Jaguar XJ-S. Manual - part 47

 
 

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lines now, too.  This stuff seems to offer pretty much the best of both worlds -- it's not quite as long-lived as straight 
OAT, but it is much better behaved in operation than OAT, much like conventional coolant. 

“Note that these three different additive packages are not really cross-compatible.  No, they won't eat the insides of your 
radiator if you mix a little of one in with another in a pinch, but you'll be better to get the system flushed out and a fresh 
mix of 50/50 whatever your car needs put back into it. 

“If I owned a car that came with OAT or HOAT from the factory, I'd likely stay with it.  The anti-corrosion additives, 
in particular, leave residues on the walls of the various coolant passages (that's how they work -- the residues coat the 
base metal and prevent corrosion), and it's tricky to convert an engine that's been run with one style of package to use 
another package and get the full benefit. 

“Switching from conventional to OAT, for instance, requires a mild acid flush of the cooling system after removal of 
the conventional coolant and before pouring in the OAT if the long-life corrosion benefit of the OAT coolant is to be 
realized.  Just pouring the OAT in after draining the conventional won't gain the full measure of added coolant life the 
OAT marketers (notably Texaco) like to use as selling points.” 

Somewhere in the midst of all this, the labelling on the containers of DEX-COOL quietly changed, making far more 
modest claims for durability and combatibility. 

Quoting from the 

Popular Mechanics web site

:  “Now let's look at "retrofit" and "drain and fill" and explain what you 

can and can't do safely, and a bit of why.  If a vehicle has a copper-and-brass radiator, forget a retrofit, says General 
Motors, because the organic acid (orange) antifreeze may not provide adequate protection for the lead solder in that 
radiator.  The Chrysler orange hybrid combination of silicates and organic acids is meant to provide special protection 
for the water pump.  Sorry, you can buy it only at a Chrysler-brand dealer. 

“Prestone believes you can retrofit to its organic acid orange almost any vehicle with an aluminum radiator and cooling 
system that has been well-maintained and is in good condition, if you do it right.  However, the antifreeze maker 
recognizes the possibility of a problem with Dodge truck 5.9-liter V8 water pumps, for which green or yellow/gold 
U.S. antifreeze is recommended–if you don't get Chrysler's specific orange.” 

A 1998 magazine article quoting Applied Chemical Specialities Co. (a competitor of Havoline) put the retrofit issue 
more strongly:  “DEX-COOL is an excellent antifreeze to be used in brand new cars in which traditional 
phosphate/silicate antifreeze has never been used.  However, if any traditional antifreeze has ever been used in your 
car's cooling system, it is strongly advised to avoid using DEX-COOL.  This is because, short of a dangerous strong 
acid cleaning or complete replacement of ll parts within the cooling system, it is physically impossible to remove all 
residuals of phosphates and silicates - even with repeated flushing.  If DEX-COOL is used in such a system, deposit 
formation will be almost instantaneous and will seriously affect your car's cooling system efficiency and performance.  
Because most corrosion occurs under such deposits, it will also affect the long-term corrosion prevention in such a 
system as well.” 

In a more Jaguar-specific vein, Al Askevold reports some problems with DEX-COOL:  “Besides the bad report about 
DEX-COOL from the radiator shop who did my recore, I could not remove my rad caps without damaging the rubber 
seals, and my Tefba filters - I had to remove them and clamp them in a vice to remove the tops.  I tried several different 
kinds of lubricant on the parts with no effect.  I finally flushed the DEX-COOL, I am now using a different brand, so 
far no problems.” 

 

WATER:  The antifreeze needs to be mixed approximately 50/50 with water, but if you care about your cooling system 
you might want to be careful about what water you use.  The garden hose may be convenient, but it’s not necessarily 
healthy for the car.  The biggest problem to be concerned with is dissolved solids, that stuff that’s left in a pot after you 
boil water away.  Since boiling is often going on alongside the hottest parts of the engine (cylinder liners, head), this 
same stuff is left there.  As a scale on the metal surfaces, it can insulate them from the coolant and make the cooling 
system less effective.  When the scale flakes off in chunks, it can find its way into the radiator and plug up some of the 
tiny tubes.  Finally, the scale is somewhat abrasive, and therefore is not particularly good for the seal in the water pump. 

