Jaguar XJ-S. Manual - part 10

 
 

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OCTANE:  ...ain’t octane.  If only things were simple.  John Littler picked the following explanation up from an 
internet discussion; I presume that ULP stands for UnLeaded Petrol and PULP stands for Premium:  “The US does 
things differently to the rest of the world (I bet you've heard that before!).  In Australia the octane reported at the pump 
is the RON (research octane number).  There is also another octane that you never hear about called MON (motor 
octane number).  The MON is determined in a similar way, but under more severe conditions (so it yields a lower 
number).  In the US they report AKI (anti knock index) which is an average of MON and RON. 

“The ratings for ULP and PULP are typically: 

  

MON RON AKI 

 ULP 

82.5 

91 

87 

approx 

 PULP 

86 

95.5 

91 

approx 

You can see that the US 91 is actually our PULP.  This is the US mid-grade (also the main grade in Europe).  They also 
have a premium grade which has an AKI of 93 or a RON of about 98.” 

Littler adds his own editing to this snippet:  “I wish it were true that the US had a mid-grade rated at 91 AKI/95 RON.  
Then it would be suitable for my Jag which requires 95 RON.  Unfortunately the US mid-grade is only 89 AKI/94 
RON, so I have to pay for the 93 AKI/98 RON Super-Unleaded.  If the Jag had a knock sensor I would risk trying the 
94 octane stuff, but it doesn't, and the dealer has warned me to always use 98. 

“BTW, prior to 1973 the US reported octane as RON like the rest of the world.” 

 

UNLEADED FUEL:  XJ-S owners in the US have been using unleaded fuel for decades, but many owners in other 
countries are only recently facing the prospect of leaded fuel being no longer available.  Since some of their owner’s 
handbooks specify the use of “Four Star” leaded fuel only, they are understandably concerned. 

Addressing this concern, Roger Bywater of AJ6 Engineering (and formerly with Jaguar Engine Development) wrote an 
excellent article for Jaguar World magazine (see page 721), Vol 10 No 3, January/February 1998, page 42.  Some of 
the high points are summarized here. 

There are two reasons for concern regarding the use of unleaded fuel:  Octane and valve seat lubrication.  Valve seat 
lubrication is simply not an issue in the Jaguar XJ-S; every engine the car has ever been built with has an aluminum 
head with hardened valve seat inserts, and the part numbers for these parts are the same for US-spec models always 
intended to run on unleaded fuel.  If you have shoehorned in a Chevy engine old enough to have a cast iron head with 
no inserts and designed to require leaded fuel, well, shame on you. 

Octane is another story.  The US-spec engines have lower compression ratios to run on lower octane fuel.  To be able to 
run the high compression engines on lower octane fuel, either the compression ratio will need to be lowered or the 
timing will need to be retarded.  If the engine is being rebuilt anyway, lowering the compression ratio (different pistons, 
thicker head gaskets, machining the combustion chamber, whatever) may be workable, but most people will prefer to 
simply retard the timing.  Bywater suggests that there’s very little difference between a high compression engine with 
retarded timing and a low compression engine with advanced timing anyway. 

With the Lucas ignition systems, retarding the timing is simply a matter of turning the distributor to a new setting.  
Bywater says that for the 95 octane unleaded that will be available in the UK and Europe, about three degrees should do 
it, although as many as five may be necessary for older cars with a lot of carbon buildup.  On Marelli ignition cars, 
retarding the timing only requires pulling a jumper to switch the ignition ECU to a more retarded timing map. 

 

UNLEADED FUEL -- AJ6 ENGINE:  If you own an early (‘83-87) AJ6-engined car with the Lucas 8CU EFI, you can 
retard your timing by simply turning the distributor.  However, if you have a later (‘87-on) 3.6 with the 9CU engine 
control system, the ignition timing is controlled electronically and not so easy to change.  According to Bywater’s 
article, there are two options:  You can remount the crankshaft position sensor using the mount from the later 4.0 AJ6 
engine, or you can remove the crank pulley and reposition the toothed rotor by drilling new mounting holes. 

