Jaguar XJ-S. Manual - part 77

 
 

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IDLE STUMBLE:  The mid-80’s H.E. tends to “stumble” at idle when warm, even when it runs perfectly smoothly 
when cold or at higher throttle.  This appears to be normal, although it is definitely disconcerting to an owner that spent 
the kind of money this car costs and expects better from his 12-cylinder engine.  Fuel injector cleaning and/or 
replacement, spark plug replacement, and oxygen sensor replacement have no effect. 

The only thing that might help is to readjust the throttle linkage as described on page 269, starting with the butterfly 
stop adjustments. 

 

SUPPLEMENTAL AIR VALVE / SOLENOID AIR SWITCH:  In addition to the coolant-operated Auxiliary Air 
Valve under the back end of the left intake manifold (page 263), the H.E. has an electrically-operated air valve at the 
front end of the right side air filter housing to help control idle speed.  The first challenge is to get the name right.  In 
most places in the Supplement to the ROM as well as in the Haynes manual and in the Electrical Guide, it’s called a 
“Supplemental Air Valve”, but in the emission control diagrams it’s called Item H: “Solenoid Air Switch”.  Most 
people seem to prefer Supplemental Air Valve, so that’s what it’ll be called in this book.  Please remember that it’s not 
the same thing as the Auxiliary Air Valve.  Obviously, these names are often shortened to SAV and AAV. 

To avoid future confusion over the name, it is recommended that you go through all the emission control diagrams in 
your manuals and scratch through the words “solenoid air switch” for item H and write in “Supplemental Air Valve”.  
This would include page 17-1 in the Supplement to the ROM and Fig. 13.31, 13.32, 13.33, and 13.34 in the Haynes.  
Also note that all these diagrams seem to indicate that the valve is physically nearer the rear end of the manifold, but it 
is in fact near the front. 

The Supplemental Air Valve allows air to go from the air filter housing directly into the right side intake manifold, 
bypassing the butterfly, and therefore boosts the idle; the balance pipe serves the left bank.  The original intention is to 
serve as part of a cold start scheme; a timer operates solenoids in vacuum lines that kill vacuum advance to the 
distributor so the idle speed drops, and this valve brings the idle speed back up. 

Operation of this valve is described in the discussion of the H.E. emission controls in the Supplement and the Haynes 
manual, where it is item H on the schematics.  On B Emission cars (UK and elsewhere) the timing retard system, 
including the supplemental air valve, is operated for 15 minutes if the coolant was below 45°C at startup.  This kills 
efficiency and causes the engine to generate a lot more waste heat, apparently to help the 700-pound engine warm up 
faster so the cabin heater works before you get to your destination.  On A and C Emission cars (North America) it’s 
operated for 45 seconds regardless of coolant temp.  This short period of inefficient operation helps the catalytic 
convertors light off.  The timer -- either 45-second or 15-minute -- is located above the passenger side footwell, just 
inboard of the fuse block. 

Another correction to note: according to the Digital P EFI schematics (Fig. 13.9 and 13.11 in the Haynes), the SAV and 
the “Vacuum changeover switch” are controlled by an oil temperature switch.  As of yet, nobody has reported finding 
an oil temperature switch in an XJ-S. 

On later cars, somebody at Jaguar got the wise idea to use this existing valve to help stabilize the idle when the A/C 
compressor kicks in.  If the cold start timer activated the SAV by sending 12V to it, it’d be simple enough to connect 
the wire from the A/C compressor clutch to it as well with a diode or two to prevent backfeed.  However, that’s not 
how the timer controls the SAV.  The SAV is provided 12V whenever the fuel pump is running, and the timer provides 
a ground connection to open it.  So, just connecting up the wire from the A/C compressor clutch won’t work; instead, 
the wire from the clutch goes to an “idle relay” and the relay in turn provides a ground circuit for the SAV. 

But Jaguar went yet one more step.  If the car is in Neutral or Park, opening this valve when the A/C compressor kicks 
in boosts the idle too much.  So they provided an “idle override relay” to prevent the SAV from boosting the idle when 
the starter safety switch indicates the shifter is in N or P. 

Victor Naumann adds:  “Idle relay (black) and idle override relay (blue) are located on the left side of the radiator 
support in front of the A/C suction hose.” 

The system that controls the supplemental air valve is covered on page 134. 

 
 

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If your supplemental air valve is NFG, it’s not really designed to be repairable.  If you want to give it a try, the way to 
get it apart is to grab the two hose nipples and pull them directly apart.  It’s gonna take quite a pull, it’ll probably 
require a puller of some sort plus a good way to grip those nipples without ripping them to shreds. 

Michael Minglin says, “Best price I could find on a supplemental air valve was $139.  I decided that was too much, 
considering it was a simple 12V normally closed valve with 3/8" hose nipples.  Checked with a local industrial valve 
supplier and found a 12V normally closed valve with 3/8" hose nipples for $26.70.  These valves come with 
interchangeable coils; 12V, 24V, etc.” 

