EJ25 Engine in Phoenix, Arizona

The head gasket decision, honestly

Head gasket failure is a genuinely common outcome on the naturally aspirated EJ25, particularly on earlier builds, and it is the point at which most Subaru owners have to make a real decision rather than authorise a repair.

The decision is closer than it looks. A gasket job on a boxer engine is labour-heavy because of how the engine sits, and if the car overheated badly the heads may need machining as well. At which point the cost climbs toward a replacement engine. Get the heads assessed before choosing, because that assessment is what actually decides it.

And if you do replace: ask what has been done to the gaskets on the replacement unit, and with which parts. A used EJ25 can arrive carrying exactly the same latent problem, which is the one outcome that leaves you worse off than when you started.

Counting camshafts tells you which half you are in

The naturally aspirated variants, EJ251, EJ252, EJ253, are single-overhead-cam engines. The turbocharged EJ255 and EJ257 are twin-cam. That is the quickest structural check when a listing is vague or the paperwork is missing.

The other date that matters is 2019, when the EJ25 gave way to the FB25. The FB25 uses direct injection and different engine management; it is a different engine that shares a displacement and a boxer layout, and it is not a drop-in replacement.

What to ask about a used EJ25

Ask what has been done to the head gaskets on the replacement unit, and with which parts. Multi-layer steel gaskets are the standard repair for this failure, and an engine that has already had the job done with the correct parts is a different proposition from one that has not. If the supplier cannot say, that is an answer in itself.

Ask whether the engine overheated, and what that did to the heads. A head gasket failure that led to sustained overheating can warp the cylinder heads or score the deck surface, and a replacement engine that went through the same sequence may carry the same damage. A straight answer on the donor vehicle's history tells you more than a listing that omits it.

Ask about the timing belt, tensioner and water pump. The EJ25 is belt-driven, and all three are items that belong in the installation on any used unit. The labour overlaps almost entirely with fitting the engine, so doing them at the same time costs a fraction of doing them separately. A belt failure on this engine is not a minor event.

Ask whether the unit came from a running vehicle or a non-running one. An engine pulled from a car that was still in service is a known quantity in ways that one sitting in a yard may not be. Compression figures, even rough ones, are worth more than the family code on their own.

The FB25 as the later alternative

From 2019 the FB25 replaced the EJ25 in naturally aspirated Subaru applications. It uses direct injection and a timing chain rather than a timing belt, and it was designed in part to address the head gasket problem that defined the EJ25's service history.

The FB25 is not interchangeable with the EJ25. The block, heads, intake, exhaust and engine management are all different, and fitting one to a car built for the other is a conversion rather than a repair. If your vehicle is near the 2019 changeover boundary, confirm which engine it has before ordering anything.