Jaguar XJ-S. Manual - part 172

 
 

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In either scheme, remember that the Jaguar V12 is 700 pounds and it’s made of aluminum which conducts heat very 
well.  You’re probably going to need the biggest wattage heater you can find.  Or, perhaps a better idea, two heaters; 
perhaps one in the lower radiator hose and one in the oil pan. 

There are some who claim that the dipstick heaters, especially high-wattage ones, can be harmful to the oil and result in 
accelerated breakdown. 

Those who live in cold climates already know one method that is not recommended: build a campfire on the ground, 
then roll the car over it.  Every winter some morons make the paper by burning their cars to the ground.  

 

RADIATOR COVERS:  Would you believe your car can overheat when it’s freezing out?  The answer is definitely 
yes.  The radiator is designed to efficiently transfer heat to the incoming air.  When very cold outside, the coolant in the 
radiator may be cooled to the point where it begins freezing, and a slush containing chunks of ice starts collecting at the 
bottom.  When the chunks get big enough, they can plug up the hose taking the coolant back to the engine.  Once the 
flow of coolant stops, the coolant in the radiator freezes even more solidly -- and the coolant in the engine overheats. 

If it’s really that cold, the best prevention is to block some of the air coming through the radiator.  This is easily done 
with a piece of cardboard.  You’ll be amazed at how much blockage is necessary to restore proper operation; it seems 
you have to cover three quarters of the radiator. 

If this proves to be a regular problem, there are actually products available that provide a radiator cover that is 
adjustable from within the passenger compartment by means of a cable. 

 

OIL:  Many winter drivers have found that use of synthetic oil helps with cold weather operation.  Synthetic oil 
apparently does not thicken as much as natural oil when cold. 

John Goodman:  “A tip passed on to me many years ago was to put a waterproof thermal jacket over the oil cooler in 
the winter months (bit hard to get at in a Jag !)” 

 

FROZEN DOOR LOCKS:  You’re freezing, and you can’t get into your car!  A revoltin’ development.  Jim Wood 
says, “There are little black boxes - somewhat near the size of your car door lock "beeper" - that has a little round rod 
that fits into your key opening and its sole purpose is to heat the key opening up & melt the ice - in just a few seconds - 
at least in New York's coldest weather so my sister and bro-in-law say - so that your key will turn and open the door 
lock.  They've been using these battery-operated things successfully for years.  They fit very well just in the palm of 
your hand.  They're not very expensive either!!!  I'd keep a spare battery in my pocket or purse tho just in case - not 
inside the glove box ...” 

 

 

LPG

 

 

People in Australia and some other areas like to convert their gasoline-powered automobiles to run on Liquified 
Petroleum Gas (propane) because it winds up costing about half as much in fuel costs to drive their cars.  A complete 
coverage of all the implications of such a modification would add considerably to the already hefty poundage of this 
book, but mentioning a few of the concerns is in order.  John Fergus says, “I would recommend you get a copy of 
Propane Fuel Conversions by Jay Storer (SA Design Books); this will tell you in detail all you need to know. 

Eric Vuurman describes an LPG system:  “The conversion leaves the original setup unchanged and the car can run on 
both LPG and petrol.  Changing from one to the other is a matter of turning a switch on the dash (while driving !).  The 
LPG-unit basically consist of: 

 
 

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- Pressure-tank  ( 50 - 120 liters) 

- evaporation unit  

- 2 shutter-relays to change from LPG <--> Petrol 

- nozzle between air-intake and carburettors. 

“LPG is loaded into the tank by an outside mounted filling cap, no mess, no smell or fumes in the car.  During 
operation, liquid LPG flows from the tank to the evaporation unit under natural pressure.  In this unit the LPG 
evaporates into gas.  The unit is heated by the cooling sytem of the engine (LPG needs heat to change from one phase 
to the other).  The gas flows from this unit to a special "nozzle" piece that is placed between the air filter and the carbs.  
The control of the gas flow is achieved by a combination of vacuum and throttle control.  In this way the engine is fed 
with a mixture of air/lpg and the carbs do not need to add anything to it.” 

Stig Abrahamson describes the installation on his Daimler Double Six:  “Gas is fed via a solenoid operated valve to the 
vaporizer which is located next to the brake servo.  Gas is then fed via adjustable orifices to mixers located between the 
air filter and throttle bodies.  The mixers are simple venturies and draw gas in proportion to the air flow through the 
throttle.  Switching between petrol and LPG is simply done by switching off the main ECU relay and switching on the 
solenoid LPG valve.” 

