Jaguar XJ-S. Manual - part 31

 
 

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an operation. Working through a narrow hole with long instruments, trying hard not to break something vital.  I was 
even using forceps to hold tubes and wires back to get to the areas I needed to go!”  Craig Sawyers says, “I have that 
Jag tool.  However, I have always found it impossible to use under the aircon compressor.  The problem is that the 
swivel joint is operating very nearly through a right angle, and is very inefficient as a result.  Besides which, even if it is 
possible to get the plug out with the tool, it must be fraught with danger of cross-threading when putting it back, or 
replacing with new.  In fact, the last XJS I moved the compressor out of the way to do the plugs (my brother-in-law's) 
had three chunks of broken plugs lurking under the condensor.  So I now always move the compressor (a ten minute 
job) and the throttle pedestal (a further ten minutes) and make the whole process more clinical.  Besides which, after 
vacuuming the crud from the centre of the V, you can then use a nice deep socket, and a torque wrench to get the things 
in "by the book".” 

If you decide to remove the throttle pedestal, John Napoli says, “Rather than removing the coil, throttle switch, etc. 
separately, consider removing the whole ball of wax as one.  Simply remove the four bolts holding the throttle stand to 
the engine valley, and it all comes out as one.” 

You may run into problems getting those four bolts out as well.  It’s not too difficult to remove in the midst of an 
upper-engine teardown when you’re taking everything out of the way anyway, but for the spark plug job you’d like to 
remove just the pedestal without removing things like the oil pressure sender.  The problem is the two bolts at the rear 
side of the pedestal; they are easy to see, but not so easy to get a wrench on.  First tip: obtain a 7/16” swivel socket -- or 
a 10mm swivel socket if the later engines have metric bolts here. 

The first time you have the pedestal off, saw off the lugs on the right side that were apparently originally intended to 
hold a coil.  That will make access a little easier next time. 

Napoli decided to make some access holes in the pedestal:  “On my car, I modified the throttle stand further to lighten 
it, improve airflow in the area, and make it easier yet to get in and out.  I drilled a series of large holes all over.  
Cleaning it all up and polishing a bit makes it look nice, improves gas mileage, reduces emissions, and decreases 
quarter mile times.” 

Peter Cohen says, “The last time mine was out, I slotted the holes for the two rear bolts so that I could start the threads 
and then mount the pedestal.” 

As long as you’re going to the effort of taking the plugs out, it wouldn’t be a bad idea to go ahead and perform a 
compression check.  It’s a good way to keep a watchful eye on engine condition. 

Before installing the new plugs, it is recommended the time be taken to carefully inspect the threads.  Competition is 
fierce among the plug manufacturers and they generally cannot afford to spend a lot of time and effort making perfect 
threads every time.  One small burr on a steel spark plug can wreak havoc on the threads in the aluminum head.  

Be sure to use anti-seize compound on the threads.  Do not use normal grease, as this may hamper proper grounding of 
the plug, and it also may get hot and form a crust that makes it even harder to remove the plugs.  And do not use a 
graphite-based lubricant, as graphite may react with the aluminum and weaken it. 

When installing spark plugs, it is helpful to use a piece of 3/8” hose to get them started.  Press the hose firmly over the 
top of the plug and use the hose to feed them into place and thread them in finger tight.  Then the hose can be pulled off 
and a socket used for final tightening. 

There are warnings not to tighten the plugs past 8 ft-lb. of torque.  8 ft-lb. is not enough to turn the plugs loosely in the 
threads unless they have been recently cleaned and chased, but the point is well taken.  It is an aluminum head, and 
tightening “three grunts” is both unnecessary and costly.  Just tighten until you feel them seat, make sure they’re snug, 
and quit. 

 

SPARK PLUGS:  A quick survey of a group of Jaguar experts clearly indicates that NGK is the plug to use.  The study 
may have been unscientific and the reasons may have varied, but the consensus was clear and overwhelming. 

