BMW M2: S58 intake valve carbon build-up on the G87

The current M2 uses the same S58 twin-turbo six as the M3 and M4, and inherits its one predictable maintenance fault. Direct injection sprays fuel into the cylinder rather than past the intake valves, so nothing washes them clean, and oil vapour from the crankcase ventilation deposits carbon on the valve backs instead. It becomes noticeable around 60,000 to 80,000 km as a rough idle, a slight misfire and a flat patch low in the rev range. Context matters here: the S58 is regarded as substantially more robust than the S55 it replaced and carries no equivalent rod bearing concern, so this is a service item to budget for rather than a warning sign about the engine. It is labour-heavy because the intake has to come off, and it recurs.

• 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, because that is when a carboned engine shows itself. Listen and feel for an uneven idle or a slight shake, watch the tachometer for a needle that will not settle, then check again with the air conditioning switched on, which loads the engine at idle and exposes a weak cylinder. On the drive, resist using the performance at first: accelerate gently from low revs in a higher gear, which is exactly where carbon produces a hesitation or a flat patch, and where hard acceleration hides it. Then give it one firm pull through the range and confirm it delivers cleanly all the way. Watch the dash for an engine warning light. Ask whether the intake valves have ever been cleaned and at what mileage — a normal service question on a direct-injection engine — and ask whether the car has been tracked, since track use changes what else is worth checking. If a workshop inspection is possible: ask for stored misfire counters to be read.

Parts involved: PCV / oil separator, Turbocharger.

$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 idle roughly or hesitate at low revs?”
• “Has the car ever been used on a track?”

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.