Subaru EJ (1.5–2.5)

The flat-four that defined Subaru for thirty years — naturally aspirated in the Impreza and Forester, and turbocharged in the WRX and STI, where it became one of the most modified engines in the world.

Head gasket failure is the fault the family is known for, particularly on the 2.5 naturally aspirated versions of the late 1990s and 2000s. The presentation is often external — coolant or oil weeping at the side of the block — rather than the classic mayonnaise-under-the-cap, so the layout changes what the symptom looks like.

On the turbocharged versions the additional and more expensive theme is ringland failure, where detonation damages the piston between the ring grooves. It is strongly associated with modification, poor fuel and heat, and it presents as a sudden misfire with low compression on one cylinder. Because these engines are modified so routinely, establishing what boost and software the car is running is the first question on any knock or misfire complaint, ahead of any component test.

It is a family spanning 1.5 to 2.5 litres, and the cars listed below carry the EJ15, EJ16, EJ18, EJ20, EJ22 and EJ25 between them. The head-gasket reputation belongs to the 2.5 naturally aspirated engines and the ringland failures to the turbocharged 2.0 — so a fault quoted for the EJ is almost always a fault quoted for one member of it.