Jaguar XJ-S. Service manual - part 73

 
 

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injector which then becomes a rather noisy blow lamp inside the manifold.  A single plenum engine would immediately 
stop then would easily restart probably without the condition even being recognised.  A V12 could have one bank 'lit 
up' inside but the other bank would keep it running, after a fashion, until the thermotime switch trips out when things 
revert to normal.” 

 

COLD START PROBLEMS:  William F. Trimble says, “My Haynes manual for the V12 (yeah, I know ... :-) suggests 
to remove the thermo-time switch altogether if you have cold-start problems.”  This author’s Haynes manual says “A 
problem can occur on 1980 models causing stalling immediately after cold start.  This can be overcome by removing 
the cold start relay...” 

John Braybrooks says, “I have a 1982 XJ-S in Victoria, British Columbia, Canada.  I keep my car in a garage and one 
recent winter my son drove it out so that he could use the garage space to work on his 1929 Model A.  When he 
finished he returned the Jaguar to its rightful location.  Next day my wife also moved the car to facilitate sweeping the 
floor and again returned it.  Next morning when I tried to start the car there was no life.  I called local Jaguar specialist, 
Paul McKay, and he immediately diagnosed the problem as flooding.  He came to my house and replaced the spark 
plugs which were the least difficult (5) and we were then able to get the engine running on those cylinders, later more 
and more came on line until all was well.  He informed me that this was a characteristic of this engine and it was not 
unusual for the dealers to experience this problem with brand new vehicles loaded on and off transporters during the 
winter.  Under the right conditions the cars would not start when delivered to the dealer.  Paul warns that if this does 
occur and you are able to get the engine running of a few cylinders do not "gun it" otherwise the unburned fuel will be 
burnt in the catalytic convertors and the excessive heat could cause a meltdown.  If I have to move my car for very short 
periods in cool weather I now always let the engine run until it is thoroughly warm.” 

Roger Bywater explains:  “That's a familiar story.  Back in the early 1980’s they had new H.E.’s doing that even in our 
mild winter after being shuffled around the factory car parks a few times before delivery.  That's why they got rid of the 
cold start injectors after a period of just having the connectors taped back for the dealers to hook up before delivery to 
the customer. 

“Whilst it may be the case that the H.E. V12 is more prone to cold plug fouling than some other engines, doing 
repeated cold starts in very cold conditions without getting to even part warm is maybe asking a lot of an elderly car 
probably with shaky HT leads and things.  If I had to move any car, not just a Jaguar V12, repeatedly in severe 
conditions I would at least let it run to near fully warm every other time then rev it up two or three times to make sure it 
was clear.” 

 

Digital P 

 

FUEL SUPPLY SCHEMATIC CORRECTION:  Fig. 3 on page 19-3 in the ©1982 Supplement and Fig.13.18 in the 
Haynes manual as well as the same diagram in the Handbook purport to illustrate the fuel supply system for the Digital 
P.  In fact, it’s quite accurate -- if you never had the fuel rail recall performed on your car.  Since all cars should have 
had the recall done, the left half of this illustration should look like Figure 15. 

 
 

288

 

Figure 15 - Digital P Fuel Supply Schematic 

There are now two pressure regulators (although the bottom one in the illustration is actually the only one that regulates 
pressure), and the cold start injectors are gone.  Note that for the 1992 model year, Jaguar went back to having a single 
pressure regulator. 

Fig. 4, 18, 20, and 21 and sections 19.60.01, 19.60.03, 19.60.04, and 19.60.05 in the ©1982 Supplement as well as Fig. 
13.13, 13.14, 13.22, 13.23, and much of the text in Chapter 13, Sec 5 in the Haynes manual likewise reflect the pre-
recall fuel supply system.

 

 

HOT STARTING:  The fuel cooler works great when the engine is running, but is worthless after the engine is shut off. 
 The heat rising from the engine heats the fuel in the rail, which is not moving.  If the engine is started about a half hour 
after shutoff, it may have difficulty starting. 

Jaguar has provided two different fixes for this problem.  Both involve a temperature sensor in the boss on the left side 
of the fuel rail; the boss has no opening into the fuel, but the sensor has a copper bottom that presses against the rail to 
sense the temperature.  The boss itself seems to exist on all XJ-S H.E.’s, since a recall replaced the rail after the hot fuel 
problems were found. 

The earlier type sensor is electrical, and is connected in line with the inlet air temperature sensor for the EFI system.  
The later type sensor has vacuum connections, and is connected between the intake manifold and the left side fuel 
pressure regulator.  At least one of these sensors has been known to simply fall apart, but owners have had some 
success simply gluing them back together. 

If your car doesn’t have either fix (or if it does but still has difficulty with hot starts), there is a work-around for the hot 
start problem.  Turn the ignition key on and off several times before attempting to start, allowing the fuel pump to run 
for two seconds each time.  This will flow cool fuel from the tank into the rail. 

