose coming from the
atmospheric tank. If I can see green then all is ok. …coolant recovery 101
indeed!
Trev

Regards Pknellie
1982 XJ6 Daimler Vanden Plas
1982 very very red XJS V12 coupe (Roxanne)
Mangoplah via Wagga Wagga Australia

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On my car, the overflow tank is lower than the crossover
pipe. Coolant is filled through the latter, resulting in
filling the overflow tank completely.

Welcome to the fun 'n games involved in achieving a low hoodline! This is
why the header tank includes a flow scheme, driven by an ejector connected
to the water pump inlet, and why there is an elaborate air purge system
across the radiator top rail. If you want a system where the air just rises to a
tank at the high point, you’ll end of with a front grill that looks like a Model A.

– Kirbert

// please trim quoted text to context onlyOn 1 Sep 2016 at 16:38, sbobev wrote:

Other than the HE
none of the other models bothered with the ‘‘atmospheric’’
tank and just made do with the one tank.

A cooling system has to have room for expansion of coolant. I believe the
“header tank”, the all-metal tank near the left front of the engine
compartment, was designed to serve that purpose. That would mean it’s
supposed to be treated exactly like the plastic overflow cans we see in other
cars, with a MIN and MAX level that are somewhere in the middle of the
volume of the container. It’s my suspicion, though, that owners insisted on
trying to keep it slap full. Every time they’d check it, it’d be down, so they’d
take the car to the dealer and BMC that it keeps losing coolant. Jaguar
probably added the “atmospheric catchment tank” to help alleviate the
customer complaints. Right up there with the oil pressure gauge that always
reads in the middle.

– Kirbert

// please trim quoted text to context onlyOn 1 Sep 2016 at 14:27, baxtor wrote:

In reply to a message from Kirbert sent Sat 3 Sep 2016:

Could not have said it better Kirbert and l did try.
Since coolant level should only be checked cold the level
in that tank will ALWAYS be less than full, top it up and
by the time operating temp is reached the top up is gone.
I check my c�olant level at the crossover filler neck.–
The original message included these comments:

A cooling system has to have room for expansion of coolant. I believe the
‘‘header tank’’, the all-metal tank near the left front of the engine
compartment, was designed to serve that purpose. That would mean it’s
supposed to be treated exactly like the plastic overflow cans we see in other
cars, with a MIN and MAX level that are somewhere in the middle of the
volume of the container. It’s my suspicion, though, that owners insisted on
trying to keep it slap full. Every time they’d check it, it’d be down, so they’d
take the car to the dealer and BMC that it keeps losing coolant. Jaguar
probably added the ‘‘atmospheric catchment tank’’ to help alleviate the
customer complaints. Right up there with the oil pressure gauge that always
reads in the middle.


stephen davis 1976 XJ-S, 1989 XJR-S, Sl SWB XJ12 x 3
gorae, Australia
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In reply to a message from Kirbert sent Sat 3 Sep 2016:

Yes…and in doing so they made the connection on the
catchment tank 5/16 ‘’ and the connection on the
atmospheric tank 1/4’’…or vice versa…then they added a
steel adaptor part out of view inside the guard to make
the adaption in size between the tubing. This little part
slowly rusts solid and you are back to expelling coolant
on the ground…clever stuff.–
The original message included these comments:

take the car to the dealer and BMC that it keeps losing coolant. Jaguar
probably added the ‘‘atmospheric catchment tank’’ to help alleviate the
customer complaints. Right up there with the oil pressure gauge that always


Matt Furness 85XJS-HE 5 Speed Manual
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