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Subaru Legacy: FB25 DI Intake Valve Carbon Buildup

FB25 DI Intake Valve Carbon Buildup is a common engine fault on the Subaru Legacy on the 2014–2019 cars, and we rate it major. Putting it right costs ≈A$550 to ≈A$1,290 at independent workshop rates, shown as an approximate guide in Australian dollars. Below is what goes wrong, the symptoms to notice on a test drive, where to look, and what to ask the seller before you hand over any money. It is one of 54 documented faults on the Subaru Legacy.

Vehicle
Subaru Legacy
Engine
FB25
Years
2014–2019
Fuel
petrol
System
Engine

This page covers the Subaru Legacy as a model. Some faults only affect particular engines or build years — enter a plate below to narrow the list to one car.

All Subaru Legacy faults54 documented faults on this model

What you are looking at

No photograph available

We do not hold a verified photograph for this fault yet. We would rather show none than show a picture of something else.

What goes wrong

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.

What you would notice

  • 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

How to check it

Where to look. Intake valve faces and port walls, both cylinder banks, accessed via intake manifold removal

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.

What it costs to put right

≈A$550 to ≈A$1,290 at independent workshop rates, shown as an approximate guide in Australian dollars. That is for this job on its own, not for the car.

An estimate, converted to Australian dollars, 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.

Ask the seller

  • “Has any decarbonisation or walnut blast service been performed and when?”
  • “Have there been any check engine lights or misfire codes?”

Now check the Subaru Legacy you are actually looking at

Everything above is true of the model, and it is free. What it does not tell you is whether THIS car still has money owing on it, has been written off or reported stolen, what kms it showed in past sale ads, whether it has had accident repair claims and what it is worth today. That comes from the official records for that plate. Enter the plate and its state for the A$17.90 Vehicle Check, or the full history from A$24.99.

Where this comes from

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.

Subaru Legacy buyer questions

Is fb25 di intake valve carbon buildup serious on a Subaru Legacy?

We rate it major and common on the Subaru Legacy. 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.

How much does it cost to fix on a Subaru Legacy?

≈A$550 to ≈A$1,290 at independent workshop rates, shown as an approximate guide in Australian dollars. That figure is for this job on its own. A car showing the symptoms on this page is usually worth either walking away from or negotiating down by at least the repair figure.

Which Subaru Legacy years are affected?

Our research records this fault on the 2014–2019 Subaru Legacy. Build dates move around, especially on imported cars, so treat the range as the years to look hardest at rather than a guarantee either way. The official records tell you the exact build and first registration date for the car in front of you.

How do I check for it before I buy?

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. Look here: Intake valve faces and port walls, both cylinder banks, accessed via intake manifold removal