Jaguar XJ-S, Known Issues and Common Problems
Overview
The XJ-S was built for 21 years, which means the problems vary significantly depending on when your car was made. Early pre-HE V12 cars from the late 1970s share almost nothing, in terms of reliability profile, with a 1995 facelift 4.0 coupe. What they do share is complexity. The XJ-S was Jaguar’s flagship grand tourer, loaded with equipment, and every system adds another potential failure point.
The good news is that none of these problems are mysterious. The XJ-S has been around long enough, and sold in sufficient numbers (over 115,000), that every issue is documented, every fix is known, and parts are available for virtually everything. The JDCA and online forums are gold mines of specific knowledge.
The bad news is that neglect is the XJ-S’s killer. These cars were expensive to service when new, and many owners simply stopped spending money on them. A $15,000 XJ-S that hasn’t been properly maintained for five years can easily need $10,000 in catch-up work. Factor that in when you’re buying.
Engine, V12
Cooling system failure
What happens: Engine overheats, temperature gauge climbs into the red, coolant boils over. In severe cases, head gaskets fail and coolant enters the oil.
Why it happens: 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.
How to fix it: 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.
Severity: Critical. An overheated V12 costs $10,000+ to fix. Cooling system maintenance is the single most important thing you can do.
Head gasket weeping
What happens: Coolant appears around the head gasket line (external weep), or coolant contaminates the oil (internal failure, mayonnaise under the filler cap, white exhaust smoke).
Why it happens: 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.
How to fix it: 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.
Severity: External weep: monitor. Internal failure: critical, do not drive.
Lucas ignition system failure (pre-1989)
What happens: Misfiring, rough idle, hard starting, power loss, sometimes total ignition failure.
Why it happens: 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.
How to fix it: 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.
Severity: Needs attention. Ignition failure will strand you.
Marelli ignition issues (1989 onwards)
What happens: Similar symptoms to Lucas, misfiring, rough running, but less frequent and usually easier to diagnose.
Why it happens: 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.
How to fix it: 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.
Severity: Needs attention. Better than Lucas but not immune to problems.
Fuel injection faults (V12)
What happens: Rough idle, hesitation, poor fuel economy (even worse than usual), hard starting, stalling.
Why it happens: 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.
How to fix it: 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.
Severity: Needs attention. The car runs badly and wastes fuel until it’s sorted.
Exhaust manifold cracking (V12)
What happens: Ticking or blowing noise from the engine bay, exhaust smell, sometimes visible exhaust gas.
Why it happens: The cast iron exhaust manifolds crack from thermal cycling. They’re close to other components and can be hard to inspect visually.
How to fix it: Replace with stainless steel aftermarket manifolds, they last much longer than the originals. Budget $2,000-4,000 AUD for a pair including fitting.
Severity: Needs attention. Not dangerous but unpleasant and will worsen.
Engine, Six-Cylinder (AJ6/AJ16)
Timing chain tensioner failure
What happens: 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.
Why it happens: 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.
How to fix it: Replace the tensioner and chain. It’s a front-of-engine job, significant labour but well-documented. Budget $1,500-3,000 AUD.
Severity: Needs attention. A jumped chain can bend valves.
3.6 cylinder liner issues
What happens: Coolant loss, white smoke, overheating. The 3.6-litre AJ6 can develop cracks in the cylinder liner sealing.
Why it happens: Design weakness in the early 3.6 engines. The liner seal can fail, allowing coolant to enter the combustion chamber.
How to fix it: Engine rebuild or replacement. Not cheap. This is one reason the 4.0-litre is the preferred six-cylinder variant.
Severity: Critical on 3.6 engines. The 4.0 is largely free of this issue.
Oil leaks (cam covers and sump)
What happens: Oil weeps from the cam cover gaskets and sump gasket. Drips on the exhaust and smokes.
Why it happens: Gaskets harden with age and heat cycling. The cam cover gaskets are cork or rubber and deteriorate over time.
How to fix it: 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.
Severity: Minor annoyance. Fix it before it drips on the exhaust.
Transmission
Borg-Warner Model 66 automatic problems (early cars)
What happens: Harsh shifting, slipping, delayed engagement, overheating. The BW66 is simply not strong enough for sustained V12 power.
Why it happens: Under-engineered for the application. The BW66 was adequate in lighter, less powerful cars but struggles with the V12’s torque and heat output.
How to fix it: 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.
Severity: Needs attention. The BW66 is a known weakness. Many have already been replaced.
GM TH400, generally bulletproof
The TH400 is one of the most reliable automatic gearboxes ever made. Regular fluid changes keep it happy. If it does develop issues (rare), rebuilt units are readily available and affordable.
Severity: Minimal concern. This is one of the XJ-S’s strong points.
Electrical
Air conditioning system failure
What happens: A/C doesn’t blow cold, blows warm on one side, or doesn’t work at all. Compressor clutch won’t engage.
Why it happens: 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.
How to fix it: 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.
Severity: Needs attention. An XJ-S without A/C in an Australian summer is miserable.
