De-mystifying the Jaguar V12

The Jaguar V12 engine is one of the greatest iconic powerplants ever made, powering the E Type, XJ12 and XJ-S models plus numerous successful race cars from Touring Cars to LeMans GTs. But for some the apparent complexity makes it a very scary option. Having raced these magnificent units I feel it’s time to set the record straight.

Looks simple enough....

The first time I opened the bonnet on my XJ-S V12 I just stared at it in disbelief for a few minuets, then gently shut the bonnet again and walked away. To say it was full would be an understatement, it looked like someone had emptied a very large bucket of automotive spaghetti into the engine bay until it was full, then smoothed it off with a very big oily trowel.
Which is a very great shame, because the engine itself is a fairly straight forward design, just with 12 of everything. Owning and driving one is a fantastic pleasure, round town the engine is silent from within the cabin and it is arguably more refined than a Rolls. On the open road it has a briskness that defies its size, and given enough space has a relentless thrust that just keeps on going.
So if you have a desire to own one then don’t let the tales of woe put you off. It is only that the parts bolted on to the engine, such as the fuel system, air control system and electrics that seem shockingly complex, and that is only until you get the hang of what does what.
The good news is that the engine itself is very reliable and can cope with a huge amount of abuse, the bad news is that because of this many older cars have many missed services, giving the new owner quite a lot to do. The base engine has gone through three main changes over the years; the original E Type and XJ engine had flat cylinder heads and is the one preferred by the racing fraternity, but it had an alarming thirst for fuel. The fuel crisis of the 70’s led to the High Efficiency HE engine, introduced in the early 80’s, with a very high compression ratio and very special combustion chambers with a heavily recessed intake valve for very high swirl rates, this limits tuning potential slightly but gives amazingly reasonable fuel consumption, I used to achieve about 25mpg on a run from my XJ-S HE. The HE stayed in production into the 90’s when it was replaced by the 6 litre bock which was largely the same but with a longer stroke.
All these engines are very robust, as they are derived from the XJ13 race engine the 5.3 revs to 6500rpm and the 6 litre goes to 6000rpm. Because they are designed to be quite happy running at these speeds, in an every day road car they are totally unstressed. Of course this pedigree means it is still great for classic racing today, many racers take engines straight out of scrap cars and merely change the oil before heading for the track.
Speaking of oil, it does take quite a lot of the slippery stuff, over 10 litres is usual making an oil change a substantial investment. The oil tends to last quite well however, an oil cooler is standard and the engine’s quick warm up system minimises oil contamination. When changing the oil it is best done with warm oil, as with most engines, but leave the exhaust for a while to cool down because the oil filter is right next to the down pipes and contact is inevitable.
The simplest incarnation was in the carburettor fed E Type and XJ12 models, but the engine was designed for fuel injection from the start, and as soon as a suitable system was available it was fitted to the XJ12, the XJ-S was injected from the start. The systems was very sophisticated with little extra features to enhance performance and emissions, all stacked on top of the engine, making it look so complicated. Breaking it down a bit, there are six main systems in that spaghetti; fuel, vacuum, engine management, ignition, cooling system and air con.
I will take a typical S3 XJ12 as an example, then go over differences on other models later. Opening the bonnet reveals a network of small bore tubing, some of this is the fuel system with the U shaped fuel rail feeding all 12 injectors, but most of the small tubing is the complex vacuum operated control system for the distributor. The front of the engine is dominated by the air con pump with its cylindrical silencer, yes that’s right the air con pump has a silencer, very refined.
You will also notice it has two cooling system pressure caps, one at the front of the engine and one on the expansion bottle on the wing. Nearer the rear of the engine is the throttle pedastle, this is where the throttle cable operates a circular device that in turn operates the two throttle links, more of that later.
With a gearbox on its over 7 foot long and weighs half a ton
Lets start with a potted history of the ignition system, it has gone through several major changes over the years, most models of the 70’s and 80’s use the 12 cylinder Lucas Opus distributor, similar to many other BL cars of the time. The HE engine’s high compression ratio required much more spark energy at high speed than the older engines, and feeding a spark to 12 cylinders at engine speeds up to 6500rpm require the coil to store a lot of energy, it was too big a job for just one coil back in those days so the cunning chaps at Jaguar doubled up the coils, one coil has the king lead to the dizzy and the other sharing only the low tension 12v side of things. This secondary coil looks a bit odd, as the king lead post is blocked off, the main coil is located next to the dizzy but the secondary coil is mounted right at the front of the car, in front of the radiator, if its faulty or missing then the engine will misfire at high rpm but may well run perfectly round town.
