Jaguar XJ-S. Manual - part 45

 
 

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3) 

If your needle seems to wander around a lot, you have air in your cooling system.  See the 
section on filling and bleeding on page 179. 

4) 

If your needle is above the N, your car is overheating.  If your needle is halfway between 
the N and the H, your car is severely overheating.  And don’t worry about the needle being 
on the H; it will never get that far. 

Note that these guidelines apply to the XJ-S as configured from the factory -- with the gauge sender located in the right 
side thermostat housing.  As mentioned above, it’s the left side that is likely to have overheating problems -- which may 
explain why trouble occurs when the gauge reads just above the N where any reasonable person would presume it’s still 
within normal operating range.  If the sender were moved to the left thermostat housing as proposed below, perhaps the 
gauge would have to be reading closer to the H to indicate serious trouble. 

Believe it or not, it appears that the vertical gauge is actually quite reliable.  In many (but not all!) of the reported cases 
of the needle being above the N due to problems with the gauge or sender, it was plainly obvious that there were gauge 
problems -- often with all four gauges, and sometimes pegging them against the upper end of the gauge seconds after 
cold start.  More subtle faults in the electrics, such as corroded connections, always seem to cause the needle to read 
low (often all four gauges at once -- see page 588) or to not read at all, staying at the C end.  A high temp reading that 
appears even remotely plausible is usually a cause for concern. 

A lot of owners describe the situation when talking about their temp readings, such as how hard they’ve been driving 
down the freeway or how hot it is outside today.  Unfortunately, such qualifiers are usually an indicator of trouble.  
Engine coolant temperature is controlled by the thermostats, and should not vary beyond their control range regardless 
of conditions.  And their control range (from fully closed to fully open) is pretty tight indeed; it has been described as 
“three needle widths” of travel on the gauge.  Since the cooling system should always have some margin (excess 
cooling capacity) so the thermostats can control the coolant temp, the indication should always be within this control 
range.  If the gauge ever indicates more than a couple of needle widths above the regular operating temperature with 
whatever thermostats are installed -- including reading on the N with 180°F thermostats -- it means that the thermostats 
are no longer in control, they are wide open, and it’s absolutely everything the cooling system can do to hold the 
temperature indicated.  Obviously, if things get just a hair worse -- the outdoor temperature rises a couple of degrees, 
one more tube inside your radiator gets plugged up, you sit in traffic a few more minutes, whatever -- the temp is going 
to rise some more.  Overheating damage is imminent, you need to be taking corrective action now. 

The only way “corrective action” would include replacing thermostats is if the thermostats are bad.  Merely switching 
to colder thermostats is not corrective action; if the cooling system can’t hold 190°F, it won’t be able to hold 180°F 
either.  Replacing 190°F thermostats with colder thermostats won’t fix anything unless the 190°F thermostats were 
defective. 

 

HOW HOT IS TOO HOT? - LATER XJ-S ROUND GAUGE:  James Teston says, “I have a '92 with the round 
temperature gauge. (Not the barrel gauge)!  It looks like this: 
                                        C  \  N  /  H 
                                               ^ 
                                               | 

And this is where the needle stays  (Between the N and the right hash mark).” 

Steve Gallant says, “My 93 XJR-s with 6.0L engine runs the same regarding temperature -- middle of the N to just 
slightly to the right.  It has been this way since I purchased the car with 5k miles on it.” 

Howard Gladman concurs:  “My 95- 6.0L exhibits same reading.  Once the the needle reaches about a needle width to 
the right of N the Aux fan switches on and the needle stabilizes.  56,000 on the clock and running strong.” 

It’s a pretty safe bet that all three of the above cars are running 190°F thermostats.  If you’re running colder 
thermostats, expect readings farther left than these -- and if you don’t see them, fix your cooling system. 

