XJ-S

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.

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.

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.

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.

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.