 
 

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John Littler says, “Well, for around $50US you can get a little gizmo from a number of vendors which then gives you a 
dial to advance or retard the stock timing by up to about 5 degrees either way (does it on the fly so it just takes 1 degree 
off whatever the ECU says etc).  Definitely available from Summit (MSD and Crane), also available in Oz from two 
other suppliers as well.  Lastly, seeing as I believe the AJ6 use Delco ECU's (is that right?), then a copy of the 
Kalmaker software will let you change anything you want on the EPROM.” 

 

THE DANGERS OF RUNNING LEAN:  Most people tend to think of their car’s fuel system as an on/off proposition: 
either you’ve got fuel, or you don’t -- and if you don’t, it won’t run, so you’ve gotta fix it.  There is, however, a third 
possibility that bites the Jaguar V12 owner only too often, and that is running lean. 

Running lean is no big deal if you’re driving gently; in fact, there are modern automobiles designed for “lean burn” 
operation, which means that at light throttle they can operate quite a ways leaner than stochiometric in order to optimize 
fuel economy.  But running hard -- in any gasoline engine, Jaguar, lean burn, whatever -- with an air/fuel ratio leaner 
than stochiometric is just asking for engine damage.  A Jaguar V12 that has been running lean for a year but has never 
seen full throttle is probably as good as new, but hold that pedal to the metal for 30 seconds and a couple of new pistons 
will probably be needed. 

Why am I making a big deal out of this?  After all, the XJ-S has EFI, which should provide reliable fuelling at all 
throttle conditions.  The later models even have oxygen sensors that will correct for either lean or rich conditions on the 
fly.  Why worry?  Well, there’s plenty to worry about.  If the fuel pump or other parts of the fuel system aren’t up to 
par, the EFI system may provide correct fuelling under normal driving conditions, but when floored the fuel supply 
can’t keep up -- and the engine runs lean just when it is most dangerous!  And, horror of horrors, there are absolutely no 
warnings that you’re in trouble; everything on the dash looks just great, the engine sounds just dandy, and the car has 
plenty of pep -- right up until it starts missing on a couple of cylinders because the pistons are toast. 

If you ever intend to explore the lower half of the pedal travel on the Jaguar V12 (and who doesn’t?  Why else would 
you own the car?), it is recommended that you install air/fuel ratio indicators, which are commonly available; they are 
very popular with performance enthusiasts for exactly the same reason: they provide a warning before it’s too late.  
These indicators operate off the signal of a regular oxygen sensor.  If your car doesn’t have oxygen sensors, you can 
purchase generic sensors (the heated type are recommended), and it’s a simple matter to have bosses welded into the 
exhaust downpipes to install them.  If your car already has oxygen sensors, there’s no need to install more; the 
indicators will read the signal from the existing sensors without interfering with EFI operation. 

Both Jeg’s (page 714) and Summit (page 717) offer LED air/fuel indicators, and they’re even reasonably priced.  
You’ll probably want two indicators, since you have two oxygen sensors -- or perhaps you can rig a switch to read one 
sensor and then the other with a single indicator.  The indicators usually feature three colors of LED.  If the engine is 
running near stochiometric mixture, LED’s near the center of the display are lit, and they are usually green.  The rich 
and lean sides of stochiometric are usually red and yellow LED’s, but some indicators have red as rich and some have 
red as lean; make sure you understand that it is lean that means trouble, whichever color the LED’s are. 

When the engine is running in closed-loop mode (warmed up, light throttle), the indicators should be reading green.  
When floored, the ECU goes into open-loop mode, which means it should run richer than stochiometric.  A rich 
indication when floored is OK.  If the gauge ever starts to read on the lean side of stochiometric while you have your 
foot in it, lift your foot now and investigate problems in the fuel system before you romp on it again. 