 

 
 

176

COOLANT TEMPERATURE SENSOR RELOCATION:  As mentioned in the description of the cooling system 
above, the coolant temperature sensor for the EFI system is on the left thermostat housing while the coolant temperature 
sender for the gauge is on the right thermostat housing -- and both are on the wrong side.  It would be better if the EFI 
coolant temperature sensor was on the right side and the gauge sender was on the left side, for two different reasons.  
As a result, an owner could rationally decide to move the EFI sensor from left to right, or the gauge sender from right to 
left, or to move both, essentially exchanging the two.  All moves are covered here; make your own choice. 

It’d be really nice if both sensors were the same thread so you could just swap them.  Unfortunately, the gauge sender is 
Smiths with a 5/8” UNF, 18 threads per inch, while the EFI sensor is Bosch, metric, 12mm x 1.5mm thread. 

To move the gauge sender to the left thermostat housing, you’re gonna have to drill and tap a 5/8”-18 hole.  This will 
require two tools you probably don’t have: a 9/16” drill bit, and a 5/8”-18 tap.  9/16” drill bits are not too difficult to 
find, but note that they are only available to fit 1/2” or larger drills!  They make 1/2” drill bits with reduced shanks all 
the way down to 1/4”, but the idea of a drill bit any larger than 1/2” reduced to 3/8” shank seems to have completely 
eluded the market.  A 14mm drill bit might also work, but they aren’t available to fit a 3/8” drill either. 

One possibility is obviously to rent a 1/2” drill.  Another idea:  you can carefully grind six flats on the bottom 1/2” of 
the shank of the bit.  Then you can chuck up a socket driver in your 1/4” or 3/8” drill, snap a socket onto the socket 
driver, and insert the hex on the bottom end of the drill bit into the socket.  It might be a little wobbly, but if you drill 
the hole as large as possible with conventional drill bits beforehand (1/2”), this hokey arrangement should do an 
acceptable job of enlarging the hole. 

This author bought a 9/16” bit with a 1/2” shank and three long flats to help it fit securely in a chuck.  By grinding three 
short flats in between the existing flats, a 12mm socket fit quite well -- and doesn’t hurt the bit one iota, since the 
original flats are intact and would still fit properly in a 1/2” drill. 

Another option, of course, would be to replace the OEM gauge sender with a generic sender with similar electrical 
characteristics but threads that fit the existing EFI sensor port on the left thermostat housing.  Good luck.  If you wish to 
try this, note the OEM sender registers 412Ω at 40ºC. 

You can install the gauge sender anywhere in the left thermostat housing that will see coolant flow from the heads.  It 
should not go in either the bypass passage or the passage to the radiator, since either may be closed off at one time or 
another.  On a pre-8S44060 engine, the obvious place to put it is on a flat that was clearly intended for such a boss but 
never drilled, right on top and near the rear end of the housing.  From engine 8S44060 on, there’s already a hole here 
with a sensor in it, so this location isn’t an option. 

There’s another flat surface apparently intended for a boss just rearward of the EFI sensor, but this port location 
connects to the bypass passage.  Also, the boss is too small for drilling and tapping a 5/8” hole. 

Another option, of course, is to just drill out the EFI coolant temperature sensor boss and retap it for the gauge sender.  
That leads to the next problem: you now need somewhere to put the EFI coolant temperature sensor.  And over on the 
right side is ideal. 

There are at least two existing ports on the right side; the one where the gauge sender goes, and the one where the 
thermotime switch goes.  On the author’s ’83, there is no thermotime switch; there is a plug in the hole.  Doesn’t help, 
unfortunately; both holes are too large for the EFI coolant temperature sensor.  So, you have two options: either drill 
and tap one of the existing holes out to a significantly larger size, install a plug, and drill and tap the plug to fit the EFI 
sensor; or just install a suitable plug in the old gauge sender hole and drill and tap an entirely new hole in the coolant 
manifold for the EFI sensor.  There are a couple of locations that will serve.  Note that the EFI sensor could just as 
easily go on the rear coolant manifolds. 

For a plug for the old gauge sender port, I’d suggest: another sender.  In fact, just leave the original sender there and 
buy a new sender DAC2583 for the left side.  The senders are less than ten bucks each. 

