Common Mercedes Diesel Problems | CDI & BlueTec

Diesel Mercedes-Benz cars, SUVs, and Sprinter vans equipped with BlueTEC engines are among the most advanced diesel engines. High-pressure fuel injection systems and variable geometry turbochargers ensure optimal combustion.

This gives better power output and lowers exhaust gas emissions. Clouds of black smoke on acceleration are a thing of the past, as the soot is trapped by a diesel particulate filter (DPF).  On top of that, Diesel exhaust fluid (DEF) is injected into the stream of exhaust gasses. This reduces NOx emissions.

This is how Mercedes met the highest emission standards while retaining high engine power outputs and fuel economy. On the downside, all these advanced systems can break down. It is essential to be familiar with the usual weak points.

Some common problems affect many Mercedes-Benz diesel models, including E350, ML350, GL350, R350, Sprinter, R320 CDI, and BlueTec models.

Common problems

Diesel Particulate Filter

As Mercedes DPF traps and holds soot, with time, it reaches a point when it needs to be ‘emptied out.’ This is done by regeneration. During this automated process, the engine computer raises the exhaust gas temperature to 630°C. This burns up soot particles. However, if the engine is not running correctly, automatic regeneration will not be started.

Some of the possible causes are a faulty temperature sensor, swirl flap motor, or EGR valve. Also, make sure you only use approved engine oil.  Wrong engine oil can cause excessive soot buildup. The regeneration process can be initiated manually using an appropriate diagnostic tool. In most cases, this will purge a clogged DPF.

DPF Exhaust Pressure Sensor

Like many diesel cars, Mercedes uses a DPF Exhaust Pressure Sensor to determine how much the DPF restricts the exhaust flow. More restriction equals more clogged-up DPF. As with any other sensor, this one can fail for various reasons. In some cases, incorrect readings can give a misleading representation of the DPF state.

In most cases, the sensor will stop operating. This will give a ‘check engine’ light and a corresponding code. This usually triggers limp home mode, so you can expect a noticeable loss of power. DPF exhaust pressure sensor is not expensive. It is located underneath your Mercedes, next to the DPF. It is relatively easy to replace.

Follow this guide to learn how to change the Mercedes DPF Pressure Sensor.

Diesel Exhaust Fluid Heater

DEF heater is another known weak point. All Bluetec vehicles use Diesel exhaust fluid, also known as AdBlue. It is stored in an external tank and would freeze in cold weather. To prevent this, the DEF tank has an electric heater, which can burn out. If this happens, you will be warned with a ‘check engine’ light. While the outside temperatures are not very low, this will not affect the engine performance.

Yet, don’t forget that DEF will freeze in a cold climate. The cost to replace the DEF heater at the mechanic ranges between $2500-$3000. If you have DIY skills, you can purchase the DEF heater online and replace it for under $1000.

Limp Mode

If your Mercedes loses power, it may be in ‘limp home’ mode. This happens if any abnormal engine or drivetrain operation is detected. The engine computer will reduce the power output by switching off the turbocharger, although not caused by it. In extreme cases, it will even limit the gear selection to one gear and reverse. This is done to prevent any damage to your engine and force you to fix the problem without delay.

The usual causes are faulty EGR valves, swirl flap motors, turbocharger actuators, or boost leaks. Make sure you have traced down the actual cause of the problem before changing anything. Remember that a turbocharger that is not working doesn’t necessarily mean it is broken.

Oil Leaks

Oil leaking from the oil coolers is a problem that affects 2007-present V6 diesel engines. This is a common oil leak source on all Mercedes engines. Hardened oil cooler gaskets cause it. The good news is that the oil cooler itself can be reused. In addition, there are upgraded seals available.

This means that once replaced, seals will not leak anymore. The bad news is that the oil cooler is located between engine heads, in the depth of the engine Vee. This means the intake manifold, turbocharger, and much more must be removed. This makes it expensive to repair if you take it to the garage. However, you can save a lot of money if you do it yourself.

Glow Plugs

Glow plugs are an essential component for a cold start of every diesel engine. These are, in essence, strong electrical heaters. As the car ages, the glow plugs can fail. The symptoms are a long cranking time and rough running in the first several minutes.

Although changing them is a relatively simple job, remember that a stuck glow plug can break during removal. You will need a plug removal tool for this cause. The glow plug module can go bad, as well.

Conclusion

Whatever problems or symptoms you are experiencing, avoid guessing. Always read the codes before replacing anything. A good Mercedes OBD2 scanner can scan all the codes caused by problems mentioned in this text.

These problems affect various models. Mercedes-Benz makes some of the best diesel engines out there. If you want proof, check out all the Sprinter vans with over 500,000 miles on the OM642 engine.

What is the Best Scanner to Use for Diagnosing Mercedes-Benz Problems/Issues?

The YOUCANIC full system diagnostic scanner is a valuable tool for Mercedes-Benz owners who want to troubleshoot problems with their vehicles. This advanced scanner can read and interpret fault codes from the car’s computer system, providing detailed information about any problems with the engine, transmission, brakes, or other critical systems. The scanner can also perform diagnostic tests on various systems, including the ABS, airbag, and steering systems, to help pinpoint the root cause of any issues. With this information, you can make more informed decisions about repairs and maintenance, saving time and money on costly repairs.

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