Mercedes Benz Truck

Mercedes Benz Truck

There are four primary elements to the Mercedes-Benz aftertreatment system which include: -

  • Diesel particulate filter
  • Diesel oxidation catalyst,
  • Selective Catalyst Reduction system,
  • Exhaust gas recirculation system.

Mercedes’ line-up of vehicles suffers from EGR valve, cooler and intake soot issues like most diesel vehicles on the market. Throw in DPF issues just for good measure if you are just pottering around the burbs at lower speeds as well. If SCR isn’t properly maintained it can cause grief too!

Diesel Particulate Filter (DPF)

All Mercedes-Benz vehicles have a DPF fitted to help in the reduction of harmful contaminants being expelled into the atmosphere.The soot emitted by diesel engines is particularly harmful to people who suffer with respiratory illnesses such as asthma, bronchitis and emphysema.

Over time the DPF can become blocked with a residue of ash from the soot trapped in the filter being burnt off in a process called regeneration. Periodic maintenance is required to keep the DPF functioning correctly which in turn will keep the running costs of your Mercedes Benz’s low, maintaining performance and reliability.

Exhaust Clean Australia are your experts in DPF cleaning with our revolutionary, market leading equipment ensuring clean emissions and trouble-free motoring. When you or your mechanic send your DPF to us for a clean please ensure that the sensors aren’t removed as we will clean them as part of the service for no extra charge!

While all major car manufacturers have made great strides in reducing diesel exhaust emissions, and particularly NOx (oxides of nitrogen) in recent years, Mercedes, has taken the lead in this field with BlueTec, its version of NOx reduction technology that was first introduced in 2006.

Here is a brief outline at the package of emission control technologies that constitute BlueTec emission control on Mercedes-Benz diesel vehicles, how the system works, what can go wrong with it, as well as how to diagnose at least some issues that can prevent BlueTec technology from working properly.

What is BlueTec?

The basis of BlueTec technology is SCR (Selective Catalytic Reduction), and while SCR is not unique to Mercedes, the technologies that supplement SCR, and Mercedes’ monitoring/control systems make Blue Tec arguably the most efficient and effective NOx reduction system in use today. The principal components of BlueTec systems are as follows: -

High-pressure common rail fuel injection

The heart of the system is the fuel injection system that injects fuel directly into the cylinders at pressures around 1500 bar through injectors that use Piezo-electric crystals as valves. This makes it possible to split fuel injection events up into multiple separate injections to optimise each stage of the combustion process.

Variable geometry turbocharger

Variable geometry turbochargers contain moveable vanes in the turbine side of the housing that control drive pressure in order to control boost pressure more effectively. A variable geometry turbocharger matches the boost pressure to an engine’s exact boost pressure requirements throughout the engine’s operating range by maximising exhaust flow through the turbocharger at all engine speeds. This greatly improves boost pressure control, which in turn improves the engine’s power delivery while using less fuel.


AdBlue is also known as DEF (Diesel Exhaust Fluid), and while allNOx reduction systems depend on the injection of this mixture of de-ionised water and urea into the exhaust stream, the use of AdBlue in conjunction with the fuel injection strategies and precise boost control Mercedes-Benz have developed produces NOx reductions of up to 50% on most vehicles across the range.

How BlueTec works

BlueTec is a compromise between two other NOx reduction systems, but it accomplishes the goals of bothby via a different route. The first involves reducing the amount of NOx that can form in the engine using an EGR system (Exhaust Gas Recirculation) that affects power and fuel consumption. The second involves the use of catalytic converters to trap NOx for a period before it’s burned off by injecting raw fuel into the converter which obviously affects fuel consumption big time.

BlueTec on the other hand, injects precise amounts of AdBlue (Diesel Exhaust Fluid) into the exhaust gas stream just upstream of the catalytic converter, where the ammonia (derived from urea) in the fluid initiates a process of chemical reactions that ultimately converts NOx into water and nitrogen. This process is known as SCR (Selective Catalytic Reduction), and while it is very effective and does not affect fuel consumption, the system is extremely complex.Sometimes it can be very difficult and time consuming to diagnose and/or repair.