In fact, as long as you have two gauge senders, just install another wire and a switch somewhere so you can switch 
from one to the other to monitor both sides of the engine with the same gauge.  If you go this route, you might want to 
replace an original C40106 sender with another DAC2583 just to make sure any difference in indicated temp is due to a 
real difference in temperature rather than a difference in sender. 

 
 

177

If you have just read through this section and have decided to make this mod, note that unless you can figure out how to 
use taps and drills down in some fairly tight places, you will need to remove the thermostat housings to do the drilling 
and tapping.  Probably the easiest way to do that will be to remove the air filter housings and then unbolt the entire 
manifold assembly -- thermostat housing, coolant connecting pipe, and rear coolant manifold -- and move the entire 
assembly rearward far enough to disengage the hose connecting the thermostat housing to the crossover pipe.  So, while 
you’re preparing for this job, go ahead and buy a total of eight coolant manifold gaskets EBC 9634, two air filter 
housing gaskets EBC 9635, and both thermostat cover gaskets EBC 8330 and EBC 8331.  You may also want to get 
four “top hat” seals C37990 for the coolant connecting pipe, and maybe an AAV gasket C42165 or whatever a newer 
gasket number is.  And you will want to get a new copper washer for reinstalling the EFI sensor, C2296/3, although 
you can probably find a suitable sealing washer locally.  Reportedly a new gauge sender comes with a washer, so if you 
don’t remove the existing sender you won’t need to buy one separately. 

 

BLOWING COOLANT:  In general, if you are blowing coolant, your car is running too hot.  If you have a pre-H.E., 
pressure relief at the radiator cap on the header tank on the left wheel well will blow coolant or steam out the little tube 
into the wheel well.  Note that, if you fill the system right up to the cap, it will blow a little coolant the first time you run 
simply due to expansion.  You should be able to tell when it’s actually overheating, since it will blow much more 
energetically. 

If you have an H.E., pressure relief at the radiator cap on the header tank on the left wheel well will cause coolant or 
steam to blow into the plastic “atmospheric catchment tank” within the compartment behind the left front wheel.  This 
tank, in turn, overflows via a tiny tube out the bottom of the car at the very rear of the left front wheel well.  If the tank 
is empty or nearly empty, you may not be able to tell that the cap is relieving pressure; steam may condense within the 
plastic tank, and nothing visible comes out the tube.  If the overflow tank is totally full -- which it can get to if the 
owner continues to top up the system on a regular basis -- any pressure relief can push liquid out of this tube, even if 
it’s steam that’s coming out of the header tank.  And, again, if the system is topped up, it may push a little liquid out the 
first time it’s run without overheating. 

Finally, as noted on page 185, the overflow tube coming out of the atmospheric catchment tank is a classic example of 
poor design, so more than likely any flow out of this tank will be all over the inside of that compartment and come 
leaking out of nooks and crannies, such as out the joint between the rocker panel and the fender. 

 

OVERHEATING DAMAGE:  In any aluminum-block engine, severe overheating can result in a warped block or 
warped heads, which in turn normally call for an engine replacement.  In the Jaguar V12, a more common symptom of 
an overheated engine is a dropped valve seat.  Since the heads are aluminum, the valve seats are sintered iron rings that 
are pressed into the aluminum.  Since iron and aluminum have different coefficients of thermal expansion, overheating 
will cause a loose fit and the seat can just fall out.  After that, it holds the valve part way open and bangs around in 
there.  Amazingly, reports of broken valves are rare; more often the owner who continues driving despite the annoying 
ticking under the hood allows the valve to beat the seat to pieces, which in turn bang up the piston, the other valve, and 
the head. 

Do not continue to drive when the car is overheating.  If no other options are available, drive it short distances at a time, 
shutting it off and allowing it to cool before starting again.  And don’t continue to drive if it sounds like a Chevy with 
bad lifters, either -- you’ve already dropped a seat, but you might as well quit before you do even more damage. 

