Subaru Impreza: EJ20 Turbo Head Gasket Failure — Coolant/Oil Cross-Contamination

The EJ20 turbocharged engine in GD-chassis WRX models uses a composite head gasket that is prone to failure at the coolant-to-oil interface, particularly on the exhaust side of cylinder 4. Thermal cycling from the turbocharger, combined with the boxer engine's horizontal cylinder orientation that pools oil against the gasket face, accelerates delamination of the gasket layers. Once breached, coolant and engine oil mix, forming a brown emulsified sludge in the coolant reservoir and on the oil filler cap, and the engine rapidly loses coolant without visible external leaks. Left uncorrected, bearing journals wash out within weeks due to oil dilution.

• Brown or milky emulsion visible on the oil filler cap or dipstick
• Coolant level drops steadily with no external puddle under the car
• White or sweet-smelling exhaust smoke under sustained throttle
• Engine coolant temperature rises irregularly or gauge creeps above normal

1. 1

   Remove oil filler cap and inspect for brown mayonnaise emulsion.

2. 2

   Check coolant reservoir for oily film or discolouration.

3. 3

   Perform a block test (combustion gas sniff test) on the coolant reservoir.

4. 4

   Inspect for white exhaust smoke under sustained load.

5. 5

   If a workshop inspection is possible: Pressure-test the cooling system with engine cold and watch for pressure loss with no external leak.

Parts involved: Head gasket, Turbocharger.

$2,200 to $4,800 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, which gasket set and head-bolt torque procedure was used?”
• “Can you show me the coolant reservoir and oil filler cap right now?”
• “Does the car have a full cooling system service history including thermostat and water pump?”

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.