Mercedes-Benz CLA: M282 intake valve carbon build-up — rough idle, misfires and flat spots

The M282 petrol is direct injection, which means fuel is sprayed straight into the cylinder rather than past the intake valves. Nothing then washes those valves clean, so carbon deposits build on them over time and gradually choke the airflow. It shows as a rough idle, misfires and flat spots in the power delivery, and it comes on so slowly that owners usually adapt to it rather than notice it. Short journeys and gentle driving accelerate it. The repair is a cleaning job rather than a mechanical failure, but it is a labour-heavy one because the intake has to come off, and it will need doing again — so it is a running cost to price in rather than a one-off. A car with a full service history and regular open-road use is meaningfully better here.

• Rough or uneven idle, worst from cold
• Misfire — a stumble or hesitation when accelerating
• Flat spot in the power delivery at low to mid revs
• 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 it as it idles, because that is when a carboned engine is at its worst. Listen and feel for an uneven idle, a slight shake, or the tachometer needle refusing to settle, then check again with the air conditioning switched 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 flooring it — carbon shows as a hesitation or a flat patch in that part of the range, and hard acceleration masks it. Then accelerate firmly once and confirm it pulls cleanly through. Watch the dash for an engine warning light throughout. Ask how the car is used, since short gentle trips accelerate the build-up, and ask whether the intake valves have ever been cleaned — on a direct-injection engine that is a normal maintenance question rather than an accusation, and the answer tells you where in the cycle you are buying. If a workshop inspection is possible: ask for stored misfire counters to be read.

$600 to $2,200 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?”
• “What sort of driving does it do — short trips or regular open-road runs?”
• “Does it ever idle roughly or hesitate when accelerating gently?”

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.