 
 

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This problem might be considered minor.  There are only about two gallons of water introduced to the system at each 
coolant change, and that amount of water only has a finite amount of dissolved solids in it.  This appears to be the 
attitude the auto manufacturers take, since there are no radiator hose filters in a car from the factory (see page 190).  
However, if you change your coolant often or use really crummy water, you might be contributing to the plugging of 
your radiator.  If your car loses coolant and you add water daily from a garden hose, you might as well go ahead and 
pull your radiator and have it rodded out. 

To minimize problems with dissolved solids, you should use distilled water, deionized water (also known as 
demineralized water), or water that’s been purified using a reverse osmosis process.  All three types are generally 
available at the local grocery store for perhaps 75 cents a gallon; at some places, you can take your own jug and get it 
even cheaper from a dispensing machine.  Deionized water is sometimes called “purified water”, and is usually labelled 
that it “serves distilled water uses”. 

Grocery stores also sell various types of drinking water, including spring water.  These may not be any better than the 
tap water, since they may contain lots of dissolved solids -- they just contain dissolved solids that taste good. 

Other processes advertised include ozonated water, which is a process to kill germs, and carbon filtered water, which is 
a process to remove volatile compounds that affect the flavor.  Neither process has any significant benefit for cooling 
system use.  The first time the engine is warmed up to operating temperature, the germs will be dead and the volatile 
compounds will have boiled away and been removed from the system by the air purge system. 

There are arguments about using rain water or condensate from an A/C system.  The argument is that they are both 
“distilled”; however, the other side of the argument is that they apparently pick up quite a load of dissolved solids 
somewhere along the way, either from pollutants in the air or from dust or whatever.  Look at the container that collects 
the water and judge for yourself if it’s OK for the inside of your cooling system to have similar stuff in it. 

Peter Smith reports from “...a series of brochures from Tectaloy.  It says "the preferred water is demineralised.  Do not 
use spring or bore water."  The brochure illustrated that the corrosion effects on welded aluminium were serious, and 
that as little as 1 volt earthing through the cooling system could chew out an aluminium radiator in weeks (probably do 
the cylinder head a power of no good as well).  It also notes that true distilled water is getting harder to obtain due to the 
cost of production and the energy required in the distillation process.  Also "if you believe rain water is the answer just 
reflect for a second on the damage it does to gutters and tanks, and they're usually galvanised against corrosion.  Spring 
water is totally unsuitable as it is full of minerals and salts".  Summary was that distilled, demineralised or reverse 
osmosis water were the most suitable.” 

One thing you should definitely not use is “softened” water from one of those water softeners that is recharged with 
salt.  Dave Lokensgard says, “Never use softened water in a cooling system containing aluminum.  Water softeners are 
ion-exchange units, not de-ionizing units.  The main bad actor coming from the water in aluminum corrosion is the 
chloride ion, and water softeners work by replacing all anions in the water (negatively-charged ions, like phosphate, 
sulfate, and so on) with chloride, and all positively-charged ions with sodium ion.  This is why they are recharged with 
salt (sodium chloride).  So softened water is the worst thing you can use in a cooling system.” 

Jason Korke:  “The coolant I bought was designed to be mixed with water.  It was made by Castrol which in Australia 
is I believe fairly well regarded.  It wasn't cheap!  The bottle said, "Mix with demineralized or soft water."”  It is 
unknown why Castrol’s directions would say such a thing; perhaps they meant naturally soft water (water with few 
minerals) as opposed to softened water (water in which the minerals have been exchanged for salt). 

If you work in a science lab, an electric power plant, or some other type of industrial facility that uses clean water, you 
may actually be able to get ahold of some really pure water -- something chemically much closer to unadulterated H

2

O 

than the distilled, deionized, or RO stuff you can buy locally.  Believe it or not, this may not be good.  Pure water is a 
solvent; it would actually be quite effective at corroding your aluminum, iron, and brass parts if it weren’t for the 
protective additives in the antifreeze.  So, although the total lack of dissolved solids is nice, perhaps it’d be better if you 
added some dissolved solids before using; drop a few chunks of scrap aluminum and copper into the jug and let it sit a 
while before using it in your cooling system. 