 

CLOSED-LOOP OPERATION:  The earlier D-Jetronic EFI systems were strictly open-loop.  This means that the ECU 
contains a schedule of values (a “map”) for how much fuel the engine gets at a given operating condition, and that’s 
final.  The Digital P systems in markets that did not limit emissions likewise were open-loop only.  For North America 
and other markets where emissions were regulated, however, the Digital P system featured a closed-loop fuel control.  
There is still a map providing “baseline” fuel scheduling, but there are also oxygen sensors in the exhaust system that 
monitor the exhaust for excess oxygen.  If an oxygen sensor finds excess oxygen, the ECU presumes the engine is 
running too lean and trims the fuel scheduling a little richer.  If it doesn’t find excess oxygen, it presumes the engine is 
running too rich and trims the fuel scheduling a little leaner.  The oxygen sensors are essentially an on-off device -- they 
either detect oxygen or they don’t -- so the closed-loop control mode is neurotic, always either correcting one way or 
the other; it is never satisfied.  When operating normally, it is constantly cycling from a hair rich to a hair lean and back 
again.  As a result of this constant feedback correction, the engine is always kept very close to stochiometric fuelling in 
this mode. 

 
 

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Oxygen sensors are also known as Lambda sensors; Lambda (λ) is the Greek symbol generally used to denote relative 
air/fuel mixture, with λ=1 corresponding to stochiometric, λ>1 meaning lean, and λ<1 meaning rich. 

Of course, stochiometric is not necessarily ideal.  For max power, you’d like it a little rich.  At max power, it’d better be 
a little rich, because running near stochiometric is a good way to burn pistons.  So, there are two switches -- a roller 
microswitch on the turntable and a vacuum-operated switch nearby -- either of which will kick the EFI system out of 
closed-loop mode at high throttle.  This causes it to run on the baseline map with no trim, which is of course designed 
to be a little rich just for this purpose. 

A photo showing the microswitch can be seen at: 

 

http://www.jag-lovers.org/xj-s/book/Microswitches.html

 

Michael Neal sends this info on checking the Digital P EFI system:  “Monitoring O

2

 sensor feedback voltage just lets 

you know the EFI is in closed loop operation.  This should be checked with a high input impedance digital volt meter.  
A cheap meter will fry the system.  Check the voltage at the O

2

 sensor connector with the sensor hooked up and the 

engine running.  The voltage will fluctuate from 0 to 1 V.  With a fairly fast updating meter you should get a voltage 
swing of at least .3 V.  A .1 or .2 V swing shows a problem with the O

2

 sensor, open loop operation, or a problem with 

the sensor driver in the ECU. 

Robert Dingli says: “It is pretty obvious when the system goes into closed loop control.  While the open loop controller 
may be tuned to near lambda = 1 the voltage output will ‘flicker’ around 0.5V or else sit at a low or high output.  When 
under closed loop control, the perturbation due to the closed loop system can be clearly seen as a low frequency 
oscillation in the output.” 

Before checking that sensor output voltage, take note:  if your car has a red 2-conductor socket on the end of a wire 
near the ECU with a jumper in it, you will need to remove the jumper to get it to go into closed-loop at idle in neutral or 
park.  See below. 

Besides monitoring the oxygen sensor outputs, you could also check for closed loop operation by monitoring the 
feedback voltages within the ECU itself.  No, you don’t need to open it up -- Jaguar provided a 4-connector plug in the 
trunk near the ECU specifically to enable monitoring of the feedback signals.  See page 292. 

 

OPEN-LOOP OPERATION IN PARK OR NEUTRAL:  One pertinent fact regarding oxygen sensors: they have to be 
hot to work.  The Jaguar V12 at idle with no load and the timing properly advanced is turning over so easily that the 
exhaust may not be hot enough to keep the sensors working.  When the sensors quit working, the closed-loop control 
will richen the engine up, and the idle speed will start to drop off.  Sometimes the sensors will start working again, and 
the closed-loop control promptly leans it back out, and the idle comes back up.  You can end up with an unstable idle.  
Typically, if you put the car in gear, the load of the torque convertor is enough to keep the exhaust warm and the 
sensors working, so the idle is steady. 

To avoid the unstable idle problem, early emission-controlled Digital P cars had the starter inhibit switch on the shift 
lever also connected to the EFI system.  In Park or Neutral (starter permitted), the EFI operates in an “open-loop” 
control mode, where the oxygen sensors are ignored and the EFI controls to a fixed fueling map.  When in Drive or any 
other moving gear (starter inhibited), the EFI operates in “closed-loop” mode, adjusting the fuel mixture to obtain the 
correct oxygen sensor feedback. 

