Subaru EJ25 (SOHC) — External head gasket leaks

English

Scope

Vehicles: Subaru Legacy/Outback, Forester, Impreza (non-turbo) with the SOHC EJ25 — roughly 1999–2011 depending on model
Engines/components: EJ251/EJ252/EJ253 2.5L SOHC flat-four — composite head gaskets at the head-to-block joint (the earlier DOHC EJ25D, ~1996–1999, had a different internal leak pattern)
Production window: Late 1990s through late 2000s; later FB-series engines are a different design
Markets: Global; especially well documented in North America where these models sold in volume

The issue

The composite head gaskets weep externally at the head-block seam — typically oil on the left (driver’s-side US) bank and coolant on the right — long before any dramatic failure. Slow coolant loss can progress to overheating, and the boxer layout means the seep is easy to miss until it drips.

Symptoms an owner can detect

Oil or coolant weep/crust along the head-block seam, visible from under the car at the bottom of each cylinder head
Slow coolant level drop with no visible puddle; sweet smell after hot shutoff
In progressed cases: temperature creep on hills or in traffic, bubbles in the overflow tank

Severity

Major. The leak itself starts as an annoyance, but ignored coolant loss becomes overheating, and overheating kills these engines.

Frequency (honest assessment)

Genuinely common at high mileage — a defining trait of the SOHC EJ25 era. Many owners run years with a minor weep and vigilant coolant checks; the risk is neglect, not suddenness.

Fix and cost

Repair: Head gaskets both sides with machined/checked heads and updated (typically MLS) gaskets; timing belt, water pump, and seals done in the same job because the labor overlaps almost entirely
Typical cost: USD 1,800–3,000 / JPY 25–45万円 at independent Subaru specialists, including the timing-belt-service overlap

Buyer checklist

Flashlight inspection along both head-block seams from below — crusty trails are the tell
Ask whether head gaskets were ever done, with which gasket type, and whether the heads were machined
Check coolant condition and level, and the overflow tank for oil film
Price an unfixed high-mileage example as if the job is due, because it likely is

Sources

Ubiquitous in Subaru owner communities and independent Subaru specialist documentation across two decades. Details vary by year and market — verify against the specific engine code.