If you want to learn more about water, there is a “WaterNet” forum on the Internet. 

 

 
 

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RADIATOR CAPS:  The XJ-S H.E. has two radiator caps, but only the one on the header tank (left side of the engine 
compartment) is actually meant to operate as a conventional radiator cap; namely, to control the pressure in the system. 
 The one on the cross pipe (at the top right of the engine) is really just a place to add coolant, using a standard radiator 
cap because they’re available.  If one or both of the caps go bad, they may be replaced with standard coolant-recovery 
radiator caps. 

The early XJ-S H.E. originally came with two different caps that were chained in place to make absolutely sure you 
didn’t mix them up.  However, as Alex Dorne points out (and Jaguar eventually figured out), there is no opening out of 
the chamber between the lower seat and the upper seal in the fitting on the cross pipe.  As a result, it doesn’t really 
matter what pressure rating the cap is you install there; the upper seal will totally seal that opening, no pressure relief is 
possible.  The cap on the header tank will always establish the pressure limit within the cooling system.  So, Jaguar now 
offers two identical caps as a replacement so it doesn’t matter if you mix them up. 

Nowadays all radiator caps are coolant-recovery type, but I will point out the difference anyway.  In non-recovery 
systems, any coolant that was relieved by the radiator cap merely blew overboard, and when the system cooled back 
down air would be drawn back in.  Radiator caps made for non-recovery systems usually had a brass diaphragm under 
the top cover that primarily served as a spring to keep the cap from rattling; it didn’t matter if it didn’t seal, since 
coolant was just going overboard and air was being sucked in anyway. 

In a recovery system, coolant released is collected in a reservoir and sucked back into the system on cooldown.  While 
the configuration of the radiator opening hasn’t changed, it now becomes more important that the top cover of the cap 
actually seal.  When the engine is cooling down and drawing coolant back in, any leaks at this joint will cause it to draw 
air instead.  So, modern coolant-recovery caps have a rubber seal in place of the brass diaphragm.  Since this type cap 
works just fine on non-recovery systems, it is doubtful if anyone actually makes the older style anymore. 

 

COOLANT RECOVERY SYSTEM:  Hey, all cars have them nowadays, it’s not rocket science.  Each time an engine 
heats up, the expansion and pressure buildup blows air, gasses, and coolant past the radiator cap and into a recovery 
tank, where the air and gasses bubble to the top.  When the engine cools down, the contraction draws coolant back from 
the bottom of the recovery tank.  However, on the XJ-S the coolant return line from the pressure cap to the 
“atmospheric catchment tank” behind the left front wheel is unusually long.  Since the expansion/contraction of an 
engine only moves a little water at a time, it requires several thermal cycles to purge 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. 

Even the tiniest leak in the coolant circuit will screw up a coolant recovery system.  You may not be concerned about a 
very slight loss of coolant, but you should be.  The engine takes several hours to cool down, so the rate at which coolant 
is drawn back from the recovery tank is glacial indeed.  With even a tiny leak in the system, this suction will draw in air 
from outdoors instead.  Once you have pockets of air in the system, the next event in your life will be an engine rebuild. 
 Steve Haley says, “I found that even being a cup and a half low on coolant makes a big difference in the location of my 
gauge needle.” 

 

ATMOSPHERIC CATCHMENT TANK:  On the H.E., the atmospheric catchment tank is a plastic container located 
directly behind the left front wheel, within the bodywork.  “Atmospheric” means that it’s not pressurized; it’s basically 
an open container to pour coolant into.  To get to it, remove the left front wheel and remove the sheet metal panel at the 
rear of the wheel well. 

The vent on the atmospheric catchment tank is somewhat unusual.  On most cars, the coolant recovery container is 
within the engine compartment, and when it overflows (like, when your car is overheating big time), the fluid coming 
out the vent just dribbles out onto the ground.  In the XJ-S, however, such leakage would result in antifreeze throughout 
the bodywork -- unacceptable.  So, the container has a vent line that is routed out the bottom of the car.  For this vent to 
work as intended, the container must be airtight.  The design is really lousy, however, and it is likely to leak throughout 
the bodywork when overheating anyway. 

