11:34 01-12-2025

Mercedes-Benz OM651 diesel engine problems and reliability

Unveiled in 2008, the Mercedes-Benz OM651 was billed as a next-generation, high-tech diesel and found its way into a wide span of models, from the A-Class to the S-Class. Yet the first years in service quickly showed that alongside its strengths, the modern unit carried some serious drawbacks that still draw criticism.

Timing chain troubles

At launch, the timing drive used a straightforward roller chain. That simplicity, however, bred a host of headaches:

• The chain stretched quickly, producing a distinct rattle during engine operation.
• Elongation accelerated wear on related parts and increased the risk of engine failure.
• Replacing the chain was technically complex and costly, as it required removing numerous components.

In short, the modest roller-chain layout translated into real-world inconvenience and sizeable bills for owners.

Engine oil leaks

Another major issue was widespread oil leakage caused by sealing defects. Because of the engine’s design and insufficient gasket tightness, oil could escape, dirtying the engine bay and exposing internal mechanisms to additional risks:

• Contaminated components sped up the wear of friction surfaces.
• Oil spots could ignite if fluid reached hot elements of the car.
• Regular seal replacements were also far from cheap.

All of this added notably to the cost of running an OM651-equipped vehicle.

Cooling system issues

The cooling system had several weak points, including an unreliable cylinder head connection. Under heat and vibration, joints often lost integrity:

• Coolant loss led to overheating and could deform pistons and valves.
• Owners frequently faced full replacement of certain cooling components, including the head gasket and hoses.

The result was extra expense and a tangible hit to day-to-day reliability.

Unreliable piezoelectric injectors

The engine’s piezo injectors brought more trouble. They proved sensitive to fuel quality and operating conditions:

• This heightened sensitivity caused erratic fuel delivery, rough running, and higher consumption.
• In some cases, injector failure was critical and demanded expensive repairs.

Taken together, this undermined confidence in the unit’s long-term dependability.

Emissions compliance

To meet modern emissions standards, engineers introduced measures to cut pollutants. It later emerged, however, that the software governing these systems contained errors:

• Faulty sensor logic and component control led to shutdowns of emissions systems.
• Fixing the issue required an engine software update that became mandatory for all vehicles with this unit.

Such updates complicated ownership and pushed maintenance costs higher.

Other weak spots

Beyond the headline faults, the OM651 also showed issues typical of many contemporary diesels:

• Frequent diesel particulate filter (DPF) clogging.
• Failures within the exhaust gas recirculation (EGR) system.
• Heat exchanger failures.

These components likewise demand regular attention and add to running costs.

For all its positives—strong torque, good economy, and quiet manners—the Mercedes-Benz OM651’s list of shortcomings makes ownership demanding. High service bills, and the price of delaying fixes when early symptoms appear, inflate the cost of keeping the car on the road and inevitably dull its appeal to buyers.