Subaru STI: EJ25 STI Head Gasket Failure

The EJ25 2.5-litre turbocharged flat-four used in WRX STI models has a well-documented multi-layer steel (MLS) head gasket failure pattern, most pronounced on the rear cylinder on each bank. The combination of high cylinder pressures, a wide bore, and aluminium heads on a cast-iron block produces differential thermal expansion that causes the head gasket to weep or blow — typically between coolant and oil passages, or externally. Failure accelerates on vehicles that have been driven hard, run lean, or have experienced any overheating episode. The factory torque-to-yield head bolts further complicate repair, as they must be replaced on reassembly.

• Coolant loss without visible external leak — coolant is consumed into combustion chamber
• White sweet-smelling steam from exhaust, particularly after the engine reaches operating temperature
• Brown or milky contamination in coolant reservoir or on oil dipstick
• Overheating or fluctuating temperature gauge, especially under load
• Rough running or misfires on one or more cylinders due to coolant entering combustion

1. 1

   Inspect coolant reservoir for brown oily residue or emulsification.

2. 2

   Check oil filler cap and dipstick for white mayonnaise-like deposits indicating coolant ingress.

3. 3

   Look for white steam from exhaust at operating temperature.

4. 4

   Inspect the underside of the engine for external coolant seepage around the head/block junction.

5. 5

   If a workshop inspection is possible: A cooling system pressure test and combustion gas (CO2) test of the coolant are the definitive checks.

Parts involved: Head gasket.

$3,200 to $7,500 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, were OEM-spec head bolts and ARP studs used on reassembly?”
• “Has the car ever overheated or has the temperature gauge ever risen above the normal midpoint?”
• “Can you provide a current coolant system pressure test and a combustion gas test result?”

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.