Subaru Impreza: FB20 DI intake valve carbon buildup

The GP/GJ Impreza with the FB20 2.0-litre direct-injection petrol engine does not wash intake valves with fuel as port-injected engines do. Crankcase vapours routed through the positive crankcase ventilation system deposit oil residue directly onto the back of intake valves and inlet port walls. Carbon accumulates progressively from around 60,000 km onward and can significantly restrict airflow, causing rough idle, low-RPM hesitation, and measurable power loss. Walnut shell blasting or manual mechanical de-coke of the intake tract is the accepted repair and should be considered a scheduled maintenance item on high-mileage examples.

• Rough idle that worsens when the engine is cold
• Hesitation or stumble under light throttle at low RPM
• Reduced fuel economy compared to early ownership baseline
• Occasional misfires logged as fault codes P0300 through P0304
• Noticeable power loss from 60,000 km onward that worsens progressively

1. 1

   Look for hard black carbon deposits more than 1 mm thick on valve stems and port walls.

2. 2

   Check for vacuum leaks around intake manifold port gaskets which can worsen mixture issues.

3. 3

   If a workshop inspection is possible: Using a borescope camera, inspect the back of intake valves via the intake port with the intake plenum removed or via an inspection port if fitted.

Parts involved: PCV / oil separator.

$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 an intake valve de-carbon or walnut-blast service been carried out and at what mileage?”
• “Has the PCV system been inspected or the PCV valve replaced?”
• “Are there any stored misfire or lean-mixture fault codes in the ECU?”

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.