The H.E. engine uses taper seal plugs; the pre-H.E. engine uses washer-seal plugs.  For the H.E., NGK used to make at 
least two plugs, TR5 and BPR6EFS.  Both were resistor type, but the TR5 was a “V-Power” plug with a little notch cut 

 
 

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in the center electrode.  Jim Moore says, “They go by numbers now; an NGK 2238 is recommended by NGK for my 
'85 XJ-S, turns out it has TR5 written on other end of box and on the plug.” 

There are many gimmicks used to sell spark plugs.  One common feature that may be of some benefit:  It is easier for a 
spark to jump to an edge or point than to a flat surface.  This is why lightning rods are pointed.  Many of the cute 
configurations of spark plugs result in there being more edges on the electrodes instead of flat surfaces.  The notch in 
the center electrode of the NGK V-Power plug is such a feature. 

Supposedly, it’s actually more important that the sharp edges be on one electrode than the other.  With old-fashioned 
ignition systems, the center electrode was always the anode and the ground was always the cathode -- and the sharp 
edges really need to be on the cathode, which means they’re on the wrong electrode with the V-power plugs.  
Apparently NGK fixed that; Richard Mansell quotes from a Jaguar publication on the changes for the 1992 model year: 
 

“A new design sparkplug, BR7EF, incorporating a V groove earth electrode, is fitted to improve efficiency and 
provide better firing consistency.” 

Note that very little else changed about the Jaguar V12 for the 1992 model year; one can safely conclude that the 
upgraded spark plugs would work well in older H.E. engines. 

NGK certainly isn’t the first company to offer multiple sharp edges on the ground electrode.  ND plugs have had a 
grooved ground electrode for years.  A highly-advertised plug called the Splitfire charges several times as much as 
regular plugs for providing a couple of extra edges for the spark to jump to. 

If you happen to have a distributorless ignition with double-ended coils (only offered on Jaguar V12’s in the mid-90’s, 
and only in the saloon) then half the plugs have their polarity reversed, and the sharp edges need to be on the center 
electrode again!  You can’t win. 

Another idea in spark plugs that gained popularity in the 90’s is platinum electrodes.  Platinum is an obscenely 
expensive metal -- makes gold look cheap -- but it doesn’t take much of it to make a spark plug electrode, just the 
tiniest dot at the gap.  And that dot makes the plug last several times as long, which is the reason some modern cars can 
claim 100K miles before the first tune-up.  Note that while platinum-tipped plugs may cost considerably more than 
standard plugs, they do not offer any improvement in performance at all; their entire benefit is in durability. 

Following the crowd, NGK has introduced new plugs for the V12, similar to their earlier offerings but with platinum in 
the tips.  As a result, it is increasingly difficult to find non-platinum NGK plugs.  David Buchner doesn’t like platinums 
because he feels you should get in there and check things more often than every 100K miles, but even with such an 
attitude one might be forced to use platinum plugs and just regap and reinstall them!  Might wanna be careful about 
filing the gap, though; platinum isn’t all that hard, a few passes with a file and the little dot might be gone! 

Bosch Platinums were among the first platinum-tipped plugs to make a name for themselves -- and it was a bad name.  
Randy Wilson says “I’ve found that Bosch standard plugs do not perform quite as well as Champions or NGK, but are 
better than most common American plugs.  I have, however, never had any success with their platinum plugs.  If the 
engine is running well, they don’t seem to make any improvement, and if the engine is not well, they foul easily and are 
not cleanable.”  But don’t let the bad reputation sour you on either platinum or Bosch plugs; Bosch has since introduced 
the “Platinum +4” plug, which has four ground electrodes (more edges!) but also seems to have corrected the problems 
the earlier Platinums had. 

There is no appreciable disadvantage to using resistor plugs.  The resistance limits the current flow, which in turn 
reduces spark plug wear and radio interference.  Prior to the actual spark, there is no current flow so the resistance has 
no effect on the voltage available at the electrodes. 

 

SPARK PLUG GAPS:  The proper gap for the H.E. engine is 0.025” -- you can confirm this by checking the decal on 
the underside of the hood.  Most plugs come out of the box with a 0.032”-0.035” gap.  The gaps must be checked prior 
to installation.  Oversize gaps can cause poor performance at high RPM, but worse they can contribute to rotor failure 
and resulting catalytic convertor meltdown in cars equipped with Marelli ignition -- see page 160. 

