Question 1

Cooling system failure

Accepted Answer

Engine overheats, temperature gauge climbs into the red, coolant boils over. In severe cases, head gaskets fail and coolant enters the oil. The V12 generates enormous heat and the cooling system is barely adequate even when everything works. Twin electric fans, a viscous coupling fan (some models), a complex network of hoses, a header tank, and two thermostats all have to function perfectly. The header tank cracks (a known weakness), hoses perish internally (collapsing and restricting flow), fan relays fail, and the radiator clogs with scale. Any single failure can cascade into overheating. Comprehensive cooling system overhaul. Replace every hose (there are dozens), fit a new aluminium radiator if the original is tired, replace the header tank (aluminium aftermarket units are available and far superior to the original plastic), check both thermostats, verify both electric fans operate (including the relay), and flush the system thoroughly. Budget $3,000-5,000 AUD for a complete cooling system refresh. This is non-negotiable for Australian ownership.

Question 2

Head gasket weeping

Accepted Answer

Coolant appears around the head gasket line (external weep), or coolant contaminates the oil (internal failure, mayonnaise under the filler cap, white exhaust smoke). The V12 block and heads expand at different rates, stressing the gasket. Repeated thermal cycling, especially overheating events, causes the gasket to fail. External weeping is common on high-mileage engines and can be lived with if it's minor. Internal failure requires immediate action. Both heads off, gaskets replaced, heads pressure-tested and skimmed if warped. Use quality gaskets (Payen or genuine Jaguar). The job is 30-40 hours of specialist labour. Budget $8,000-12,000 AUD.

Question 3

Lucas ignition system failure (pre-1989)

Accepted Answer

Misfiring, rough idle, hard starting, power loss, sometimes total ignition failure. The Lucas CEI (Constant Energy Ignition) system uses an amplifier module, a pick-up in the distributor, and associated wiring. The amplifier fails (heat-related), the pick-up air gap drifts, and wiring connections corrode. Each bank of six cylinders has its own distributor, so problems can affect one bank while the other runs fine, a distinctive symptom of V12 ignition faults. Replace the amplifier module (carry a spare, they fail without warning). Check pick-up air gaps. Clean all connections. Fit electronic ignition conversion (Pertronix or similar) for long-term reliability. Upgrade to the later Marelli system if possible, it's a significant improvement.

Question 4

Marelli ignition issues (1989 onwards)

Accepted Answer

Similar symptoms to Lucas, misfiring, rough running, but less frequent and usually easier to diagnose. The Marelli system is more reliable than Lucas but still has failure points. The distributor cap and rotor arm degrade, the coil packs can fail, and the wiring connections corrode. The system is also sensitive to moisture. Regular maintenance, replace caps, rotors, and leads at recommended intervals. Check coil packs. Keep connections clean and dry. Budget $300-600 AUD for a full ignition service.

Question 5

Fuel injection faults (V12)

Accepted Answer

Rough idle, hesitation, poor fuel economy (even worse than usual), hard starting, stalling. The Lucas/Bosch digital fuel injection system relies on multiple sensors and vacuum lines. Common culprits include: air leaks in the intake manifold (the gaskets shrink and crack), faulty coolant temperature sensor (sends incorrect data to the ECU, causing rich or lean running), stuck injectors (varnish from old fuel), and perished vacuum hoses. Diagnose systematically. Check for vacuum leaks first (spray carb cleaner around joints while running, if the idle changes, you've found the leak). Test the coolant temperature sensor. Clean or replace injectors. Replace all vacuum hoses (they're cheap, do them all). Budget $500-2,000 AUD depending on what needs doing.

Question 6

Exhaust manifold cracking (V12)

Accepted Answer

Ticking or blowing noise from the engine bay, exhaust smell, sometimes visible exhaust gas. The cast iron exhaust manifolds crack from thermal cycling. They're close to other components and can be hard to inspect visually. Replace with stainless steel aftermarket manifolds, they last much longer than the originals. Budget $2,000-4,000 AUD for a pair including fitting.

Question 7

Timing chain tensioner failure

Accepted Answer

Rattle from the front of the engine on cold start, which may or may not settle when warm. In severe cases, the chain jumps a tooth and the engine runs badly or stops. The hydraulic chain tensioner loses its ability to maintain tension. The chain stretches over high mileages. The tensioner relies on oil pressure, so low oil level or old oil compound the problem. Replace the tensioner and chain. It's a front-of-engine job, significant labour but well-documented. Budget $1,500-3,000 AUD.

Question 8

3.6 cylinder liner issues

Accepted Answer

Coolant loss, white smoke, overheating. The 3.6-litre AJ6 can develop cracks in the cylinder liner sealing. Design weakness in the early 3.6 engines. The liner seal can fail, allowing coolant to enter the combustion chamber. Engine rebuild or replacement. Not cheap. This is one reason the 4.0-litre is the preferred six-cylinder variant.

Question 9

Oil leaks (cam covers and sump)

Accepted Answer

Oil weeps from the cam cover gaskets and sump gasket. Drips on the exhaust and smokes. Gaskets harden with age and heat cycling. The cam cover gaskets are cork or rubber and deteriorate over time. Replace the gaskets. Cam covers are a straightforward job, $200-400 in parts and a couple of hours' work. Sump gasket requires more access. Use quality gaskets and don't overtighten.