There are four primary elements to the Mercedes-Benz aftertreatment system including an oxidation catalyst, SCR technology, a diesel particulate filter and an exhaust gas recirculation system.

Mercedes’ line-up of vehicles suffers from EGR valve, cooler and intake soot issues like most diesel vehicles on the market. Throw in DPF issues just for good measure if you are just pottering around the burbs at lower speeds as well. If SCR isn’t properly maintained it can cause grief too!

Diagnosing BlueTec issues

Apart from running out of DEF (Diesel Exhaust fluid), there are many other issues that will cause a BlueTec-equipped vehicle to enter either a limited start, or a no-start mode. This is not to protect expensive components in the exhaust after treatment system but is purely to prevent excessive amounts of NOx from being released into the atmosphere when the SCR system does not work as intended.

DEF depletion

DEF is stored in a dedicated tank, and during normal operation of a BlueTec-equipped vehicle, the volume of a full tank of DEF is calculated to last the distance between scheduled services. However, sustained aggressive driving can deplete the supply of DEF at a higher rate.The DEF usage is usually about 1-4% of diesel, so for every 100L diesel used expect to use around 1-4 litres of DEF.

AllBlueTec-equipped vehicles display a visual warning to the driver, which warning clearly indicates not only the level of the DEF tank, but also how many more times the vehicle can be started with the remaining DEF in the tank. If the DEF tank is run dry, the engine will enter a no-start mode immediately, and this mode will persist until the DEF tank is refilled and a system reset is carried out.

To do the reset procedure you need a scan tool with Mercedes-specific software that can monitor and display Mercedes-specific live data, as well as perform BlueTec adaptations and resets.

You need to extract all active and pending fault codes but take note that the codes you find must NOT be cleared. If you clear any codes, the system will enter what is known as “Tamper Mode”and it’ll automatically enter a limited start cycle even if all repairs are carried out successfully.

Contaminated / expired DEF

All high-quality DEF consists of a mixture of 62.5% deionised water and 32.5% high-grade urea, which is the source of the ammonia that converts NOx into water and nitrogen. Every important aspect of SCR systems, such as the injection rate and storage capacity is designed around these percentages; in this proportion, the water and liquid urea freeze and thaw at exactly the same rate, which prevents one component in the mixture separating out of the mix in freezing temperatures. Moreover, since the DEF tank on some Mercedes applications is under the bonnet, the DEF on these applications is particularly susceptible to contamination with water, windscreen washer solution, and even engine coolant. Even trace amounts of any contaminant can cause the engine to not start.

DEF injection system issues

While freezing temperatures are not an issue in Australia, majority of SCR systems now have a heating element in the tank to keep the DEF at a set temperature. All DEF mixtures sold in the world have the same relative concentrations of water and urea on the one hand and have a maximum shelf life of only 12 months.Most SCR catalyst faults are the direct result of issues in the DEF injection system. Blockages and leaks anywhere in the system (particularly leaking valves/injection nozzles) cause too much or too little DEF being injected and both conditions can cause issues with the catalyst filter itself requiring it to be cleaned which we can do at Exhaust Clean Australia utilising our patented industry leading machines. It is also recommended for the SCR catalyst convertor to be cleaned at regular service type intervals to maintain optimum efficiency.

Faulty NOx sensors

SCR systems (BlueTec, included) use upstream and downstream NOx sensors to monitor the efficiency of the catalytic converter, and as with oxygen sensors, the ECU/SCR control module compares the output of both sensors to calculate the efficiency value for the SCR catalytic converter.

However, since NOx sensors are very expensive, all other possible causes must be investigated before any NOx sensor is deemed to be faulty.