Air/fuel indicators can help you spot a lot of ills, including totally unheard-of problems not covered in this book.  
You’ll be able to monitor how your fuel system is working at all times and notice when something goes wrong. 

The May 1996 issue of Skinned Knuckles has an article, “Oxygen Sensors & Air/Fuel Ratio” by Hugh Poling, in which 
the author points out the limitations of air/fuel indicators that use a conventional oxygen sensor for input.  Some A/F 
indicators have three LED’s, indicating lean, stochiometric or rich.  Others have more than three, apparently indicating 
different degrees of lean or rich; according to Poling, these indicators are wired so that particular voltages from the 
oxygen sensor light up particular LED’s.  The indicators Poling objects to are the ones that are actually marked with 
different Lambda values; “From the SAE Transaction pertaining to oxygen sensors, this relationship of voltage to LED 
markings is bogus...Since the standard oxygen sensor covers a very narrow range of only a few percent around 

 
 

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stochiometric, this labeling is fraudulent.”  In other words, an LED indicator connected to a conventional oxygen sensor 
is quite reliable at indicating whether you are running lean or rich, but do not rely on it to tell you how lean or rich. 

Note that there are instruments on the market that can provide accurate indications of mixture within the lean or rich 
ranges, but you’ll know if you buy one -- they are very expensive, and they do not use a conventional oxygen sensor for 
a signal. 

 

OIL CHANGING:  If you buy an oil drain pan from the local auto parts store, slide it under the Jag, and open the plug 
in the sump, you may be in for a messy surprise.  The Jag V12 will drain around 11 US quarts of oil, and this is more 
than the capacity of many conventional oil drain pans.  Either be sure to get a really big pan, or figure out how to drain 
the sump into two pans. 

Here’s an idea:  Get one of those plastic oil drain tubs that features a spigot to pour the oil into bottles without a funnel. 
 When the tub is sitting on the floor, the spigot acts as a spillover; if too much oil is drained into the tub, it’ll come out 
the spigot rather than spilling over an edge.  With the tub sitting under the engine, set another container under the 
spigot.  Your total drain capacity hence becomes the total of both tubs. 

A disposable aluminum turkey broiling pan works great for that second pan.  If it doesn’t fit neatly under the spigot as 
is, it’s not difficult to mangle the edge a bit to allow it to sit right. 

 

OIL FILTER REPLACEMENT:  The oil filter on the Jaguar XJ-S mounts nearly vertically, the way they all should; the 
dirt stays in the filter as it’s being removed.  Considering the position, you can do your engine a favor by filling the new 
filter with oil prior to installing it.  When you start up, you will get pressure that much sooner.  There are also reports 
that prefilling may help protect the element itself from rupture when the engine is started.  You will find it helpful to 
use a small screwdriver or some such to poke through the smaller holes in the base of the filter and push open the flap a 
little to provide a vent while trying to pour oil in the center hole.  And be patient; it takes a minute or two for oil poured 
into the center hole to soak through the element and fill the outer chamber. 

It has been noticed that some filters for the XJ-S are different from others.  Some are very large, extending almost level 
with the bottom of the pan.  Others are of conventional size.  Some have a series of flats around the bottom edge to fit 
the socket-type filter wrench that goes on the end, and some don’t.  Since you can’t get to this filter from the side, it is 
recommended you get a filter wrench that can be used from the end, and make sure the filters you purchase can be 
removed with your wrench.  The socket-type filter wrench works well and is cheap, but requires the series of flats on 
the filter.  There are also coil type and strap type filter wrenches that will work on most any filter; note that the coil type 
cannot be used to tighten the filter. 

Peter Cohen says, “I have found that if I hand tighten them as much as I can with one hand with the little bit of leverage 
available down there, I am able to hand remove them as well.”  Peyton Gill adds, “I got a box of disposable latex rubber 
gloves (used in the medical industry).  They provide additional grip and make hand removal and installation much 
easier plus keep your hand clean when the oil runs down the filter.  These things are cheap and can be bought at 
drugstores.  It really makes a big difference applying torque.” 

