Please Note: The advice contained in these pages was given in response to individual letters sent to David Marks in his capacity as Technical Advisor to the Jaguar Enthusiasts’ Club.  As these are edited excerpts from written replies to such letters and their individual circumstances, such situations may not directly apply to your vehicle.

David Marks Garages can therefore accept no liability in the event of any unsatisfactory results through following this advice.

We recently bought a 1986 XJS V12. When starting from cold the engine revs up and down between 400 and 1.600rpm. This continues until the engine has warmed up. Once hot, it starts every time without problem.

I have flushed out the cooling system. fitted new thermostats and changed all twelve plugs. The engine normally ticks over at 900rpm even with the idle speed adjusting screw fully in on the auxiliary air valve.

Incidentally the electric cooling fan comes on at the radiator when the dash fan switch is in any position but off, no matter what setting is used on the temperature control.

Dave’s Response:- You are describing the classic symptoms of a sticking idle speed control valve, called the auxiliary air valve. This is located at the rear of the engine on the left-hand bank.

You may be lucky if you remove the valve and clean it out with carburettor cleaner, and then alternate by immersing it in hot and cold water, you could shock’ it back into working order.

If this fails, purchasing a new one is the only option. They retail at about £125.00 from Jaguar.

The reason that your engine electric cooling fan comes on. is that it is designed to operate when the a/c compressor is engaged. and this can occur when either hot or cold air is demanded as part of the dehumidification process. I advise you to check if the fan is running only with the compressor and if the compressor is running all the time. You may have an a/c control fault, but trying to diagnose that by post will prove almost impossible.

Having read the excellent article on leaded/unleaded fuels I am still a little unclear about my own 1986 V12. I have received conflicting reports on the issue from my local Jaguar main agent who claims I should have the ignition regarded. A recent letter in Autocar from Roger Bywater of AJ6 Engineering seems to agree with this.

I have also noted a "rattling" noise from the engine under light throttle when pulling up a slight hill. This does not occur under heavy acceleration. Again the Jaguar dealer suggested that this may be pinking.

Dave’s Response:- In answer to your query on unleaded, I feel I have stated my views in my article, and need not expand further — except to say that I run all my cars, ranging from a 1952 MkVII, via two pre HE V12 XJSs, a Sill V12 saloon and an XJ4O 3.6L to a S1 V12 saloon, on unleaded petrol and all at their original ignition settings. I experience no problems at all, but do maintain all the cars very carefully. The transient pinking could be one or all of many faults. These range from a weak vacuum or mechanical advance system, to reduced flow through the fuel injectors. If the latter is the case then ultrasonic cleaning of the injectors is the only solution.

Series 2 Cooling under Load

My car is a 1973 XJ6 Series 2 4.2-litre saloon with which I tow a luxury caravan but I find when climbing hills the car overheats.

To avoid the expense and complications of additional radiators for the auto gearbox fluid and coolant, is it

possible for the Torquatrol unit to be fixed in any way to avoid slip or even replaced by a solid unit? If so what make could I use or what other alternative do you suggest?

Dave’s Response:- There is no short-cut or easy solution to this problem. First, you have to determine why the car is over-

heating. This could be something simplistic like the thermostat or a blocked radiator or the block silted up. All these areas must be checked.

It is not practical to fix the torquatrol fan or replace it with a solid one. This is not the answer I am afraid.

My XJ12 is a 1986 mode and I recently fitted a new wiper assembly from a later car with heated screen washer nozzles. Now while the wipers work on slow or fast speed, they do not work on intermittent or "flick" wipe.

Dave’s Response:- The wiper assemblies on these XJSs are notoriously prone to problems, and sorting out the correct motor for the year of the car is not always easy.

Unfortunately, I cannot give you the part numbers of the relevant parts, as these largely depend upon VIN codes.

Also, a lot of these parts are now no longer available from Jaguar.

My best advice would be to find a reconditioned item that matches your year of car, and to fit this.

I own a 1986 3.6-litre Sovereign saloon and it suffers from fuel starvation on hills and after running at above 55mph. However, it does this erratically and sometimes there is no provocation to the problem, the car just loses power.

