Jaguar AJ6 / AJ16

Jaguar's last straight-six — the AJ16, a heavily revised AJ6 — at 3.2 and 4.0 litres in the X300 XJ and the final XJS. It was replaced by the AJ-V8, and the two share nothing but the marque's AJ prefix, which Jaguar has used for every engine it has ever built.

It is the better of the two generations and the difference is worth establishing. The AJ16 gained a new cylinder head, sequential injection and a distributorless ignition, and it addressed most of what the AJ6 was criticised for. A car from the crossover years may be either.

The timing chain tensioners are plastic and they fail, which on a chain-driven six of this age is the fault to look for first: the noise is a rattle at the front that is loudest cold. Left alone the chain wears its guides and the tensioner debris circulates.

The head gasket is the other recurring item at high mileage, and the supercharged XJR version of the 4.0 adds an Eaton blower with its own oil that nobody changes — the same neglect that kills the supercharger on every other engine here fitted with one.