Reports are that some dealers are installing spark plugs with 0.035” gap.  If your dealer is doing this, the least you 
should do is never darken his doors again.  Better yet, file a complaint with the EPA, since spark plug gaps are an 

 
 

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“emission critical” specification (that’s why it’s on the decal under the hood) and it is illegal for them to be violating it. 
 If you happen to suffer a catalytic convertor meltdown as a result, please sue the dealer and help put him out of 
business. 

 

STRIPPED SPARK PLUG HOLES:  Since you have read this book, used anti-seize compound on the spark plug 
threads, and didn’t overtighten, you will never strip spark plug threads.  However, the PO

3

 or the idiot you’ve been 

taking the car to for service may not be so careful and leave you with plugs that simply won’t come out without 
bringing the threads with them.  Aaargh! 

The official Jaguar fix involves recessing, drilling and tapping the holes for larger threads, installing an insert, and 
installing a small pin to prevent the insert from unscrewing.  Note that the inserts will be different for H.E. and pre-H.E. 
engines, since the pre-H.E. used washer seal plugs while the H.E. uses taper seal plugs.  Generic inserts may work fine, 
but regardless of insert used the head surface should be recessed prior to installation to ensure that the tip of the plug 
will be positioned at the same level as before.  If the surface is not recessed, the plug will seat in a slightly retracted 
position and result in a lower compression ratio on that cylinder as well as possible effects on the swirl pattern in the 
H.E. 

Mike Morrin reports success using helicoils to repair the H.E. plug threads -- something one wouldn’t expect would 
work, since it would appear to require drilling out the hole far enough to destroy the taper seat.  “If drilled carefully, 
about 30% of the taper seat is lost.  I think the remaining seat is still enough for a good seal.” 

 

SPARK PLUG INDEXING:  Does “anal-retentive” have a hyphen?  If you answered yes, you may be interested in 
knowing that “indexing” spark plugs may improve performance in the H.E. engine.  In the H.E. combustion chamber, 
the compression stroke squeezes the fuel/air mixture out of the area over the inlet valve and into the area over the 
exhaust valve, causing a swirl.  This fuel/air mixture is swirling when the spark occurs.  If the ground electrode on the 
spark plug happens to be positioned with its back side into the swirl, then the spark is partially hidden from the fuel/air 
flow, possibly hindering proper ignition. 

To avoid this, mark the side of each spark plug with an indelible magic marker to indicate where the ground electrode 
is.  The position you want to avoid is with the back side of the ground electrode aimed toward the nearest end of the 
head; in other words, you don’t want the electrode on the spark plugs in the front three cylinders on the forward side 
and you don’t want the electrodes on the spark plugs on the back three cylinders on the aft side.  If any end up in these 
positions when installed, then exchange that plug with another. 

Please do not use a lead pencil to mark the plugs.  Graphite conducts electricity, and will cause “tracking” down the 
side of the ceramic and short out the spark plug.  Your engine will run much worse than before you indexed the plugs.  

 

SPARK PLUG WIRES:  If your ignition wires need replacing, there’s really no reason to buy the Lucas originals; 
several companies offer aftermarket sets to fit the Jaguar V12.  Or you can purchase two 6-cyl or 8-cyl ignition wire 
sets from any auto parts store and combine them into a set for the V12.  Here are some tips: 

You can get anything from a roll of wire and a package of connectors all the way to completed, ready-to-install sets;  I 
used a “semi-tailored” set -- one end of each wire comes with the spark plug connector attached, but you cut the other 
end to length and attach the distributor cap connector yourself.  The wires on the V12 are shorter than most, and this 
permits the right length so you don’t have lots of excess wire laying around. 

Another feature to look for is the angle of the spark plug connector.  Some are straight, some are 90 degree, a few are 
45 degree.  What the V12 needs is a slight bend; the ideal kit has connectors that are straight to begin with but can be 
bent to whatever angle is needed.  This allows providing different bends at different cylinders -- working on these 
plugs, you need all the help you can get! 

