Subaru CB18

Subaru's newest flat-four — a 1.8-litre turbocharged, direct-injected engine introduced with the current Levorg and now in the Forester and Outback in some markets, and the company's first genuinely new boxer architecture in a decade.

It uses a lean-burn strategy with a very high compression ratio for a turbocharged engine, aimed at efficiency rather than the outright output the EJ and FA turbo engines chased. That changes what its normal operating data looks like: fuel trims and knock-sensor activity read differently from an FA20F, and interpreting one by the other's expectations will mislead.

It is recent enough that no failure pattern is established, and saying so is more useful than speculating. What carries over from the rest of the family is the layout's consequences — head gasket leaks presenting as external weeping rather than internal mixing, and access that makes routine work larger than it looks.