Subaru Legacy: FB25 DI Intake Valve Carbon Buildup

The BS-generation Legacy fitted with the FB25 2.5-litre direct-injection engine develops carbon deposits on intake valve stems and faces over time. Because direct injection bypasses the intake valves with fuel, no solvent washes the valves clean and deposits accumulate from normal combustion blow-by. Symptoms begin appearing from around 60,000 km and include rough idle, cold-start hesitation, and occasional misfire codes. The Legacy shares the FB25 engine and platform with the contemporary Outback BS and is equally susceptible. Walnut blasting or chemical decarbonisation resolves the condition but treatment may need repeating every 60,000 to 80,000 km.

• Rough idle on cold start
• Stumble or hesitation under light acceleration from rest
• P0300 to P0304 misfire diagnostic codes
• Slightly increased fuel consumption over time
• Occasional Check Engine warning light

Review service records for any prior decarbonisation treatment. If a workshop inspection is possible: Run an OBD scan for misfire fault codes. Borescope the intake valve faces for visible carbon deposits via the intake manifold runner ports.

$600 to $1,400 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 any decarbonisation or walnut blast service been performed and when?”
• “Have there been any check engine lights or misfire codes?”

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.