P0018 on Jaguar: How to Diagnose & Fix

The Jaguar AJ-V8 engines in the XF suffer from hydraulic timing chain tensioner failure, causing chain slack on cold starts. The tensioner bleed-down allows the chain to rattle against the guides, and prolonged neglect leads to stretched chains and jumped timing, risking catastrophic valve-to-piston contact.

The AJ-V8 engine in the XK shares the same timing chain tensioner weakness found across the Jaguar V8 range. The hydraulic tensioners bleed down when the engine is off, causing the timing chains to rattle on startup and progressively stretch until timing is compromised.

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.