Electric window and mirror motor failure
What happens: Windows won’t go up or down, or move slowly. Door mirrors don’t adjust.
Why it happens: Electric motors burn out. Regulators seize. Wiring connections corrode.
How to fix it: Replace the motor or regulator. Parts are available. Not a difficult job but requires door trim removal. Budget $200-500 per window.
Severity: Minor annoyance but can leave you with a window stuck down in the rain.
Central locking failure
What happens: Doors won’t lock or unlock centrally. Some doors lock, others don’t. Boot won’t unlock.
Why it happens: Individual door lock actuators fail (vacuum-operated on some cars, electric on others). Vacuum lines perish. Microswitches wear out.
How to fix it: Diagnose which actuator has failed and replace it. Check vacuum lines and replace if perished. Budget $100-300 per door.
Severity: Minor annoyance. You can still lock the car manually.
Instrument cluster and gauge failure
What happens: Gauges read incorrectly, flicker, or die. Warning lights come on randomly.
Why it happens: Printed circuit boards behind the instrument cluster develop cracks and corrosion. Voltage stabilisers fail. Sender units fail.
How to fix it: 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.
Severity: Needs attention. A non-functional temperature gauge on a V12 is dangerous, you won’t know when it’s overheating.
Bodywork and Rust
Sill corrosion
What happens: Outer sills bubble and perforate. In severe cases, the car sags and doors bind.
Why it happens: Stone chips compromise the paint, water sits in the sill box sections, drain holes block.
How to fix it: 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.
Severity: Needs attention to critical, depending on severity. Structural component.
Rear wheel arch corrosion
What happens: Bubbling and perforation of the rear wheel arches, both inner and outer skins.
Why it happens: Stone chips, trapped mud, blocked drainage. The arch lips trap moisture.
How to fix it: Cut and replace. Repair panels are available. Budget $1,000-3,000 AUD per side.
Severity: Needs attention. Cosmetic initially, structural if ignored.
Boot floor rot
What happens: Boot floor rusts through. Spare wheel well fills with water. Electrical components in the boot (fuel pump, ECU on some models) corrode.
Why it happens: Boot seal failure, blocked drains, condensation from temperature changes.
How to fix it: 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.
Severity: Needs attention. Water in the boot damages electrical components.
Suspension and Steering
Front suspension bush wear
What happens: Clonking over bumps, wandering at speed, uneven tyre wear.
Why it happens: Rubber bushes deteriorate with age. The XJ-S front suspension uses upper and lower wishbones with multiple bush points.
How to fix it: Replace all bushes. Polyurethane upgrades are available and last significantly longer. A complete front suspension bush kit runs $400-800 AUD in parts.
Severity: Needs attention. Worn bushes affect handling and safety.
Rear suspension compliance mount failure
What happens: Vague rear end, clunking, unpredictable handling over bumps.
Why it happens: The rear suspension uses large rubber compliance mounts (cages) that deteriorate and collapse. They’re critical to rear suspension geometry.
How to fix it: Replace the compliance mounts. It’s a significant job, the rear subframe needs attention. Budget $1,500-3,000 AUD.
Severity: Needs attention. Collapsed mounts affect handling and tyre wear.
Power steering rack leaks
What happens: Power steering fluid leak, heavy steering, groaning noise from the pump.
Why it happens: Rack seals fail with age. The pump wears. Hose connections corrode and weep.
How to fix it: Reconditioned racks are available. Replace hoses and check the pump. Budget $1,500-3,000 AUD for a reconditioned rack fitted.
Severity: Needs attention. Loss of power steering on a heavy car is hard work.
Convertible-Specific
Hood mechanism failure
What happens: The electric hood won’t raise or lower, gets stuck halfway, or operates unevenly.
Why it happens: The hydraulic mechanism loses fluid, cylinders leak, the pump fails, or microswitches that control the sequence malfunction.
How to fix it: 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.
Severity: Needs attention. Being stuck with the roof down in an Australian thunderstorm is character-building but not recommended.
Water ingress around hood seals
What happens: Damp carpets, musty smell, wet boot.
Why it happens: Hood seals perish and shrink. Drain channels block.
How to fix it: 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.
Severity: Needs attention. Water rots the floor and damages the interior.
Summary
The XJ-S is a complex car with a lot of systems, and age means all of them need attention eventually. The cooling system on V12 cars is the number one priority, everything else is secondary. After that, electrics, rust, and the usual age-related suspension and brake wear form the bulk of maintenance demands.
The six-cylinder cars are simpler and cheaper to maintain. The facelift cars are better built. The GM TH400 automatic is virtually indestructible. Parts are available for everything. The JDCA and specialist forums are full of people who’ve dealt with every problem listed here and will share their knowledge freely.
Don’t buy the cheapest XJ-S you can find. Buy the best-maintained one. A car with documented service history and a known specialist relationship is worth far more than a low-mileage barn find with unknown maintenance status. The deferred maintenance bill on a neglected XJ-S can easily exceed the car’s purchase price.
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