The mystery is compounded by the apparent cure, switching fuel tanks. This immediately restores the fuel supply and I can then return to the original tank immediately. The situation applies to whichever tank I am using at the time.

In my own attempts to rectify the fault I have ensured that both submerged fuel pumps run strongly when selected, replaced the in-line fuel filter, located the tank breathing system and ensured a flow of air through the pipes and blown through the petrol feed pipe to the carburettor float chambers in an attempt to dislodge any foreign matter.

Dave’s Response:- I think the problem will lie with in-tank filters. Over a period of running, or under high load conditions as you describe, fuel drawn through these filters will tend to gather debris around the gauze. Once no more fuel is drawn through, if when you change tanks, the motion of the car will tend to "wash" the filter partially clear and so it will function for a short while again at a later time.

The filters are accessed by removing the screw-in bung at the base of the tank and pulling the filter off the pick-up pipe.

Beware that if the tank is slightly corroded, the action of unscrewing the bung may tear the bottom of the tank.

Make sure the tanks are fairly empty, and once clear drop 1/2 a gallon or so of fuel into the tank with the bung still out in order to clear any more debris.

Make sure you have a suitable container to catch any fuel, and that the tanks are fairly empty before you start, and definitely no smoking!

On a serious note, do not use an inspection pit for this, as fuel vapour settles and will make a very big bang if it ignites.

Smell of Petrol

Mike Daffin has an annoying problem with his 1985 XJ6 Series 3 saloon.A smell of petrol when the vehicle is moving with either the sunroof or any windows open and it makes no difference if the vehicle is running hot or cold. He has looked for the obvious leaking pipe hose joints and can’t find anything.

Special Car

Garry Adams from Bath has purchased a Vanden Plas Series 3 saloon and wants to know if there were many of these special cars. His is a Double Six version in Mistletoe with dark green interior. Also the wheels are chromed pressed - should this be or should they be the Kent alloys? Another “puzzle”, as he puts it, is that the front and rear screens are not bonded but have normal screen rubbers although the car has a vinyl roof.

Dave’s Response:- There were only 401 Series 3 Vanden Plas saloons produced with the Double Six engine. On the matter of windscreen rubbers, although the screens were bonded on these models they still used a rubber seal. The earlier Series 3 VDP models did not feature the Kent alloys which were an extra cost option, instead the old style steel wheels were used during this initial period.

Compatibility problems

Alan Mc.Harrie is currently renovating a Series 1 XJ and wants to know about the compatibility of the Series 3 engine and gearbox to a Series 1 car. He is already aware of problems with the propshaft and wiring to the fuel injection, but is there anything else or is it too much of a change from originality.

He also owns an XJS which has water coming out of the vents on the gearbox tunnel and on the passenger side vent next to the door. He has already cleaned out the drain tubes in the heater box. He also has another problem in that the ABS warning light is permanently on.

Dave’s Response:- The Series 3 engine, gearbox and rear axle will drop straight into a Series 1 bodyshell. It is also best to use the Series 3 propshaft and have it shortened for what is obviously a short wheelbase bodyshell in your car. Fitting the fuel injection is also not difficult as it is a completely separate, stand alone section of the engine. It plugs into the positive supply of the battery, although you will have to fit return fuel lines to the petrol tanks. Also you will find the fuel pump in the boot will not be fitted with its return valves, etc.

The whole job is not that difficult but it can be complex and needs to be done fully and well. So, if you have never done anything like this before you will need some constant advice and help on site.

With the XJS problem it seems impossible that enough water could be leaking to force its way out through the passenger side vent. I therefore think the problem may be from a different area than the air conditioning. Does the windscreen leak? Perhaps there is a major heater matrix leak. Is the water coming out fresh or is it anti-freeze based?

It is difficult to say more without actually seeing the car, but it is obvious that this needs urgent investigation because water and electrics don’t mix and the amounts in question would seem to be excessive.

