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The four-cylinder diesel engine called the OM651 which is a factory code for the 2.1/1.8 L diesel direct-injection engine that Mercedes-Benz introduced in 2008.
Common Mercedes OM651 engine problems include:
- Fuel Injector Problems
- Timing Chain And Timing Chain Tensioner Problems
- BlueTec Fuel Dilution Problems (Rising Oil Level)
- 2017 And Newer OM651 BlueTec NOx Sensor Issues.
- Coolant Leaks
- Bi-Turbo / Twin-turbo problems
The engine came in two displacements, a 1.8L and a 2.1L displacement. It develops anywhere from 136 hp to 204 hp, depending on the engine electronics and turbo configuration.
Which Cars Use the OM651 Diesel Engine?
The OM651 engine can be found in almost all Mercedes-Benz vehicles from 2008 and onwards:
- Mercedes-Benz A-Class (W176, W177)
- Mercedes-Benz B-Class (W246)
- Mercedes-Benz Vito (W447)
- Mercedes-Benz C-Class (W204, W205…)
- Mercedes-Benz GLC (X253)
- Mercedes-Benz CLA (C118)
- Mercedes-Benz GLA (X156)
- Mercedes-Benz Sprinter
- Mercedes-Benz S-Class (W222)
- Mercedes-Benz GLK (X204)
- Mercedes-Benz E-Class (W212, W213…)
- Mercedes-Benz CLS (W218)
- Mercedes-Benz Viano
OM651 Diesel Engine Common Problems
If you are buying a used or a new Mercedes-Benz with the OM651 engine today, let us give you the good news first. Today, this is a highly reliable and tested engine, with very little of the typical Mercedes engine problems.
We would even dare to say that it is one of the best diesel engines on the market today. Remember, the OM651 engine had to follow the legendary engines that were the OM648 and OM646, so people had high expectations.
And yes, like all engines, it had certain issues that were discovered and taken care of early on, just a year after the introduction of the OM651 in 2008. We will make sure to alert you on the issues that are still common today.
Fuel Injector Problems
When the first owners of vehicles with the OM651 diesel engine started covering some miles, it was immediately clear that the OM651 diesel engine had problems with Delphi Piezo fuel injectors/nozzles.
Some 220 CDI and 250 CDI encountered problems before even reaching the 40,000-mile mark. Mercedes-Benz admitted that there was a problem with those piezo fuel injectors and they launched a customer service action that allowed everyone with this engine to have the faulty Delphi fuel injectors replaced with more “standard” magnetic solenoid injectors.
Along with the fuel injectors, Mercedes also replaced the ECU, a new fuel return line and an updated engine cover were fitted.
The problem cost Mercedes-Benz over 1 billion dollars and it actually resulted in Mercedes stopping the manufacturing of the OM651 engine because they simply could not acquire enough fuel injectors to supply the replacements and new engine assembly lines. From 2012 and onwards, Mercedes started using electromagnetic fuel injectors as standard.
If you buy a pre-2012 used Mercedes-Benz with this engine, make sure that it had the faulty fuel injectors replaced under warranty. To learn more about piezo fuel injectors, read our guide on fuel injector problems.
Timing Chain And Timing Chain Tensioner Problems (+ Replacement Cost)
Just like the 6-cylinder OM642 diesel engine, the OM651 four-cylinder diesel engine also suffers from what is considered to be premature timing chain wear and stretching.
The opinions are a bit divided on this subject, and while a number claims there are no timing chain problems with the OM651 diesel engine until the car reaches high mileage numbers (200,000 miles), there are still a number of owners who did experience problems with the timing chain and there are a lot of cases where the timing chain and tensioner had to be replaced before the car reached 100,000 miles.
In order for the engine to run more calmly and smoothly, Mercedes-Benz fitted the timing chain at the back of the engine, at the transmission side. This makes the replacement process a lot more complicated and expensive.
The cost to replace the timing chain on the OM651 engine costs at least 1000$ and more if done at an official dealership service center.