Owners subject to emissions testing need to be aware of this.  The regulations require the car to meet emissions in D, 
which it will if it’s running right -- but unless the idle mixture has been properly adjusted, it might not meet them in N 
or P because the EFI system is in open loop and is not correcting for rich or lean fuelling.  Of course, Jaguar’s stand is 
simple: bring the car in and let our authorized service technicians adjust the idle mixture.  Once properly adjusted, it 
won’t matter if the system is in closed loop or not, since the mixture is right and needs no correcting.  However, if they 
test for emissions in N or P in your area and you don’t feel like having the idle mixture adjusted right now, you can just 
pull a jumper from a red 2-connector socket near the ECU in the trunk to put the EFI into closed-loop mode in N and P. 
 If the emissions testing is done in D on a set of rollers or some such, you probably don’t need to worry about it, but it 
couldn’t hurt to pull the jumper anyway. 

 
 

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Starting with VIN 134286, the XJ-S was fitted with “3-wire” oxygen sensors.  These work the exact same way as the 
single wire sensors, except that they have a built-in electric heater.  The heater keeps the sensor working at all running 
conditions.  Eventually the circuit that puts the EFI into open-loop at idle in Park or Neutral was eliminated as well as 
the jumper in the trunk, but this change didn’t happen at VIN 134286. 

When the 3-wire sensors were first introduced, Jaguar wanted $150 each for replacements -- and even finding a generic 
replacement could cost considerably more than the older 1-wire sensors.  As a result, some owners installed 1-wire 
sensors in their cars, and either just put up with unstable idle problems or fiddled with timing or idle mixture to avoid 
them.  If you have a car that should have 3-wire sensors but you find 1-wire sensors in it, you probably should go ahead 
and install 3-wire sensors; they’re no longer expensive.  At least put the right sensors in when the 1-wire sensors die of 
old age.  After you get the correct sensors in, you might want to check the timing and adjust the idle mixture just in case 
they were fiddled with to deal with an unstable idle.  And if you have to adjust either of those, you’ll probably need to 
reset the idle speed afterward. 

 

OXYGEN SENSORS:  First, a clarification: these sensors don’t sense oxygen; they sense the absence of oxygen.  
Essentially, a Lambda sensor is a battery that generates a voltage based on the potential difference between the oxygen 
outside the exhaust pipe and the oxygen (or lack thereof) inside.  If there is oxygen on the inside, there is oxygen on 
both sides, so the potential difference is zero -- no voltage.  When there’s no oxygen on the inside, there is a difference 
between inside and outside and the sensor generates a little under 1 volt. 

What happens when your oxygen sensors go bad?  Well, obviously they will put out no volts regardless of oxygen in 
the exhaust pipe.  The EFI system will either decide, based on the lack of voltage, that the car is running lean and will 
trim it in the rich direction, or it will default to the baseline schedule with no trim.  Roger Bywater:  “Later fault logging 
systems would just ignore a faulty signal, flag a fault warning and either run at mid-range as mapped or run 'safe rich' 
depending on the strategy.”  Either way, it’s gonna run rich.  This is a relatively safe failure mode, as running rich can 
cause rough running or bad fuel economy but it usually won’t cause permanent damage to anything.  It sure will impact 
your fuel economy, though. 

Oxygen sensors are available at local parts stores.  There is no need to insist on a Jaguar part; a generic replacement 
sensor will work fine.  Brian Sherwood suggests Bosch universal sensor p/n 11025 for the earlier one-wire sensor.  
Take an old sensor in with you (if possible) to make sure the new ones are the right thread.  Don’t worry if the fluted 
sensor portion doesn’t look exactly the same.  If the wire is the wrong length or has the wrong connector on the end, 
use a crimp connector to connect the new sensor with the wire cut off the old sensor; the generic sensors usually come 
with suitable crimp connectors for this purpose. 

You really don’t want those generic one-wire sensors though.  What you really want to do is retrofit the later 3-wire 
heated sensors.  See below. 

A generic sensor may come out of the package with the wire wrapped tightly around the body of the sensor itself.  
Don’t unwrap it yet!  This wrapping makes it possible to use a normal deep socket to install this sensor, rather than the 
special sensor sockets sold with a slot down the side for the wire.  Of course, on the XJ-S it’s not too difficult to just use 
an open end wrench. 

The installation of an oxygen sensor is one of the most important places to use anti-seize compound -- see page 28.  
Fortunately, the sensor manufacturers know this, and even the cheapest generic sensor usually comes with anti-seize 
compound already on the threads and a little plastic cover over the threads to keep it there until ready for installation. 

 

OXYGEN SENSOR WARNING LIGHT & SERVICE INTERVAL COUNTER:  The package a generic oxygen 
sensor comes in may say something like “Remove negative battery cable for 15 seconds to clear codes” but this won’t 
turn off the dash warning light -- at least, not on the pre-90’s XJ-S.  The dash indicator on the early 80’s XJ-S isn’t 
really a “fault code” per se.  There’s no point to disconnecting the battery on these cars; it’ll just force you to reprogram 
your radio stations.