W211 & W212 E Class: Servicing, Repairs and Tuning

The E 300 CDI BlueTEC sits at the top of the W211 diesel range and it earns that position. The OM642 is a 2987cc V6 producing 211 hp and 540 Nm, with a 16.5:1 compression ratio and bore and stroke of 83.0 by 92.0 mm. It is managed by a Bosch EDC16 ECU and it is genuinely one of the better diesel engines to come out of Stuttgart, but there are a handful of known failure points that every owner should understand.

The most common fault we see on the OM642 is the oil cooler seal. It sits between the cooler and the block, and when it goes it drops oil into the valley of the engine and down onto the starter area. You often don't see it at a glance because it's buried, but you might notice oil consumption, a smell after a long run, or a slow drip from underneath. Fixing it properly means the intake has to come off, which is a reasonable job but not a small one. It is absolutely worth doing before it migrates further.

The swirl flaps and their linkage inside the intake runners are another known issue. These are common across several Mercedes diesel engines of this era. The linkage can corrode and seize, or break, and in worst case scenarios fragments can be drawn into the engine. We check the condition of these routinely on any OM642 that comes in for service. Turbo actuator wear and EGR cooler issues are also on the list, along with the expected diesel consumables: injectors and glow plugs that wear with age and mileage.

The BlueTEC system adds an AdBlue circuit into the mix. If the AdBlue pump, injector or NOx sensor develops a fault, the car will start counting down warning messages and eventually refuse to start. We diagnose the entire system properly, not just clear the codes and send you on your way.

The E 220 CDI with the OM651 is a different animal to the V6 E 300. Where the OM642 is all smooth torque from a big displacement, the OM651 is a near square 2148cc four cylinder, bore and stroke of 88.0 by 88.3 mm, compression of 17.5:1, rated at 170 hp and 400 Nm. It is managed by either a Bosch EDC16C2 or Delphi CRD11 / DCM3.2 ECU depending on the build. The result is an engine that feels punchy and efficient rather than lazy, and in a W211 body it makes for a genuinely impressive everyday car.

But compared to the OM642, the OM651 has a couple of additional vulnerabilities that owners need to know. Injector issues are the big one. Early OM651 units fitted with Delphi injectors are known for leaking or outright failure, which shows up as rough running, hard starting, or a significant drop in power. This isn't a cheap fix, but ignoring it turns into a much bigger problem. When an injector is failing on this engine, we want to catch it before it damages the bore or puts excessive fuel into the oil.

Timing chain stretch is another concern specific to the OM651 that wasn't really a factor on the OM642 V6. If oil change intervals have been stretched or the oil level has been neglected, the chain and its tensioner can wear faster than they should. We check chain condition as part of any thorough inspection on these. Like the V6, the OM651 also runs swirl flaps and an EGR, and both can cause trouble, particularly if the car has done a lot of urban driving or short trips that never fully warm the engine through.

And then there's this. The W212 E 63 AMG moves the conversation somewhere else entirely. Where the W211 diesels are about efficiency and long distance composure, the W212 E 63 AMG is about 6208cc of naturally aspirated M156.985 V8, a compression ratio of 11.3:1, bore and stroke of 102.2 by 94.6 mm, 525 hp and 630 Nm of torque through an MCT 7-speed wet clutch transmission. It is managed by a Bosch ME9.7 ECU. On the outside it looks like any other well dressed executive saloon. On the inside it is genuinely one of the fastest four door cars ever built.

The M156 is a spectacular engine but it is not without its own specific failure modes. Camshaft and cam adjuster wear is the headline issue. The cam adjusters can develop rattle and wear on higher mileage examples, and when they go, they go properly. A full cam adjuster job on an M156 is significant work and significant cost, so buying one with a documented service history and checking for any top end noise matters a great deal. The early head bolts on these engines are also known to stretch, which can cause seeping at the head. Not catastrophic if caught early, but something to look for.

Beyond the top end concerns, the M156 is hard on oil. Consumption is normal to a degree on these engines, but it needs monitoring because running low on oil accelerates exactly the cam wear we've just described. Oil leaks from various seals are also common on older examples. Coil packs and the high pressure fuel system are worth inspecting on any car of this vintage. The MCT transmission is generally robust but it benefits from correct fluid service and an adaptation reset through XENTRY, especially if the car has developed any hesitation or a rough take off.