Mercedes OM651 Engine Problems: Causes And Solutions

The OM651 is a four-cylinder diesel engine developed by Mercedes-Benz, manufactured in Kölleda since 2008. It made its debut in the Mercedes C-Class W204, shortly thereafter in the E-Class, and in many other models of the Stuttgart brand.

Unlike its predecessor, the OM651 was developed from scratch. The more powerful variants (220 CDI, 250 CDI) are equipped with two turbochargers and Piezo injectors, while the weaker variants utilize a single turbocharger with variable turbine geometry and magnetic injectors from Delphi. Additionally, the valve train is no longer controlled by a duplex chain, but rather a simplex roller chain.

Cars Equipped With Mercedes OM651 Engine

Mercedes OM651 Engine Problems

The OM651 experienced numerous issues at the time of its launch, many of which have been resolved over the years. Below are the common weak points of the engine:

Stretched Timing Chain

Unlike the predecessor, the timing chain has become an issue due to a faulty chain tensioner, causing premature elongation and potentially severe engine damage. The chain tensioner has been revised multiple times, primarily affecting older OM651 engines.

Regardless of the year, attention should be paid during vehicle inspection to any clattering or rattling noises at engine start (preferably cold) and load changes. If this occurs, the chain is likely already stretched, and engine damage may be imminent. It might be wise to replace the chain tensioner preventatively on older vehicles.

Oil Loss at Chain Tensioner

Over 100,000 vehicles with the OM651 engine were recalled in Germany. Manufacturing tolerances in the sealing rings could cause oil loss at the chain tensioner. Affected are CDI models produced from February to November 2014, although older models with a replacement engine may also be affected.

Defective Injectors

The 220 CDI and 250 CDI variants were equipped with Piezo injectors, which became known for their high susceptibility to failure, resulting in symptoms such as strong vibrations during acceleration and increased consumption.

Initially unresponsive, Daimler yielded to customer pressure and began installing magnetic injectors from Delphi in some variants of the 220 CDI starting in mid-2010. Starting in the spring of 2011, magnetic injectors were used in all 220 CDI variants.

The 250 CDI never underwent the change to more reliable injectors, so even newer models may experience problems. Conversely, the 200 CDI was never affected by this issue, as Delphi injectors were used from the beginning.

Coolant Loss

Some 200, 220, and 250 CDI models may suffer from coolant loss. During customer service action 1204P59A93, the affected engines' coolant connection nozzle on the cylinder head was replaced, eliminating the problem. Since the manufacturer did not proactively communicate the measure, some engines may still be affected by this issue.

Diesel Scandal

Volkswagen was not the only manufacturer that manipulated its diesel engines' emission values. Illegal shut-off devices for manipulating emission values were also detected in the OM651, leading to a recall of all vehicles with this engine. During the workshop stay, a software update will be carried out to eliminate the fraudulent software.

Before purchasing a Daimler with the OM651, make sure the recall action has been performed. If not, it should be carried out promptly to avoid the vehicle being immobilized.

Mercedes OM651 Engine Specs

Conclusion

Those interested in a Daimler with the OM651 should opt for a newer model if possible, as older models may have issues ranging from timing chain problems to defective injectors.