Peter Cave says, “The simple venturi mixers that I have seen have all been mounted on the engine side of the throttle 
body.” 

 

COST BENEFITS:  LPG costs less per gallon than gasoline, but that’s only part of the story.  There is less energy per 
litre in LPG, so your Km/litre numbers will reportedly drop by perhaps 10-15%. 

Of course, you will also need to estimate the costs in additional maintenance (or reductions in maintenance!), and 
balance all of that against the cost of system installation.  The cost of a professional conversion seems to be in the range 
of $2500US, but clearly you will need to check local sources for better estimates. 

 

TAXES:  A large part of the advantage of LPG is in the difference in taxation compared to gasoline.  Since LPG is used 
to run stoves, heaters, and the like, it generally is not subject to “road use” taxes.  As the LPG conversions get more 
popular, however, it can be expected that the governments will decide to close this particular loophole -- and eliminate 
much of the cost benefits you hoped for when you spent your money on the conversion!  Governments may decide to 
start taxing all LPG or to find a way to tax only that used in automobiles, but one way or another they will get their cut. 

 

SPACE:  An LPG conversion takes up space in the car, already at a premium in the XJ-S.  Eric Vuurman says, “The 
main disadvantage is the space taken by the tank.”  Peter Smith adds, “I imagine in the XJ-S the tank may restrict access 
to the petrol tank/spare wheel.” 

Even accepting the loss of trunk space, John Fergus says, “The main problem is finding room to place the converter.  
The converter may be known in the US as a vaporizer/regulator.  Basically the liquid propane is warmed by the 
circulation of engine coolant and this converts the propane to a gas vapor.  If it was allowed to vaporize without the 
heat it would freeze the lines.  The LPG systems are very simple but there is really a space problem in the engine bay of 
the V12 XJS.  One installer I spoke to recently said he puts it next to the brake reservoir on our right hand drive cars 
and the rest is fairly straightforward.  I have yet to see a car under the bonnet to see how tight it is but I can't see that 
there will be enough room there.” 

 

SAFETY:  Arguably a non-issue; after all, driving around with 20 gallons of liquid gasoline in a flimsy sheet metal tank 
isn’t exactly the safest thing you can be doing, so having a heavy-duty pressure vessel containing a flammable gas may 
be considered an improvement in safety.  Eric Vuurman says, “It is extremely safe.  There are very simple but strict 

 
 

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safety measures that apply to both the installation and the operation.  LPG tanks will survive the most severe crashes 
and to date no report is known of any tank exploding after a crash.” 

 

POLLUTION:  There are limited-production cars that are imported into the US in small numbers and meet the EPA 
requirements by converting to LPG!  Stig Abrahamson says, “My car is a Daimler Double Six 1980 Series 3, Pre H.E. 
Digital P injection and the 10:1 engine.  It started its life in Germany and was imported to Sweden in 1983.  During that 
time Sweden had very peculiar emissions regulations shared only by Switzerland.  A loophole made it possible to 
import cars and convert them to LPG.  This is what happened to mine.  The conversion was made with the sole purpose 
of getting it past the emissions test.  I can have the engine running in my garage (door open of course) without feeling 
any odours apart from burnt oil dripping from various leaks. 

“LPG in Sweden is now almost extinct.  I happen to live close to one of the five filling stations in the country.” 

Where this author used to work at Pratt & Whitney Aircraft, the fork lift trucks used indoors ran on LPG to avoid 
indoor air quality problems.  Honda has developed cars to run on LPG to meet the Very Low Emission requirements 
proposed for some areas. 

Note also that LPG doesn’t have any additives like gasoline does; it wouldn’t be a good idea to put additives in a fuel 
that might be used to grille hamburgers. 

 

POWER:  Along with the reduction in Km/litre, there is also an estimated reduction in power of about 10% when 
operating on LPG.  Many drivers claim they don’t even notice. 

Stig Abrahamson adds, “One advantage is that it has a sharper throttle response on LPG compared to petrol.” 

 

OPERABILITY:  Eric Vuurman says, “LPG gives instant smooth engine running. Especially with older engines/carbs 
in a wet/cold climate this is a huge advantage.  There is no need of mixture adjusting with (automatic) chokes or other 
gadgets when the engine is cold.  Just start the car and it runs great.” 

