🟡 Mercedes OM651 timing chain tensioner failure — engine rattle

A rattling noise from the front of the engine on cold start that may persist until oil pressure builds. The OM651 secondary balance shaft timing chain and its plastic tensioner fail with age. The primary timing chain is rarely the cause — it is the secondary chain that causes most issues. The repair requires removal of the front timing cover and is labour intensive. Using 5W-30 fully synthetic oil and changing it every 10,000 km helps extend chain life.