 
 

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To prevent such localized flooding, try this idea: install a pan underneath the atmospheric catchment tank to catch any 
coolant that spills out of it, and provide a drain from this pan out the bottom of the car. 

According to Mike Morrin, the pre-H.E. doesn’t have an atmospheric catchment tank; relief from the radiator cap on 
the header tank just drains into the wheel well.  He suggests that an overflow tank might be a useful retrofit.  
Considering the problems with the OEM tank noted above, it might be wiser to simply purchase a generic “coolant 
recovery system” such as those offered by J.C. Whitney.  Note, however, that such generic tanks may be designed to 
overflow all over the place rather than out a tube; either mount them somewhere that this isn’t a problem, or provide a 
tray to catch the overflow and route it out the bottom of the car.  Tank volume may also be a concern, so if given a 
choice get the largest container you can find. 

 

ATMOSPHERIC CATCHMENT TANK FILLING:  Ideally, you’d like to have a minimum of a couple of inches of 
coolant in the bottom of the atmospheric catchment tank.  However, you’ll notice there’s no convenient way to put any 
coolant in it!  Apparently, Jaguar expects you to just add coolant at the header tank, and eventually some of it will get 
into the catchment tank eventually.  Or maybe you’re supposed to disconnect the hose from under the cap on the header 
tank, put a funnel in the end and hold it high and fill it that way. 

Steve Haley had a better idea.  “I added a fill hose which I ran back out through the same hole as the others.”  This 
would give you a filler location in the left rear corner of the engine compartment.  Note that the point where this hose 
enters the atmospheric catchment tank should be airtight, so if it overflows the excess coolant will come out the vent 
line behind the LF wheel rather than pouring all over everything. 

Even with such a nifty filler scheme, you still don’t have any good way to tell what the level is.  If you arrange your 
filler hose to end about two inches above the bottom of the tank, you can find out by blowing into it.  If you hear 
bubbling, the level is at or above the end of the hose, and you don’t need to add coolant.  If the blowing just whooshes 
into the tank freely, you need to add coolant.  If you think you might have too much coolant in it, you might even 
connect a siphon and drain the excess through this hose; when it gets down to the two-inch level, it will suck air and 
automatically quit siphoning. 

 

COOLANT RECOVERY HOSE:  The line from under the radiator cap on the header tank to the atmospheric 
catchment tank is anything but simple -- or reliable.  In the engine compartment, where you can see it with the hood 
open, it is a sturdy thick-walled 5/16” hose.  When it gets into the compartment behind the LF wheel well, where you 
can’t easily inspect it, it connects to an adapter that steps the size up to 3/8”, which then connects to a length of thin-
walled rubber tubing (which actually appears smaller than the 5/16” hose).  This 3/8” hose then bends downward 90º 
and into the atmospheric catchment tank. 

One common problem is that the thin-walled tubing may get kinked making that 90º bend -- sometimes because 
somebody has fiddled with the hose in the engine compartment and pushed more of it into the hidden compartment, 
tightening up the bend.  Michael Aiken says, “My coolant recovery hose was kinked as it enters the atmospheric bottle. 
 This made the recovery system totally inoperable and could lead to more severe damage anywhere in the cooling 
system, as this is the only way pressure is vented as the coolant expands. 

“It can easily be tested by disconnecting the hose at the top of the neck of the expansion tank and blowing in the hose.  
It should offer little resistance to air flow and you should hear a gurgling sound behind the LF wheel.  I applied 30 lbs 
of pressure and no flow.  It was really kinked.  It could also possibly be broken which is not as severe but could lead to 
slow coolant loss.  The hose is a soft rubber and makes two 90º turns in 5 inches.  The second turn (the one that kinked) 
has a radius of less than an inch.  I cut the hose off just above the bottle cap, installed a 90º 3/8" heater elbow (plastic) 
and a non-crimping heater hose back to the original connector.  Cost less than $4. 

“If I were reading this I would take the 2 minutes and check it out.  Unlike most cars this recovery system is completely 
out of sight and problems are likely to go undetected.  This hose is really flimsy (unlike the hose from the tank neck 
into the wheel well which seems very strong) and just fell apart as I took it off.”