P0300 on Jaguar: How to Diagnose & Fix

The AJ-V8 uses individual coil-on-plug ignition coils that fail with age and heat, producing single- or multi-cylinder misfires and a check-engine light. A frequent contributor is oil pooling in the spark-plug wells from a leaking cam-cover gasket, which fouls the coil boots. Coils typically last 70k-100k miles but wear faster with heavy idling and heat cycling.

The 4.2 AJ-V8 cam (valve) cover gaskets harden with heat and age and begin to weep oil, often onto the exhaust manifolds (producing a burning-oil smell) and down into the spark-plug wells, where pooled oil can foul coils and cause misfires. It is a well-documented, near-universal age-related leak on these engines.

The 5.0-litre AJ133 V8 fitted to the X351 has a widely documented timing drive weakness that has nothing to do with the earlier 4.2 AJ34's plastic tensioner story — the X350's 4.2 already ran metal tensioners and Morse chains, and merging the two engines' histories is the standard error on this nameplate. On the AJ133, the hydraulic tensioner's hardened steel plunger bears against a guide rail with a soft aluminium core and a plastic wear face. The plunger cuts a groove into that face; the ratchet that is supposed to hold tension with the engine off then cannot, and the chain slaps on start-up before oil pressure builds. There are no fault codes at the rattle stage, which is exactly why it gets ignored. Left alone, the chain can jump a tooth — giving a restricted-performance warning and misfire codes — or break, which on this interference engine destroys it. Jaguar Land Rover revised the timing components progressively, with the setup substantially reworked by around 2015-2016, so late facelift cars are lower risk than early ones.