Subaru Impreza specialist — EJ engines and STI.

Why the EJ engine needs specific attention

The EJ-series flat-four — EJ20 and EJ25 in particular — is one of the most discussed engines in the import scene, and the discussion is warranted. These are capable, tuneable, and widely loved engines that also have documented weaknesses which are not present on most other platforms. The head gasket issue on the EJ25 is real. The ringland concern on forced-induction variants is real. Both are manageable if you understand what you are looking for and service the engine accordingly. They become expensive surprises if you do not.

We service Subaru Imprezas, Foresters and Legacys regularly, and the EJ platform is familiar ground. That means we are not learning these failure patterns on your car — we have already seen them.

EJ25 head gasket and EJ20 considerations

The EJ25 has a factory head gasket design that is prone to external oil and coolant seeping, particularly on naturally aspirated variants in the Forester, Outback and non-turbo Impreza. On turbocharged STI applications, the head gasket can fail internally under sustained high-boost use. We assess head gasket condition on every EJ25-engined car that comes in with unknown history: combustion gas test on the coolant, coolant consumption observation, and oil analysis for cross-contamination.

On WRX and STI variants, ringland failure on the EJ20 and EJ25 is a concern primarily on modified cars or cars that have been driven hard on stock internals past their limits. We do not assume every boosted Impreza is a problem — but on any car with evidence of modifications or heavy use, we look at the oil condition and listen to the engine data before recommending service work. A ringland problem found early is a manageable rebuild; found late, it is an engine replacement.

AVCS, solenoids and the turbo cars

AVCS — Active Valve Control System — is Subaru's variable camshaft timing system on the more recent EJ versions. The solenoids that control AVCS are hydraulic and sensitive to oil quality, in the same way that VANOS and VTEC systems are on BMW and Honda. Any STI or WRX with AVCS that has been running incorrect or degraded oil is a car where we recommend an oil service and solenoid inspection before diagnosing any AVCS-related fault code. The number of AVCS solenoids we have replaced after a fresh-oil interval resolved the apparent fault is not small.

Centre differential and STI drivetrain

The STI's centre differential is a mechanical multi-plate unit — not a viscous coupling, not a Haldex system. It requires specific gear oil at proper intervals. When centre differential oil is neglected, the early sign is a subtle change in the car's behaviour through corners under power: a slight hesitation or a change in the balance that experienced STI drivers will notice but may not immediately attribute to the diff. We service the centre diff as a routine item on any STI at sensible mileage intervals, and we include front and rear differential checks at the same time.

The wider Subaru family

The Forester, particularly the STi-badged XT and the earlier turbocharged variants, carries the same EJ platform considerations as the Impreza. The Legacy GT and Outback XT use the EJ20 or EJ25 in turbocharged form and share the same service requirements. We treat all EJ-platform cars the same way regardless of body style: baseline assessment, correct-spec fluids, and attention to the known failure points before they present as problems.

Where we start

A full diagnostic scan, a combustion gas test on the coolant, oil condition assessment, and a visual inspection of the drivetrain items specific to the variant in front of us. If you have just bought a Subaru with incomplete history, that is the starting point — not a service against the schedule and an assumption that everything is fine. The Japanese imports page covers the broader approach, and the diagnostics page explains how we structure the investigation work.