Later models managed to use just one, more modern, coil. But when computerized mapped ignition was installed the need to have the coil towers further apart required a novel system from Mirelli which effectively incorporated two 6 cylinder distributors in one, stacked one on top of the other in the same housing. This system has two coils next to the distributor and two king leads feeding a split level rotor arm.
The final iteration of the V12 ignition saga happened after the change to 6 litres when the distributor was deleted in the and replaced with individual coils for each cylinder.
Of these systems the most common is the Opus, and it is also the one with the most complicated vacuum advance system ever made. Its also very clever. To get the best from an engine the ignition advance must adapt to all engine operating conditions, a simple vacuum advance system is a big compromise but back in the 70’s full computer controlled mapped ignition was not an option for mass production. So the vacuum capsule is connected to several air solenoids, a vacuum regulator, a vacuum dump valve. The biggest problem with this comes from old rubber hoses perishing, the second biggest problem comes from people re-assembling the system incorrectly. One of the clever things the system does is warm the engine up quickly, and its huge amount of coolant. The ignition is retarded when cold and extra air is let in via an air solenoid, this is controlled by its own little electronic box of tricks which has been known to go wrong resulting in lost power and idle problems, but when it works it works very well. To compensate for lost power when running so retarded extra air is let into the manifolds by yet another solenoid. It is possible to remove this system completely, the only down side being longer warm up times.
One of the popular horror stories about this engine is that the spark plugs are impossible to change without a major engine strip down. Well, in fact most of them are no problem at all, but the front two are very close to the air con pump, a special tool is recommended but another option is to undo the pump belt and mountings and ease it over, without damaging the hoses, to get enough access.
Because of this difficulty it is quite common for the plugs to be left in far too long and start to corrode in place, so its worth blowing all the dirt out of the plug recess and allowing some penetrating oil to soak in before attempting removal. In fact it is quite important to blow any detritus out of the way first, the plugs are ate the top of the engine and it is easy for debris to accidentally fall down the hole during the change. Make sure you get the right plugs too, there was a change of design to taper seats when the HE engine came in.
The electrical system design also laughs in the face of convention. The fuel injection control really was cutting edge and in some ways daring. The early, pre-HE, ECU was relatively simple but very good at its job, the HE ECU is more sophisticated and has a manifold pressure sensor built in to the box. Unusually the ECU is mounted in the boot and the wiring for the injectors and sensors passes through the rear bulkhead and takes a tortuous route through the interior and finally emerges through the front bulkhead from whence it sprawls over the engine and wing edges to meet the control relays mounted on top of the radiator. The HE system also has a vacuum hose from the intake manifold going all the way back to the ECU. Obviously there is the potential for older cars wiring or vacuum hose to have expired in some way. Electrical connections are prone to corrosion which can usually be removed reasonably easily, but sometimes it works out easier to fit new connectors. The vacuum hose is worth replacing if its older than a decade.
The throttles, one on each intake manifold for injected engines and four on carb versions, are operated via a centrally mounted disc which is pulled round against a spring by the throttle cable, this disc also operates the load cable for the auto box and the cruse control, it also has a switch on some models for the auto box kick down and the throttle potentiometer for the ECU. So all in all its a complicated part, but all you usually need to do to it is check that the throttles all start to open at the same point, if they are a little out of balance the engine might only be using half its cylinders as you cruse round town, luckily its a simple operation involving undoing the lock nuts and winding the links in or out as described in the service manual.
The fuel system is quite remarkable too; on the XJ-S there is a fuel pump in the boot sitting in its own little tank, fed by gravity from the main tank. On XJ12 there are two main fuel tanks which both feed the fuel pump in the boot but with fuel solenoids to select which tank is used. The excess fuel returns from the engine to the tank, but because the fuel picks up so much heat in the massive engine bay it goes through a small cylindrical fuel cooler that is plumbed into the air con return pipe.