The Grainger catalog doesn’t have anything that perfect, largely because most of their stuff -- even with 
interchangeable coils -- is either 24V, 120V, or 240V.  You can get there, though: purchase their stock no. 2G485, 
about $32; the catalog says it’s only rated for ambient temperatures up to 122°F, but it’ll hold up in the Jaguar engine 
compartment as well as most of the other stuff in there.  Plug the “exhaust” opening on the valve.  Then drop by your 
local auto parts store and buy two brass fittings with a male 1/8” NPT on one end and a 3/8” hose nipple on the other.  
These are Brass-Tite number 43275.  Screw these fittings into the valve, install it where the original was, and connect 
the wires.  It doesn’t look anything like the OEM valve and it may not provide exactly the same amount of idle boost, 
but it’ll work. 

For a slightly neater appearance, replace one of those straight brass fittings with a 90° fitting.  This will make the hose 
to the bottom of the intake manifold shorter and more direct. 

For a much neater appearance, purchase Grainger stock no. 2G491 instead.  Buy only one brass fitting -- the 90° one is 
best -- and a couple of suitable bolts and nuts.  Yank the funny rubber bellows-looking grommet out of the air filter 
housing and drill suitable holes and bolt this valve directly to the housing.  Locate it carefully so it doesn’t interfere 
with all the other stuff in the area.  It might be nice to cut out a suitable gasket, but an even better idea is to install an O-
ring between the valve and the air filter housing.  Make very sure the nuts can’t come off the bolts inside the air filter 
housing. 

If you decide that your substitute valve -- or the OEM valve, for that matter -- provides more of an idle boost than you 
need, it should be a simple matter to restrict the flow through this passage a bit.  If you’re inventive, you can devise an 
arrangement with a screw obstructing the passage that allows you to adjust the amount of idle boost. 

This author found a really neat replacement: a valve from a Toyota.  This valve, found in a junkyard and bearing a 
Nippondenso number 084600-5510, is intended to serve as an idle boost solenoid.  But not only is it much better made 
than the OEM Jaguar part, it also has a thumbscrew for adjusting the boost.  You can see this installation at 

 

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

 

 

 

INTAKE MANIFOLD SHORTCOMINGS:  Bill White, an expert on the Helmholtz theory of intake manifold design, 
notes that the intake runners to the corner cylinders on the Jaguar V12 are longer than those serving the center eight 
cylinders.  He performed single-cylinder rig tests on the different runners and found that, at the tuned frequency of the 
manifold, the corner cylinders produced 8% more torque than the other cylinders.  This means that these corner 
cylinders are getting a larger charge of air than the others.  One would expect that at other frequencies the corner 
cylinders would get less air than the others. 

If this were a carburetted or throttle-body EFI engine, this wouldn’t really make much difference.  However, being a 
multi-port EFI, each cylinder gets the same amount of fuel.  Since these corner cylinders are getting a different amount 
of air, they are running leaner or richer than the other eight. 

If your system has oxygen sensors, they will sense oxygen from the leanest cylinders and control the fuel supply to 
maintain stochiometric there; the other cylinders will simply run a little richer than stochiometric, which shouldn’t hurt 
anything except a little fuel economy and emissions. 

Roger Bywater of AJ6 Engineering (and formerly an engineer with Jaguar) notes:  “In this connection I would point out 
that sample cylinder heads were regularly checked in the flow room at Jaguar and it was not unusual to find a variation 

 
 

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of 10% between the flow capacity of individual ports due to core shift on any one head...  Also in about 1975 I 
remember carrying out some tests with calibrated injectors which showed a wide spread of exhaust CO emissions 
largely due to flow variations from cylinder to cylinder following no particular pattern.  This was part of an 
investigation into why B cylinder bank was always a worse HC emitter than A bank.  In fact it had nothing to do with 
airflow or mixture strength but was a result of the direction of piston rock at TDC relative to the spark plug position.  
These sort of things are not at all unusual on any production engine of the period.” 

What can you do?  Actually, there is a fix for the intake manifold shortcomings -- offered by Roger Bywater of AJ6 
Engineering!  It’s called the Plus Torque Conversion, and is described on page 112. 

 

Cold Start Injectors 

 

COLD START INJECTORS:  The schematics in the manuals show that the cold start injector system was carried over 
to cars with the Digital P EFI.  However, the only Digital P cars with cold start injectors today are apparently the 10:1 
compression ratio pre-H.E. models.  H.E. cars don’t ever seem to have cold start injectors; some early cars came with 
them, but they were all removed when the fuel rail was replaced on a recall.  The square cross section fuel rails now 
used on H.E.’s has no provisions for cold start injectors; there is a different square cross section rail with connections 
for cold start injectors that was retrofitted into the 10:1 cars.  Some early H.E.’s may still have a harness and a relay in 
there doing nothing.  Early H.E.’s have little covers over the holes in the intake manifolds where the injectors once 
were, while later cars don’t even have the holes. 

