Subaru Legacy: EJ25 SOHC Head Gasket Internal Failure (Coolant to Combustion)

The EJ25 2.5L SOHC flat-four engine used in naturally aspirated Legacy variants suffers a well-documented failure of the head gasket between the coolant passages and combustion chamber. The aluminium cylinder heads and cast-iron cylinder liners expand at different rates under thermal cycling, causing the factory head gasket to lose clamping force over time; this allows combustion gases to enter the cooling system and coolant to enter the combustion chamber. The result is progressive coolant loss, overheating events, and catalytic converter damage from coolant combustion, making this one of the most consequential mechanical failures on the BP/BL platform.

• Unexplained coolant loss with no visible external leak and no oil contamination visible in coolant
• White steam or sweet-smelling exhaust at operating temperature, especially under acceleration load
• Engine overheating or temperature gauge climbing above normal mid-range
• Positive combustion-leak test (blue fluid turns yellow/green) on cooling system

Perform a chemical combustion-leak test (block tester with blue fluid) on the coolant reservoir with the engine warm; inspect exhaust for persistent white steam under load at operating temperature; pressure-test the cooling system and observe for pressure drop with no visible external leak; check engine oil filler cap and dipstick for creamy emulsion indicating coolant intrusion; scan for coolant temperature irregularities or ECU coolant sensor faults.

Parts involved: Head gasket.

$1,800 to $3,500 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “Has the head gasket ever been replaced, and if so can you show workshop receipts?”
• “Has the car ever overheated or required coolant top-ups between services?”
• “When was the cooling system last pressure-tested?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.