Mercedes-Benz C-Class: W204 Plastic Thermostat Housing Cracking and Coolant Loss

W204 C-Class models across petrol and diesel variants use a plastic thermostat housing integrated into the engine cooling circuit. The housing is subject to cyclical thermal stress and is prone to developing hairline cracks, particularly around the thermostat seat and connecting hose stubs. Cracks typically present as a slow coolant weep rather than a catastrophic failure, but if undetected can lead to significant coolant loss, engine overheating, and head gasket damage. The failure is more prevalent on higher-mileage examples and vehicles that have not had regular coolant changes, as degraded coolant becomes acidic and attacks plastic components.

• Coolant level dropping without visible external leak
• Sweet smell from engine bay or heater vents
• White residue or staining around thermostat housing
• Temperature gauge rising higher than normal or erratically

1. 1

   Inspect the thermostat housing plastic body and all coolant hose connection points for cracks, white mineral deposits, or dried coolant residue.

2. 2

   Check the coolant reservoir level and condition - it should be clear to light green without oil contamination.

3. 3

   Start the engine cold and observe the reservoir for any bubbling which would indicate a more serious head gasket concern.

4. 4

   Request records of any coolant system work.

Parts involved: Thermostat.

$350 to $1,100 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 thermostat housing been replaced?”
• “Has coolant level required topping up between services?”
• “When was the coolant last fully replaced?”

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.