If you have an H.E. with no cold start injectors, you can scratch through the cold start injection system consisting of the 
thermotime switch 298, the cold start relay 299, and the cold start injectors 300 on the following schematics:  In the 
©1982 Supplement or Supplement A, the diagrams on pages 19-1 and 19-2.  In the Haynes manual 478/49015, Fig. 
13.9, 13.10, 13.11, 13.12 and 13.93. 

 

 

Early 3.6 AJ6 Engines

 

 

Roger Bywater of AJ6 Engineering (page 710) reports that the 1983-87 3.6 AJ6 engine with Lucas 8CU EFI has a 
problem with worn engines.  A worn engine draws less vacuum at idle, but since the 8CU bases fuelling on manifold 
pressure, it believes that the lower manifold vacuum means the engine is drawing more air and responds with more fuel 
-- making the engine run overrich.  Since these cars lack Lambda sensors for trim it cannot correct itself, and the car 
runs worse and worse.  Bywater says owners complain of “shunting” in trailing throttle, bad idle, and failing MOT tests 
due to excessive CO in the exhaust, even though the engine is not really all that tired and rebuilding doesn’t appear 
justified.  Of course, at full throttle the wear problems are insignificant and the engine runs fine. 

Bywater suggests that disconnecting the vacuum advance from the throttle edge tapping and connecting it directly to 
intake manifold vacuum -- and readjusting the idle mixture and speed accordingly -- may help as a temporary measure 
if the engine isn’t too bad.  For a more permanent fix, AJ6 Engineering offers an ECU modification wherein a trimmer 
screw is added that can be used to correct for engine wear -- and can be corrected back in the event of an engine 
rebuild. 

 

 

 
 

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XJR-S 

 

John Goodman reports that the XJR-S has Zytek ignition and sequential fuel injection -- see pages 171 and 719.  “It 
is essentially the same injection system off the '80's group "C" Le Mans race cars.  Jaguar made a big thing about 
publicising it.  But I suspect TWR had a load of spare bits left over and needed something to do with them!” 

“Also there are two power resistors on the R/H front inner wing. 

“I thought Jaguar's claims for improved fuel economy was bullshine, but it is more economical than my previous 
H.E.” 

Emile DesRoches says, “The XJR-S Product Support Manual lists special software along with a serial port connector 
whereby a laptop computer can be connected to the Zytek engine management computer (unique) fitted to the XJR-S to 
perform various diagnostic, calibration functions.  Apparently this software was originally provided to dealers upon 
request from Jag North America for repairing these beasts.” 

Goodman:  “The ECU can be reprogrammed with different fuelling and timing if only we had the software!  And it is 
not Jag Dealer software, they only have the standard tune parameters!” 

 

 

Fault Codes

 

 

The later cars come with JDS (Jaguar Diagnostic System) or OBD II (On-Board Diagnostics version II) as legislated by 
the California Air Resources Board (CARB).   When those fault codes appear they can cause consternation -- that’s 
what they’re for.  The owner’s handbook provides a list of codes, but of course it never seems to provide enough real 
guidance.  Hopefully some of the reports included here will help. 

Defective relays and/or their connections are a known cause of fault codes, as mentioned in the section on relays 
beginning on page 560.  Barry Ewen says, “When I bought my car 2 yrs ago it had fault code FF67 on it and we 
swapped the air injection relay with the horn relay; the fault went away and the horn worked too.” 

Charles Randle says, “I was out to my dealers today and read up on the EPA ordered recalls for all 6.0 V-12 cars. With 
a ’94 XJS with JDS, all I get is a replacement of all major grounding connections.  But all 95-96 OBD II cars get a free 
Oil and Filter change; and replacement of all EPA components that have set an OBD II code.  Also the EPA warranty 
has been extended to 150K miles.  So if anyone out there hasn't registered their after market car with Jaguar as the 
present owner; don't pass this up. 

 

FF 3 -- COOLANT TEMP SENSOR:  Mark Barker reports on his 6-cyl:  “Trip computer displays "FF 3" (Coolant 
temp sensor failure), when the electric fan motor is dying.  Just happened this evening.  I suppose coolant temp is 
related to cooling fan. 

“Certainly was the motor -- brushes gone.  I mean gone, too.  Big pile of black dust fell out of dissasembled motor 
followed by broken bits and pieces sticking to magnet.  Found some brushes in scrap motor I had, oiled the bearings.  
Back together, no more "FF 3".  Strange, but true.” 

 

FF44 AND FF45 -- OXYGEN SENSORS:  Sam Emrick says, “Late-models (at least) seem to have a common problem 
with bogus O

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 sensor failure code reported (FF44 or FF45).  My '94 had this problem.” 

Charles Randle talks about a recall:  “It plainly says for my JDS codes that FF44 and FF45 codes are being caused by 
poor grounding connections rather than bad O

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 sensors.  I wish I had known that before changing them out a few