 

OIL FILTER REPLACEMENT -- EARLY CANISTER TYPE:  Until sometime in 1976, the XJ-S came with a canister 
filter assembly.  Not only does this make it more difficult to change, but more care must be taken to make sure 
everything is working properly.  Mike Morrin had a problem with a non-replaceable bypass valve that sits in the bottom 
of the canister:  “I noticed that the bypass valve was actually about half way open!!  This is presumably not good for 
filtration efficiency.  On dismanting the bypass valve, it became apparent that the alloy valve body had been strained, 
probably by someone tightening up the filter assembly with something not seated correctly.  I was able to straighten and 
reassemble the valve, and it looks like it should work as good as new.” 

 

 
 

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OIL ADDITIVES:  Don’t use any.  The quintessential article on the subject is “Snake Oil! Is That Additive Really A 
Negative?” by Fred Rau, ROAD RIDER, August 1992, Pg 15.  Transcripts of this article appear in literally dozens of 
places on the WWW, any search should find one.  A couple such places include: 

 

http://www.vtr.org/maintain/oil-additives.html

 

 

http://members.aol.com/brnorth3/amsoil/snake.html

 

For those who feel compelled to spend more money on oil than merely following the manufacturer’s recommended 
change intervals, the following options are suggested: 

1. 

Install a bypass filter system -- see page 114. 

2. 

Change oil more often.  In fact, changing oil ridiculously often using el cheapo oil is a very 

workable plan, but some consider it environmentally unwise.  It’s also a lot of work. 

3. 

Use synthetic oil.  Tests have indicated that synthetic oil coming out after the recommended change 

interval is often better than new regular oil. 

4. Change 

the 

filter more often.  They’re cheap, and indications are they get plugged and begin 

bypassing long before the scheduled oil change interval. 

On my car, I change the oil according to Jaguar’s recommended interval (6000 miles), use synthetic oil (Mobil 1, 15W-
50), and change the filter every 3000 miles. 

 

Leaks 

 

SEALING SUBSTANCES:  If you would prefer that your Jaguar not leak oil, the first thing you must get through your 
thick skull is to ignore all guidance on sealants and gasket dressings provided in the official Jaguar publications.  Most 
of their callouts are obsolete, many were wrong the day they were printed, and -- let’s face it -- Jaguar has absolutely no 
credibility regarding leak prevention.  To top it all off, Jaguar manuals and TSB’s have a nasty habit of calling for some 
esoteric product that’s only available in industrial quantities via special order.  Forget the Jaguar sources and follow the 
guidance in this book instead. 

Note that “gasket dressing” and “sealant” are two different things; a gasket dressing is used to help a gasket seal 
properly, while a sealant is used where there is no gasket.  Nobody is strict about the use of these terms, but the wise 
mechanic will note how a product is intended to be used and avoid trying to use products for applications they are not 
suited for. 

It’s easy to get all wrapped up in which sealant or dressing is “the best”, but all you really care about is that it doesn’t 
leak.  There are many products that will fill that bill for any particular application.  Being readily available locally in 
small quantities is a big plus.  Here are some simple guidelines: 

If it’s a gortex gasket, don’t use any substances on it; assemble it dry. 

If it’s a paper gasket, use a gasket dressing such as Hylomar on both sides. 

If the joint involves two close-fitting metal surfaces with no gasket, use a sealant such as Loctite 518. 

Those guidelines will cover almost everything that needs sealing in the Jaguar XJ-S.  There are a couple of special 
cases, though.  The sides of the rear main bearing cap require a gap-filling sealant that can be injected; for this, copper 
silicone sealant works well although it’s not particularly easy to squeeze into the slots (the top of the rear bearing cap 
should be sealed with Loctite 518).  The threads on the sandwich plate bolts with their heads inside the sump should be 
sealed with Permatex #2 non-hardening sealant.  The head gaskets and exhaust manifold gaskets are metal sandwiched 
around a fibrous core and should be assembled dry.