A common issue across all Mercedes CDI engines is carbon buildup around the injectors caused by a failing copper seal ring.
Signals failure of one or more glow plugs or a faulty glow plug control module relay. How to Access a Reliable OM646 Manual Mercedes Workshop Manual Om646 Engine
The workshop manual outlines the procedure for checking chain stretch by measuring camshaft timing. It also details the replacement of the tensioner, a job that can be performed with the engine in the vehicle. A key instruction from the manual is that when installing a new tensioner, it will be in its "zero" position, and the chain will be slack. Upon first start, the engine may rattle for 2–3 seconds as oil pressure fills the new tensioner. A common issue across all Mercedes CDI engines
This section covers structural repairs. It includes step-by-step guides for removing the cylinder head, replacing the head gasket, measuring cylinder bore wear, and installing new piston rings. It also outlines the exact procedures for balancing shaft alignment—a critical step unique to refining this four-cylinder engine's smoothness. Fuel & Injection System (CDI) It also details the replacement of the tensioner,
Use an injector seat cleaning/reaming tool to clean the bottom of the injector well perfectly flat. Any debris left behind will cause the new seal to leak.