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You may or may not be familiar with the term EGR but more than likely you have seen the images floating around the internet and social media of these blocked up filthy intake manifolds and valves.

What is EGR? Exhaust Gas Recirculation is nothing new having been around since the 1970’s (1990’s on road going passenger vehicles) it has now become a part of life and is found on nearly all diesels and some petrol vehicles. Exhaust gas recirculation is a process used in combustion engines to reduce the amount of waste nitrogen oxide released into the atmosphere, How Does Nitrogen Oxide Pollution Affect the Environment? (, it also has other functions such as accelerated engine warm up & is used on some petrol engines to minimize knock when in a lean condition (improve fuel economy).

EGR works by reducing combustion temperatures in the cylinder and therefore reducing the amount of nitrogen (present in air) & oxygen combining to form NOx which happens at a higher rate (more produced) when cylinder temperatures are higher. But wait, exhaust gas is hot right? This may seem backwards but the temperature of the exhaust gas once it has exited the cylinder is already lower than the peak combustion temperature, once passed through an EGR cooler (gas to water heat exchanger) and the temperature is further reduced it can then be added back to the combustion process to lower cylinder/combustion temperatures.


Some of the associated issues or by products of having EGR fitted on a diesel engine that can occur in as little as 50,000km include, blocking/restriction of intake manifolds and cylinder heads, increased engine wear, EGR valve failures (limp mode), reduced engine performance & efficiency to name a few.

 The viscosity of most modern diesel engine oil is a lot thinner than previous generations in order to decrease parasitic load, this leads to an increased amount of oil vapor/mist that is already present and recirculated by the PCV (positive crank case ventilation) system back to the intake, it eventually meets in the intake manifold/EGR valves and mixes with the dry exhaust gas’s causing this sticky sludge-like build up. Even with higher viscosity engine oils this is still an issue and will continue to be. 

Another problem created by EGR systems is increased cylinder/piston & ring wear due to the recirculated carbon re-entering the cylinder & having a scrubbing effect on the cylinder walls deteriorating compression and engine oil control. This added abrasive media expedites wear in the bores/piston rings, we have seen this occur to the point on failed systems where the piston rings & ring lands themselves become heavily burdened with carbon and burnt oil. 

Finally, the dreaded limp mode, whilst a diesel engine will still run with a higher percentage of EGR flow than desired we probably all know someone who has had the dreaded engine light and reduced power (queue the tow truck). Whether it be due to the lack of oxygen in the cylinder to produce power, The ecu measuring and identifying a fault & putting the car into reduced power protection mode or even the mass airflow sensor not reading the correct amount of air into the engine as the motor is scavenging exhaust through the failed valve. Either way sometimes you can limp home or sometimes you’re stuck.


There are some solutions, all hope is not lost. First and foremost, if the vehicle already has over 50,000km an inspection and thorough clean of the intake and EGR may be required. The two main ways are: an on-car chemical cleaning a procedure where a chemical that breaks down carbon is sprayed through the intake and engine to break down the associated build up, this procedure unfortunately doesn’t always reach the hard-to-get areas/baked on build-up and a lot of the carbon is forced through the engine during the procedure. We recommend this as preventative maintenance only if the car is low km and done regularly, an oil change is often required after this treatment, it’s best to plan it to coincide it with your servicing. Or secondly: Off car cleaning. The intake manifold associated pipe work and valves are removed and cleaned manually ie: hot wash, ultrasonic and dry ice blasting. This is the method we prefer as all areas are inspected/cleaned and the carbon deposits kept out of your engine. Most people opt to do this every 70-100,000km.

 Next an appropriately sized/designed catch can/oil separator and maintaining it correctly can greatly reduce the amount of oil mist and vapor that makes it back through the system. Things like a pressure relief valve, oil resistant components and a cleanable or changeable filter are things to look out for when purchasing. I would also like to make note it is important not to increase the pressure on the crank case this can lead to oil seal failures, turbocharger failures and damage to engine components. we also recommend always plumbing back to the intake to help reduce crankcase pressure if this is the route you chose to take.

EGR plates are something that have become more prevalent of recent times. A steel plate that either completely blocks or drastically reduces EGR flow can be fitted to the engine. Some vehicles are able to run like this, others will throw an engine light & some will throw a light and reduce power. The associated issues with the engine malfunctions due to the plate are sometimes resolved through the tuning of the factory ECU.

EGR delete modules and cables, there are some modules which allow you to unplug and deactivate the EGR and plug in to prevent the ecu detecting this function (stop engine lights and limp). It is possible in some cases if the valve mechanically/electronically fails the ecu may not detect the issue. EGR cables generally trick some of the critical engine data like intake temperature to force the factory ecu into a condition where the EGR is not active. We are not fans of this as it can alter other timing, boost and fueling maps in the ECU, we have recorded instances where there is a reduction in power when fitted

EGR reduction via the factory ECU, as with all ECU recalibrations we often have access to the EGR tables that determine percentages of EGR flow. Under certain conditions these can be modified & reduce to decrease the flow of exhaust gas and lengthen the intervals/prevent the build-up of carbon.

Other things to consider around EGR and the removal and/or reduction of EGR, it’s used to control and aid engine warm up. This decreases wear on a cold engine, EGR is used during the combustion process, a lot of work has gone into the engineering of controlling things like cylinder temperatures, timing, fuelling and detonation inside the factory ECU. If not already, in the future EGR will play a part in tuning higher compression or compounded turbocharger smaller capacity higher boost level diesel engines. EGR can affect and be used in conjunction with other means to modify & control boost pressure in some situations. Lastly modifying an EGR system is deemed illegal and may result in a fine or a yellow sticker (defect) on your windscreen, today it’s a necessary evil and or welcomed design for mother nature depending which way you choose to look at it.