Just like with the OM642 diesel engine, the wear and stretching of the timing chain is heavily dependent on the service history and the use of the car.
Cars that are often driven to their maximum when they are cold or cars that are only used to drive for short distances are much more prone to timing chain problems than cars that cover a lot of highway miles. Do not buy a diesel if you do not need one!
BlueTec Fuel Dilution Problems (Rising Oil Level)
From 2007 to 2018 Mercedes-Benz decided that the driver of a diesel engine that is paired with a DPF (diesel particulate filter) does not need to know when the engine ignites the DPF regeneration program.
Owners unknowingly interrupt the DPF regenerations program, which results in fuel dilution of engine oil. Mercedes acknowledged this serious problem in a technical bulletin but decided not to change anything about it but give some guidelines.
Avoid making short trips with these cars avoid idling them for extended periods and you should be alright. Driving around with a fuel diluted oil engine can lead to serious engine damage.
2017 And Newer OM651 BlueTec NOx Sensor Issues.
Modern variants of the OM651 engine, made from 2017 and onwards are experiencing common NOx sensor failures. NOx sensors are an important part of a modern diesel engine like the OM651.
NOx sensors were fitted to modern diesel engines in order to comply with the strict environmental demands. There are two NOx sensors, one measures the level of nitrogen oxide before it reaches the SCR (Selective Catalytic Reduction) and the second NOx sensor measures the amount of nitrogen oxide after the reduction in the SCR system.
Based on the readings, the car adjusts the operation of the SCR unit. NOx sensors commonly fail due to extreme exhaust heat. Replacing the NOx sensor costs anywhere from 700-1000$. Not a small deal.
Coolant Leaks
Owners of OM651 engines commonly report problems with different coolant leaks. One of the more common ones, even at low mileage, is a coolant leak at the O-ring sealed pipe junction that is located at the bottom of the diesel fuel filter.
Other standard coolant leak locations you should be aware of include:
- Heater matrix
- Water pump housing
- Thermostat housing
- Pipes and fittings around the fuel filter
- Plastic elbow on the pipe that leads to the heater
- EGR valve seals
- Radiator
If you own or plan on buying a used car with the OM651 diesel engine make sure you check for any leaks on the floor where the car is standing. You should also check the coolant level and monitor it regularly if you decide to buy the car. If you Google “OM651 coolant leak” you can have fun for hours.
Bi-Turbo / Twin-turbo problems
As we mentioned before, the 204 horsepower variant of the 2.1L OM651 engine uses two turbos to achieve amazing power output. There are generally no problems with turbos on any version of the OM651 diesel engine.
The only issue worth mentioning is the airflow control module malfunction that occurs on the twin-turbo OM651. This results in poor engine performance and clouds of white smoke behind the car as you accelerate. Repair is reasonable at about 150$.
Other issues with the OM651 engine
As these engines age and reach higher mileages you can expect “normal” issues such as high-pressure fuel pump failure, porous valve body, corrosion on the BAS control module, defective alternator pulleys, and so on.
Frequently Asked Questions
Are All OM651 Engines The Same?
No, not all OM651 engines are the same. Mercedes-Benz modified the engine with each model year as new faults were revealed. For example: from 2012 and onwards all OM651 engines came with updated electromagnetic fuel injectors which replaced the faulty piezo fuel injectors. Engine codes also indicate differences in engines (651.911 is different compared to 651.925 for example).
Is the OM651 Engine Good And Reliable?
Yes, definitely. We would say that OM651 engines are amongst the best diesel engines on the market today, especially after 2012.
Conclusion
So, should you buy a used Mercedes with the OM651 diesel engine? We would, definitely. But as always, under one condition; a FULL-service history where it is evident that all the common issues were taken care of as soon as they appeared.
You do not want a 5-6-year-old car that did not have anything replaced or fixed, because what you have then is a hot mess on your hands. You can expect to have a number of small (and big) things to fix in the coming years so avoid cars like that. Good Luck!
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