BMW M3: S58 intake valve carbon build-up from around 60,000 km

The S58 is a direct-injection engine, so fuel is sprayed into the cylinder rather than past the intake valves, and nothing washes those valves clean. Oil vapour drawn through the crankcase ventilation system deposits carbon on the back of the valves instead, and it becomes noticeable somewhere around 60,000 to 80,000 km. It builds slowly enough that owners adapt rather than notice, showing as a rough idle, a slight misfire and a flat patch low in the rev range. Worth putting in context: the S58 has a strong reliability record and carries no rod bearing concern of the sort that defines the S54 and S55 before it. This is a maintenance item to price in rather than a reason to be nervous about the engine — but it is labour-heavy, because the intake has to come off, and it will need doing again.

• Rough or uneven idle, worst from cold
• Slight misfire, or a stumble when accelerating gently
• Flat patch in the power delivery low in the rev range
• Engine warning light, sometimes with misfire codes
• Fuel economy quietly worse than it used to be

Be there for a cold start and sit with the engine idling, since a carboned engine is at its worst then. Listen and feel for an uneven idle or a slight shake and watch the tachometer for a needle that will not settle, then check again with the air conditioning on, which loads the engine at idle and exposes a weak cylinder. On the drive, accelerate gently from low revs in a higher gear rather than using the performance — carbon shows as a hesitation or a flat patch there, and hard acceleration masks it completely. Then give it one firm pull through the range and confirm it delivers cleanly. Watch the dash for an engine warning light throughout. Ask whether the intake valves have ever been cleaned and at what mileage, which on a direct-injection engine is a normal service question rather than an awkward one, and ask how the car is used — short gentle trips accelerate the build-up. If a workshop inspection is possible: ask for stored misfire counters to be read, which name the cylinder even when the car behaves on the day.

Parts involved: PCV / oil separator.

$800 to $2,800 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.

• “Have the intake valves ever been cleaned, and at what mileage?”
• “Does it ever idle roughly or hesitate at low revs?”
• “What sort of driving does it do — short trips or open road?”

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.