On the ABS issue with the XJS, it is not uncommon on cars of this age to have a fault on the nearside front wheel sensor.

Fuel Non Return Valve

Effective to all Series 3 fuel injected XJs, both six and twelve cylinder cars. However, the earlier V12s with fuel non-return valves in the boot do not suffer from this problem.

High mileage and exposure to the elements meant corrosion had set in on this Series 3 XJ12. The valve had started to weep causing the unit to be removed from the car. The valve had become so brittle it had broken as it was removed. How can you avoid or check this?

Dave’s Response: You should check this aspect. particularly if you expect to take your car on a long journey where it may not be possible to gain access to parts easily. It is therefore worth removing the

rear road wheels and shields in the wheelarch to check and checking the condition. They are easily available from Jaguar at about £25.

Be careful when fitting not to strain the return fuel line that runs at the back and slightly above the rear axle. This fuel line can become weak although is not normally prone to failure. If damaged it can easily break which would entail dropping the complete rear axle assembly about eight inches in order to remove and replace.

AED Failure

Experienced a problem with the AED unit on a Jaguar XJ6 3.4-litre 1977 model. A local mechanic tried to adjust the screws on the AED, regulating fuel and air supply and ever since the car has given problems in starting. It takes up to ten attempts on the starter to turn the engine over and then only at very low revs (about 300rpm), and then only can the car be driven after a couple of minutes warming up the engine.

Dave’s Response: The AED units are effectively non-adjustable in service. All the adjustments necessary to compensate for the decay take place on the carburettors. I would therefore recommend that you consider the purchase of another AED unit.

The AED unit draws its air in from a heated intake tucked behind the rear exhaust manifold. It is possible on the older cars that the coarse filter within this is blocked and so any lack of air being drawn through that filter can cause the problems you indicate. It may therefore be worth first taking off that manifold and filter to ensure it is clear before buying another AED.

Series 3 Auto Box

Having fitted a secondhand automatic transmission to a 4.2, this owner has been experiencing problems which he thinks is caused by the fact that he did not have the

proper equipment to set the pump pressures. At normal temperatures he has difficulty in pulling away from standstill and the car is very slow in reverse.

Dave’s Response: I suspect that this is a problem with the forward clutches within the transmission. If these are partially seized it means that if you select reverse, the transmission still wants to move forward at the same time, in effect fighting against itself. This is a problem for an auto transmission specialist, many of whom advertise in Jaguar Enthusiast but it does look as if the box does require some internal work at least.

No Reverse

A member’s Series 2 XJ6’s overdrive didn’t disengage. Unfortunately, not realising he then reversed the car for about a metre and since when he has been unable to get reverse gear.

Dave’s Response: Unfortunately by the very nature of an overdrive unit, the selection of reverse without any inhibitor to disconnect the overdrive, will cause damage inside the overdrive unit. There is no alternative other than to have the overdrive unit removed from the car and overhauled, which could cost from £250 upwards dependent on work required.

Electrical Misdemeanor on a Series 3

A 1982 Series XJ6 has an unusual symptom. When the car is opened or the courtesy light switch operated, the lights do NOT come on and the clock stops! Having looked at the wiring diagram the member has come to the conclusion that there is a short on one of the wires going from each of the courtesy lights to the interior lamp delay (item 335 on the wiring diagram). The short is not enough to blow the fuse. So his question is, where is item 335, where do the courtesy light wires run and is there another cause of the problem?

Dave’s Response: Unusually, faults of this nature can often be attributed to a fault in the aerial motor of all places. If the aerial does not work on this car I would suggest you first disconnect the multi-pin connector to the aerial motor assembly and also the fuse feeding it.

Item 335 is located under the dash liner on the left hand side. There are two of these in this area, one for the wiper delay and the other for interior courtesy light delay.

I am at a loss to understand the reason for the clock not working coinciding with the above without further information.

Series 3 Cooling Fans and Tools

After a long journey this 1991 Series 3 V12 does not engage the cooling fan when the engine is turned off. Also the car is missing many of the tools; what should be there?