                                                

 

3

 "PO" is a car collector's term for "Previous Owner".  It is often used with a derogatory connotation.  Sometimes 

the abbreviation “DPO” is used. 

 
 

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There is also the issue of bends at the distributor cap end of the wires.  Some wires have straight connectors so they 
come straight upward out of the cap, while others have 90º bends here so the wires come off the sides of the cap.  Either 
will work on the XJ-S, both are a pain to install and route. 

There is also the choice between 7mm and 8mm wires.  Ideally, one would choose 8mm wires, since the Jaguar ignition 
system is really hot and it’s all but impossible to route the wires away from other things.  Magnecor (see below) offers a 
10mm set! 

Most aftermarket kits seem to be bright colors.  Deal with it. 

Jim Belkoff writes, “I found that Pep Boys can special order a custom set of 8mm silicone wires that are made in the 
US by Borg-Warner.  I paid the $45 in advance and picked them up a day later.  They are of high quality and include 
three wires with 90-degree plug boots (for the number one wires at the compressor and another location - your choice, I 
guess).  The kit even came with a tube of dielectric grease. 

“For those of you who don’t like the bright aftermarket colors, take heart - this set has tasteful light grey wires, black 
plug boots and dark grey distributor boots. 

“The length was very close on most, and exactly spot on with some of them.  Even the ones that were a shade long 
weren’t so huge that they flopped every which way.  I have had problems with aftermarket wires in the past (mostly 
semi-custom 7mm sets) and these are the closest in length and the highest quality I have used so far.” 

John Smith of Brisbane, Australia says,  “I noticed that most parts shops have made up spark plug wires in a variety of 
lengths.  I bought a set, nice blue colour, that had 90 degree connectors for the distributor and straight connectors for 
the plug ends - the same as the original wires. 

“I measured the various original wire lengths and these are: 

1A 

38cms (15 

inches) 

 

1B 

43cms (17 

inches) 

2A 

25 

(10) 

  2B 

35 

(14) 

3A 

28 

(11) 

  3B 

17 

(7) 

4A 

30 

(12) 

  4B 

25 

(10) 

5A 

40 

(16) 

  5B 

38 

(15) 

6A 38 (15) 

 

    6B 46 (18) 

Coil 19  (8) 

These wires were 8mm and fitted perfectly.” 

Greg Benjock bought an 8mm Belden Wire set from NAPA and was not very happy with it.  “The custom ordered kit 
came with wrong terminations (all were 180deg).  All 13 terminations on the distributor cap in fact must be 90 deg 
type.  And terminations and boots for plugs 1A, 6A, 1B, and 6B are best fitted with 90 degree units.  It looked very 
much like an inexperienced person put the job together.”  Of course, maybe the cap connectors are a matter of opinion; 
this author’s are all straight connectors and work just fine! 

All that said, it should be noted that the original Lucas wires are usually not very expensive; the only valid reasons for 
using aftermarket wires are availability, a need for a dash of color, and a healthy contempt for all things Lucas. 

 

SPARK PLUG WIRE THEORY:  When the 12V power to the coil is cut off, the magnetic flux causes a spike in the 
voltage at the high tension lead.  When that voltage rises sufficiently to jump the gaps in that circuit (one between the 
rotor and cap in the distributor, another at the plug), current begins to flow. 

Once a spark begins, the air is ionized at both gaps.  This drastically lowers the electrical resistance of the gaps 
themselves.  With 50,000 volts or so to work with, the current flow can rise dramatically.  If allowed to rise 
unrestrained, this has several detrimental effects.  First, the high current flow drains the energy stored in the coil 
quickly, so the spark doesn’t last as long.  Second, the high current flow tends to erode the spark plug electrodes faster 
than necessary.  And third, the high current flow causes a considerable amount of radio interference due to EMI 
radiating away from the plug leads.  Besides all these negatives, there are absolutely no positives.  High current flow is 
not beneficial to the ignition process, only the voltage needed to start the spark is necessary.  Advertisements talk about