Early systems suffered from hot re-start problems due to having very small bore fuel rails which are prone to fuel vaporisation in the heat, the later larger bore system is a useful upgrade. When the returning fuel gets to the tank it goes into a pipe in the main tank which in turn goes back into the pump tank, that way returning fuel is mainly pumped straight back to the engine and the main fuel tank temperature doesn’t rise too much. This is all done to stop fuel evaporating, which became part of the emissions regulations in the 70’s, instead of the tank just venting to the atmosphere it vents into a can, tucked up into the right hand buttress on XJ-Ss and one on each rear pillar on XJ12s, which allows the vapour to condense and return back to the tank via yet another small bore pipe, determined gases are allowed to escape via a restricted pipe on the top of the canister which goes to the engine intake.
Needless to say its a very complicated system and is prone to problems, one of which is a strong smell of fuel in the cabin. Some owners rip the whole vent system out and replace it with conventional vent pipes.
Later models supplement this vent system with a carbon canister in the engine bay which absorbs fuel vapour and is emptied into the engine intake during part throttle running, ensuring the vapour is burnt and never vented to atmosphere. This is controlled by the engine ECU via a solenoid in the pipe from the canister to the intake.
Although all this pipework looks intimidating initially, all you really need to do is keep the hoses in good order, and if you are so inclined the system can be simplified and most of the plumbing removed.
The cooling system is another stunning work of art. Each cylinder head has an externally mounted coolant manifold with a thermostat and top hose at the end, in the E Type both top hoses go forward to the radiator, which was the traditional sort with a tank at the top and the bottom and the cores running vertically so the two top hoses went straight into the top tank, relatively simple compared to what happened next.
On XJ models it got a lot more complicated, a more modern radiator with horizontal flow and a separate expansion tank is used for greater efficiency and lower bonnet lines, and with a tank ate each side it made connecting the two top hoses a bit more tricky. The solution was to put a baffle plate one third of the way down the left hand tank, effectively splitting the radiator in two. The left top hose goes to the top of the left tank and the coolant then flows across the top third of the radiator into the right hand tank where it is joined by the right hand top hose. Then the mixed coolant from both heads goes back to the lower left hand tank via the lower two thirds of the radiator. Still keeping up? The bottom hose is attached slightly above the bottom of the lower left hand tank, and then feeds back into the water pump.
To make this set up work a small bore hose connects the top of the radiator to the expansion tank, mounted at the side of the engine bay. This tank feeds back to the radiator via a bigger hose at the bottom. It also supports one of the hoses for the interior heater, the other coming from the back of the right hand cylinder head coolant manifold. At the top of the tank is a pressure cap, but because this is not the lowest point in the system it should not be used to check the coolant level.
Amazingly it gets more complicated, between the two thermostat housings is a balancing tube which supports a second pressure cap, it is here that you should check the coolant level.
If you make the mistake of opening both pressure caps then coolant will drain from the top of the engine into the expansion tank and spill out. The highest point in the system is the interior heater matrix, so when refilling the coolant after a change it is best to have the front of the car raised a bit. You wont be able to get all the coolant in in one go, the trick is to run the engine with the heating on full and give it a few revs briefly then switch off and magically you can get the rest in.
Bizarrely the standard service schedule specifies dropping two pots of Barrs leaks with every coolant change, probably due the the huge number of joints in the system. This can lead to clogging in some of the coolant galleries and the interior heater matrix, so a good flushing can be helpful, and as ever old rubber hoses should be renewed every ten years or so.
Although this set up is genuinely complicated, all that is needed is normal maintenance such as coolant changes and checking for leaks, so don’t let it scare you off.
The front of the engine sports four belts on most models, and again although it looks complicated it really isn’t. at the back is the alternator belt, tensioned by moving the alternator, then there air con drive belt which is tensioned by a high mounted idler pulley, next we find the belt that drives the water pump and power steering pump tensioned by moving the PAS pump, lastly there is a fan belt with its own little tensioner pulley.

So there you have it, a glorious engine hidden under a complicated dressing. Although there are many neglected examples around, with some careful attention going through each system a stage at a time will ensure many happy years of sheer motoring joy.

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