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Common Mercedes Diesel Problems | CDI & BlueTec

Diesel Mercedes-Benz cars, SUV and Sprinter vans equipped with BlueTEC engines are one of the most advanced diesel engines in the world. High-pressure fuel injection system 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 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 managed to meet 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.

Here are some common problems that 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, engine computer rises exhaust gasses temperature over 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 build up. Luckily, 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

mercedes disel problemMercedes, like many diesel cars, uses a DPF Exhaust Pressure Sensor to determine how much does the DPF restrict the exhaust flow. More restriction equals more clogged up DPF. As with any other sensor, this one can fail out of various reasons. In some cases, incorrect readings can give a misleading representation on 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 Mercedes DPF Pressure Sensor.

Diesel Exhaust Fluid Heater

mercedes def heater

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

Yet, don’t forget that DEF will freeze in 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 yourself for under $1000.

Limp Mode

If your Mercedes is losing power it is possible that it is in ‘limp home’ mode. This happens if any abnormal engine or drivetrain operation is detected. The engine computer will cut down the engine power output by switching of 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, but also to force you to fix the problem without a delay.

Usual causes are faulty EGR valve, swirl flap motor, turbocharger actuator or boost leaks. Make sure you have traced down the actual cause of the problem before changing anything. Remember that a turbocharger which is not working doesn’t necessarily mean that it is broken.

Oil Leaks

Oil leaking from oil cooler is a problem that affects 2007-present V6 diesel engines. This is a common oil leak source on all Mercedes engines. It is caused by hardened oil cooler gaskets. Good news is that oil cooler itself can be reused. In addition, there are upgraded seals available.

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

Glow Plugs

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

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


Whatever problems or symptoms you are experiencing, avoid guessing. Always read the codes before replacing anything. All of the codes caused by problems mentioned in this text can be scanned by a good Mercedes OBD2 scanner.

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 that have over 500,000 miles on the OM642 engine.

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