If at all possible, don’t shut off the engine when the car is overheating; many have reported that’s when the valve seats 
drop -- when the engine is stopped.  Instead, find a water hose and leave the engine running as you hose down the 
radiator through the front grille.  Once the temp gauge comes down, then shut it down -- and call a tow truck, do not 
start it again until you have addressed its overheating problems. 

 

OVERHEATING -- H.E. vs. PRE-H.E.:  Roger Bywater indicates that the pre-H.E. cars had some tendency to 
overheat: “With regard to the marginal cooling at sustained high speeds the H.E. had a slight advantage in that the 
higher compression ratio raised the thermal efficiency and reduced the heat losses to the coolant.  It was also 

 
 

178

noticeably over-fuelled at high revs which must have helped further and the problem, slight though it was, seemed 
to be solved.  Distributor build quality was also better by this time.” 

 

OVERHEATING -- CAUSES:  Believe it or not, the XJ-S H.E. does not overheat when it’s running right -- and this 
from an owner who lives in Florida!  If yours tends to overheat, don’t ignore it.  Check the following, all of which are 
described further in this book: 

1)  The cooling system must be properly filled with no air in it. 

a)  The system must be filled with the proper coolant mixture and bled correctly to begin with, which requires 

disconnecting a hose at the heater valve (not mentioned in the manuals). 

b)  The automatic air purge system must be working properly to keep air out. 

i)  The banjo fitting on top of the radiator is probably plugged if it hasn’t been modified. 

ii)  The line to the atmospheric catchment tank may be obstructed or kinked. 

2)  The radiator must be clear, no blockage or sludge; unfortunately, this is difficult to check conclusively on the XJ-S 

without removing it from the car and removing the end tanks.  Crud in the radiator will cause overheating under all 
conditions, but usually more at speed than at idle.  Experience indicates that any radiator maintained following the 
owner’s handbook guidelines for more than ten years is likely to be plugged. 

3)  Suspend the thermostats in a pan of water on the stove and bring them to a boil.  Do not let them contact the 

bottom of the pan.  If the thermostats are not visibly wide open by the time the water boils, replace them.  Their 
usual failure mode is to open only slightly. 

4)  Retarded timing will cause overheating under all operating conditions.  See the sections on ignition timing 

beginning on page 125. 

a)  A seized centrifugal advance mechanism may be seized at any position from idle to max advance.  Usually, it 

will be correct at one particular RPM, retarded everywhere above that RPM, and overadvanced everywhere 
below.  Conditions with retarded timing will cause overheating.  Conditions with overadvanced timing can 
also cause overheating but usually not as severe; the more obvious problem is knocking.  See page 137. 

b)  A blown vacuum advance module will cause timing to be retarded at low throttle while correct at full throttle, 

so the car will overheat more when driving gently. 

5)  There may be debris obstructing airflow through the radiator, such as dirt and leaves -- either plugging the fins 

themselves or within the space between the A/C condenser and the radiator.  See page 202. 

6)  A bad fan clutch causes overheating only in stop-and-go traffic or other conditions where motion of the car doesn’t 

provide enough air flow.  See page 214. 

7)  The fan shroud needs to be properly installed and sealed up against the back side of the radiator.  See page 225. 

8)  The fan shroud flaps should be intact and free to flap as intended.  If they are missing, overheating when stopped 

and idling is likely.  

9)  There needs to be foam surrounding the radiator to prevent air from bypassing it.  See page 226. 

10)  Front spoiler -- it must be there, and it must be properly mounted.  See page 227. 

11)  Automatic transmission problems can be putting too much extra heat into the radiator.  See page 323. 

While all of the above items point to common faults, it must be said that far and away the most common cause of 
overheating on XJ-S’s that are ten years old or more is a plugged radiator.  If you are having cooling problems, address 
the easy items first, but you might as well go ahead and plan to remove the radiator and have it rodded, recored, or 
replaced.