Metering valve issues

Once you are certain that the DEF storage and injection systems are functional, you can use a suitable scan tool to perform a test of the metering valve. This test involves activating the injection pump remotely with the metering valve removed from the exhaust system. The valve should emit a very finely atomised spray pattern consisting of three clearly defined cones, and the amount of the DEF emitted during the test should weigh about 15 grams.

Diesel Oxidising Catalyst (DOC)

Your DOC is an integral part of the entire after treatment system in converting the nasty by products emitted by your Mercedes-Benz’ engine into safely breathable emissions that are no longer harmful to us or the environment. Regular cleaning intervals are recommended to ensure perfect operation of the DOC system to prevent costly failures, downtime without your vehicle and expensive repair bills. Exhaust Clean Australia, using our patented Italian made machines will keep your DOC operating like clockwork!!

Catalytic Convertors (CAT)

Catalytic convertors are an important part of the aftertreatment system on petrol powered Mercedes-Benz vehicles to reduce hydrocarbons, carbon monoxide and nitrogen oxides to drastically reduced levels that are safer for the environment. These by-products of the combustion process are fatal in an untreated state. CAT convertor health and condition is important to ensure your Mercedes-Benz is operating in a manner designed by the manufacturer to not cause harm to the environment as well as to us as human beings! Exhaust Clean Australia can clean your Mercedes-Benz CAT convertor restoring their efficiency to an ‘as new’ state drastically reducing the risk of costly repair bills, increased running costs and most importantly reducing the damage to environment!


The Intercooler on your Mercedes-Benz plays a vital role in your engine’s intake system. Cooling the incoming air temperature dramatically increases your vehicle’s performance, efficiency and contributes towards the reliability factor as well. Intercoolers are often overlooked maintenance wise till a problem appears or the vehicles performance has a noticeable drop in output. A dirty, oil filled intercooler will not be anywhere near as efficient cooling wise and if a turbo or other related engine failure has occurred the contaminants may well just be stuck in your intercooler ready to be digested back into the engine causing further possible damage. Exhaust Clean Australia’s revolutionary machines can return your intercooler to full efficiency and like new cleanliness giving you peace of mind trouble free motoring.

Exhaust Gas Recirculation (EGR)

The EGR system on Mercedes-Benz vehicles reduces the combustion temperature by diverting a small portion of the exhaust gases back into the intake manifold. The exhaust gases are no longer combustible sodiluting the intake air with exhaust gases makes the air/fuel charge less combustible.

The engine management system opens or closes the EGR valve to control the flow within the EGR system. The EGR valve connects the exhaust manifold to the intake manifold. The EGR valve is normally closed and there is no EGR flow when the engine is cold, at idle, or during hard acceleration. The EGR flow is at its peak during steady cruising under moderate load.This has a quenching effect that lowers combustion temperatures and reduces the formation of NOx.

If the EGR system is rendered inoperative because it was disconnected or tampered with, the cooling effect that was formerly provided by the EGR system will be lost. Without EGR, the engine will often knock and ping (detonate) when accelerating or lugging the engine. This can cause engine damage over time.

The EGR cooler and valve over time on your Mercedes-Benz can become totally blocked with a build up of carbon, oil and contaminants from the recirculating of the exhaust gases back into the intake system.

Common problems related to EGR system are as follows: -

  • Pinging (spark knock or detonation) because the EGR system isn’t working, the exhaust port is plugged up with excess carbon or the EGR valve has been disabled.
  • Rough idle or misfiring because the EGR valve is not closing and is leaking exhaust gases into the intake manifold.
  • Hard starting because the EGR valve isn’t closing and is creating a vacuum leak into the intake manifold.

Exhaust Clean Australia is equipped with industry leading technology to successfully clean and restore your EGR cooler and valve back to its original condition. Savings in fuel economy and the restoration for your Mercedes-Benz’ performance are the big benefits!! We also recommend having your mechanic perform an intake clean whilst we are taking care of your EGR system cleaning to receive the maximum benefits for your vehicle.

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