Dave’s Response: The first thing with the fan is to check that the fuse has not blown. This is located on the inner wing fusebox on the left hand side. If the fuse is OK then you should disconnect the wires to the fan and test that it is working correctly. It is important this fan works because not only does it ensure the efficient operation of the air conditioning but is a safeguard against overheating in hot weather and low speed traffic conditions.

On the matter of tools, it is likely that your car, despite its late manufacture date, still uses the conventional attached case tool kit located in the boot. If so, the breakdown of tools is as follows:

• Wheelbrace

• Plug Spanner with bar (on 1991 cars a one piece with universal joint)

• Translucent yellow handled screwdriver with interchangeable head.

• Four spare bulbs Three spare fuses

• Six assorted open spanners Tyre pressure gauge

• Feeler gauges

• Handle to manually wind the sunroof (where fitted)

Too Rich for Some

I am experiencing problems with my 3.4-litre Series 2 XJ6 over a rich mixture. I believe the problem lies with the AED unit. I have cleaned the manifold filter unit but the car still has a tendency to ‘hunt’ when stationary. I am also only returning about 10/12 mpg and am unable to adjust the HS8 carburettors as they’re factory set as I understand.

I’m also considering the fitment of a manual choke conversion as a remedy. Are there any pitfalls in doing so and if I proceed, can I remove the AED from the carbs without any problems?

Dave’s Response: Fitting a manual choke to a 3.4-litre XJ is often a good conversion. There are no pitfalls in doing so as the kits come with adequate

instructions. Its advised though to put the diaphragm in the bottom section of the AED which will still be needed with this conversion. Its this diaphragm that cracks and can cause many of the problems anyway.

Heating Problems

Having a 1985 XJ6 Series 3 saloon, I am having problems with the heater. Whilst getting warm air from the eye level vents on the outer ends of the dashboard, there is only cold air from the footwell vents.

I have adjusted the flap linkage as per the service manual with no real effect, although when flap Q was turned fully clockwise the air output was very slightly warmer but as it fell back with the normal play of the linkage the air flow returned to cold.

The engine runs, according to the temperature gauge at about 75 to 80° instead of the 90° suggested by the green segment on the gauge itself. Should I do something to lift the temperature and if so what?

Dave’s Response:- The engine temperature will affect the temperature of the heater matrix and therefore the amount of heat getting through to the interior of the car. The temperature indicated is rather cool. So first try fitting a replacement correct type of thermostat for the car. Make sure the thermostat has the proper bleed hole so that the air is allowed to purge out of the system when you refill. Be careful when changing this as one bolt in the manifold is prone to shearing.

The probable reason for no hot air down at the footwells is a failure of one of the vacuum modules, particularly controlling a combination of both demist and dashboard distribution. Another answer could be that the matrix itself is furred up inside and so not allowing a sufficient flow through.

One way of eliminating the latter would be to start the car, getting the engine up to full working temperature. Then turn the temperature selector to hot and turn the fans on to high speed, keeping your hands at one of the footwell vents. If the air then comes out hot for a couple of minutes and then goes cold this would mean that the matrix is at fault.

Differing Thermostats

I own a 3.8-litre E-Type Series 1 and a 3.8-litre Mark 2 and both engines are fitted with thermostats from SC Parts (part no SC 1332/74). These are definitely not the bellows type but differ slightly from the illustration in Jaguar Enthusiast in that there’s no sealing plate on the base. I was under the impression that the idea of the sealing plate was to stop the flow of water into the water rail once the engine had reached temperature.

Dave’s Response:- The sealing plate is there to close off the circular orifice which forms the outlet to the by-pass system cooling system. Your type instead of having this has a slot cut into the side of the thermostat housing that serves the same purpose. The only way of blanking this slot off is with a slide that moves up and down with the operation of the thermostat. Therefore fitting a thermostat without a bellows, whilst being slightly better than not having one at all, will not have any effect in closing off the by-pass. Fitting a later type with or without a back plate will have no effect whatsoever.

Finding these correct thermostats is not easy at present although some members have had success at some Jaguar dealers and at autojumbles.