1992 Jaguar XJS 4.0L Petrol · J4 JAG

Until mid-1993, all XJS models utilized inboard rear disc brakes mounted directly to the differential housing within the rear suspension subframe. Due to their close proximity to the hot differential and exhaust pipes, the brake calipers suffer from extreme heat soak, boiling brake fluid, and hardening hydraulic piston seals. Road grime and inaccessible location mean the calipers frequently seize, while differential output shaft oil leaks saturate the brake pads, causing severe brake fade, handbrake failure, and requiring a laborious complete rear subframe drop to repair.

V12 models manufactured between 1989 and 1993 feature a Marelli digital electronic ignition system utilizing a single distributor with a two-tiered rotor arm firing each cylinder bank independently. If the rotor arm develops dielectric breakdown or the secondary coil fails, one entire 6-cylinder bank loses spark while the fuel injection continues spraying fuel. The raw unburnt petrol pours into the glowing hot catalytic converters, melting the ceramic substrate and creating an immediate, severe engine bay and underbody fire risk.

The XJS bodyshell is susceptible to hidden corrosion, particularly on cars built before the 1994 introduction of full galvanization. Road spray and blocked scuttle drains trap moisture within the double-skinned floorpans, sills, and front chassis legs. The rear suspension radius arm forward mounting points on the floor are especially vulnerable; rust here weakens rear axle location, creating dangerous instability under heavy braking and cornering that results in immediate MOT failure.