Mercedes-Benz C-Class: OM651/OM654 EGR Cooler Internal Crack and Coolant Ingestion

Both the OM651 and OM654 diesel engines use an EGR (Exhaust Gas Recirculation) cooler that is subject to internal cracking from repeated thermal cycling. When the cooler's internal passages fail, engine coolant enters the intake manifold and combustion chambers without any external leak being visible. Separately, the EGR valve accumulates heavy carbon deposits causing rough idle and reduced power, particularly on vehicles used predominantly for short trips that prevent full combustion temperature.

• White or grey steam from the exhaust at normal operating temperature
• Coolant level dropping with no visible external leak or puddle
• Milky or foamy appearance to the coolant in the reservoir cap
• Rough idle with intermittent stumble or misfires
• P0401 EGR flow insufficient or related EGR fault codes in ECU

1. 1

   Check the coolant level and inspect the coolant in the reservoir — milky, foamy, or discoloured coolant indicates combustion gas contamination.

2. 2

   Look for white or grey steam from the exhaust at normal operating temperature.

3. 3

   Have a workshop scan for P0401 or P0402 EGR flow fault codes and inspect the EGR valve for heavy carbon deposits.

4. 4

   Request the car's usage history: predominantly short-trip urban driving greatly accelerates EGR carbon fouling.

Parts involved: EGR valve.

$800 to $3,500 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 the EGR cooler or EGR valve been replaced? Do you have documentation?”
• “Has the coolant level ever needed topping up, or has any white smoke been observed from the exhaust?”
• “Is the car primarily used for short trips, and has the EGR system ever been cleaned or inspected?”

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.