2006 — 2009
4.0L V8 (OM629) - S400 CDI Turbodiesel
Diesel
Test drive task: Perform a cold start and listen for any rattling sounds. | Description: It is worth noting that the OM629 timing chain assembly can exhibit wear in high-mileage examples, so listening for a 'rattle' at idle is a sensible precaution.
Test drive task: Shift through all gears while at a steady speed. | Description: The 7G-Tronic transmission is known for its refinement, but in cars of this age, any hesitancy or 'shudder' between shifts might suggest the transmission control module or valve body may eventually require inspection.
Test drive task: Operate every switch and button on the dashboard. | Description: The W221 is packed with complex electronics; checking that the seat adjustments, infotainment controller, and climate settings all respond promptly helps confirm the SAM units are communicating properly.
Test drive task: Gently accelerate on an incline and listen for any hissing sounds. | Description: The twin-turbo system is efficient, but hoses and seals can degrade over time, so listen for any whistling that might indicate a small boost leak.
Consider a full transmission service including fluid and filter replacement if not documented.
This is a good time to evaluate the health of the AirMatic suspension compressor and struts.
Common to review the engine mounts and transmission mounts for signs of perishing rubber.
Ensure the engine oil and high-quality filter are changed every 10,000km to protect the turbochargers.
Regularly clear the sunroof and windshield scuttle drains to prevent water ingress into the cabin electronics.
Use only high-grade diesel and keep an eye on the fuel filter condition to maintain the integrity of the injection system.
Periodically inspect the cooling system hoses for any signs of swelling or brittleness.
Keep the battery in peak condition, as these S-Class models are highly sensitive to low voltage fluctuations.
Estimated repair costs if issues arise
In cars of this age, the air struts or the compressor pump may eventually reach the end of their service life. Future budget planning should account for replacing these to maintain the famous S-Class ride quality.
The electronic brain of the 7G-Tronic gearbox can occasionally develop faults. Depending on the car's history, you might eventually need to budget for a repair of the conductor plate or a transmission service.
While the OM629 engine is robust, the turbos have seen a lot of work over the years. If this component needs attention in the future, the estimated cost in NZ covers specialized labor and high-quality components.
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Disclaimer: This report is generated with the assistance of AI analysis based on verified automotive data. While we strive for 100% accuracy, enthusiasts should always conduct physical inspections and consult service records. View Full Disclaimer