Question 10

Borg-Warner Model 66 automatic problems (early cars)

Accepted Answer

Harsh shifting, slipping, delayed engagement, overheating. The BW66 is simply not strong enough for sustained V12 power. Under-engineered for the application. The BW66 was adequate in lighter, less powerful cars but struggles with the V12's torque and heat output. If it's still working, service it (fluid and filter change, band adjustment) and hope. If it fails, replace with a GM TH400, conversion kits are available. Budget $3,000-5,000 AUD for the conversion.

Question 11

GM TH400, generally bulletproof

Accepted Answer

Question 12

Air conditioning system failure

Accepted Answer

A/C doesn't blow cold, blows warm on one side, or doesn't work at all. Compressor clutch won't engage. The XJ-S A/C system is complex and uses components that are now 30-50 years old. Compressors wear, condensers leak, evaporators corrode, and the control module fails. The system uses R12 refrigerant (early cars) which requires conversion to R134a. Diagnosis first, could be as simple as a relay or as expensive as a new compressor and condenser. A full A/C overhaul with R134a conversion runs $2,000-5,000 AUD. In Australian conditions, functional A/C is essential, not optional.

Question 13

Electric window and mirror motor failure

Accepted Answer

Windows won't go up or down, or move slowly. Door mirrors don't adjust. Electric motors burn out. Regulators seize. Wiring connections corrode. Replace the motor or regulator. Parts are available. Not a difficult job but requires door trim removal. Budget $200-500 per window.

Question 14

Central locking failure

Accepted Answer

Doors won't lock or unlock centrally. Some doors lock, others don't. Boot won't unlock. Individual door lock actuators fail (vacuum-operated on some cars, electric on others). Vacuum lines perish. Microswitches wear out. Diagnose which actuator has failed and replace it. Check vacuum lines and replace if perished. Budget $100-300 per door.

Question 15

Instrument cluster and gauge failure

Accepted Answer

Gauges read incorrectly, flicker, or die. Warning lights come on randomly. Printed circuit boards behind the instrument cluster develop cracks and corrosion. Voltage stabilisers fail. Sender units fail. Remove the instrument cluster and inspect the printed circuit board. Repair cracked tracks with solder. Replace the voltage stabiliser. Test sender units. Specialist instrument repairers can overhaul the complete cluster. Budget $300-800 AUD.

Question 16

Sill corrosion

Accepted Answer

Outer sills bubble and perforate. In severe cases, the car sags and doors bind. Stone chips compromise the paint, water sits in the sill box sections, drain holes block. Cut out the rot, weld in new metal. Replacement outer sill panels are available. Inner sill repair is more involved. Budget $2,000-5,000 AUD for professional sill repairs on both sides.

Question 17

Rear wheel arch corrosion

Accepted Answer

Bubbling and perforation of the rear wheel arches, both inner and outer skins. Stone chips, trapped mud, blocked drainage. The arch lips trap moisture. Cut and replace. Repair panels are available. Budget $1,000-3,000 AUD per side.

Question 18

Boot floor rot

Accepted Answer

Boot floor rusts through. Spare wheel well fills with water. Electrical components in the boot (fuel pump, ECU on some models) corrode. Boot seal failure, blocked drains, condensation from temperature changes. Cut out and replace the boot floor section. Check the boot seal and replace if perished. Ensure drains are clear. Budget $1,000-2,000 AUD.

Question 19

Front suspension bush wear

Accepted Answer

Clonking over bumps, wandering at speed, uneven tyre wear. Rubber bushes deteriorate with age. The XJ-S front suspension uses upper and lower wishbones with multiple bush points. Replace all bushes. Polyurethane upgrades are available and last significantly longer. A complete front suspension bush kit runs $400-800 AUD in parts.

Question 20

Rear suspension compliance mount failure

Accepted Answer

Vague rear end, clunking, unpredictable handling over bumps. The rear suspension uses large rubber compliance mounts (cages) that deteriorate and collapse. They're critical to rear suspension geometry. Replace the compliance mounts. It's a significant job, the rear subframe needs attention. Budget $1,500-3,000 AUD.

Question 21

Power steering rack leaks

Accepted Answer

Power steering fluid leak, heavy steering, groaning noise from the pump. Rack seals fail with age. The pump wears. Hose connections corrode and weep. Reconditioned racks are available. Replace hoses and check the pump. Budget $1,500-3,000 AUD for a reconditioned rack fitted.

Question 22

Hood mechanism failure

Accepted Answer

The electric hood won't raise or lower, gets stuck halfway, or operates unevenly. The hydraulic mechanism loses fluid, cylinders leak, the pump fails, or microswitches that control the sequence malfunction. Check hydraulic fluid level first. Replace failed cylinders, seals, or pump as needed. The mechanism is complex, allow time for diagnosis. Budget $1,000-3,000 AUD.

Question 23

Water ingress around hood seals

Accepted Answer

Damp carpets, musty smell, wet boot. Hood seals perish and shrink. Drain channels block. Replace hood seals. Clear all drain channels (a piece of wire and patience). Check and replace the rear screen seal if applicable. Budget $500-1,500 AUD.

XJ-S

XJ-S known problems