 

IDLE:  Stig Abrahamson explains:  “In order to feed the engine a combustible fuel air mixture all of the air has to pass 
the mixers between the air filter and throttle.  This makes it impossible to use the AAV since it bypasses the left hand 
throttle and thereby reduces the amount of fuel delivered to the engine as well as upsetting the balance between the 
banks. 

“Solution to this was to plug the AAV and set the idle using the stop screws on the throttles and balancing them with a 
standard carburetor balancer.  When this was working I had to find a way of increasing the idle when cold.  My 
solution was to carefully adjust the throttle linkage and add a solenoid to the central butterfly and energize this via a 
small thermostat opening at 60°C and preventing the throttles from closing against the stop screws when the engine is 
cold.  This gives me only one step in increasing the cold idle but it works reasonably well. 

“Another minor problem was the PCV valve which also bypasses the throttle.  I have strengthened the spring a bit to 
prevent it from opening at idle.” 

John Fergus counters, “I have often thought of blocking the AAV to see if this would give a stronger idle when cold but 
have not done so yet.  My '78 XJ-S has no mods to the AAV or the PCV and it idles quite well in all circumstances.  
However I do find it necessary to start the car on petrol for the first time each day as the LPG does not fire up a cold 
engine as instantaneously.  The one limitation is that it is not possible to get a high idle, e.g. more than 650 rpm, without 
opening the butterflies a little but I have resisted this as I still want to drive on petrol.  Overall I believe the AAV 
operation seems to help the cold idle by somehow dragging the extra LPG past the butterflies.  Perhaps the warmer 
weather in Australia doesn't place such demands on the cold start system.” 

 

 
 

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OCTANE:  LPG has gobs of octane, something like 110.  Simply not a problem any more.  However, it also has 
different flame propagation characteristics, which means that the ignition timing curves might need a little massaging to 
optimize performance.  Peter Smith says, “There is a compromise to make between the two fuels because LPG requires 
more initial spark advance and less further advance than petrol, so usually the timing is set a few degrees advanced.  
There are a couple of aftermarket black boxes which will try to reduce this compromise and Mallory used to make a 
good one.” 

 

ENGINE DURABILITY:  As mentioned above, LPG has no additives.  This concerns some people, and some even 
recommend that the car be run on gasoline on occasion to ensure that parts such as valve seats maintain a proper 
coating of whatever benefits the gasoline additives are supposed to convey.  However, note that such advice may apply 
better to engines with soft cast iron heads without valve seat inserts than to engines such as the Jaguar V12. 

Peter Smith says, “I have put more than half a million miles on LPG/petrol (dual fuel) cars and can tell you it's a better 
fuel for long life.  Being dry, it doesn't wash the lubrication off the upper cylinder walls like petrol does.  The plugs last 
longer although it needs a higher quality spark to ignite, so it won't like worn plug leads.” 

Eric Vuurman says, “I have a 1969 Volvo 164 on LPG.  It has been running for 345000 km without any trouble or 
major engine work.” 

 

OIL:  The oil manufacturers offer special motor oil for engines running on LPG.  The reasons are uncertain, but most 
people with LPG conversions just use the oil and don’t ask questions.  LPG is reportedly easier on oil; it comes out 
looking as good as it went in. 

 

IGNITION:  Stig Abrahamson says, “The LPG/air mixture acts as a better insulator than petrol/air which puts 
additional strain on the ignition system.  I had a problem with the old coil and replaced it temporarily with a generic 
one, worked fine on petrol but not on LPG.  The original works though...”  This being the situation, those with Marelli 
ignitions might want to take note and make sure to do the “silicone squirt” to avoid catastrophic failures described 
starting on page 160.  Of course, they should anyway! 

 

DIAGNOSTIC BENEFITS:  John Fergus says, “It is useful as a diagnostic tool when trying to isolate problems with 
the engine tuning as you can switch between petrol or LPG.” 

 

CLOSED-LOOP CONTROLS:  John Fergus (in Australia, where LPG conversions are big business) says, “There is a 
system available which consists of an 0

2

 sensor and a control module whereby the module will allow a direct feed of 

extra LPG to each mixer via a small additional hose when the 0

2

 sensor measures a lean mixture.  Another firm does a 

similar system but instead of running extra gas when lean it controls the vacuum being sensed at the converter and 
makes the converter supply extra gas.  All very simple and very effective and if the vehicle is a pre-H.E. without 
sensors then you end up with a computer-controlled fuel system better than the original Jag one.”