How it works

EGR (Exhaust Gas Recirculation) – How it works

You may have come across this article because you’re interested to find out how an EGR System works or because you’re trying to find out why you have EGR Valve problems. Whatever the case maybe, we’re here to let you know how it all works.

The EGR system is designed to take some of the exhaust gases, anywhere from around 5%-15% in petrols and up to a whopping 50% in diesels and put them back into the engine, the physical aspect of the gases being recirculated or not, is done by the EGR Valve.

The aim of the game for the EGR, is to reduce NOx (Nitrogen Oxide) emissions. A harmful gas that has many awful health effects for us and the planet.

By introducing exhaust gases back into the cylinder, it lowers the combustion temperature, Nitrogen Oxide is created at high temperature combustion, so this stands to reason..less would be produced.

This does however come at the cost of efficiency and power, sort of. In petrol engines the loss of power can be countered by the vehicles timing system igniting the spark at a different time to get the most power possible, so not so much of a problem.

However, due to the way a diesel engine operates, the loss of power and efficiency is inevitable. This is why people report better MPG and power when the EGR is ‘Blanked off’ in diesel powered cars, this would be an MOT failure and depending on the vehicle, will throw up your engine management light. 

The biggest concern and why you may be brushing up on your EGR knowledge, is lower combustion temperatures decrease the amount of fully burnt fuel, this creates more particulate matter/soot. It’s a double edged sword, lower NOx > higher particulates, higher NOx > lower particulates, nobody wins really as it’s all harmful stuff. 

For your car though, the increase in particulates over time cause the EGR pathway and inlet manifold to get ‘coked’ up with these carbon deposits so much so that the inlet manifold gets restricted and the EGR Valve gets stuck, causing the engine light to come on and in some cases put your car into limp mode.

The only fix, either cleaning it all out which is a horrible, dirty and sometimes ineffective job, as the EGR valves become broken due to getting stuck and then being forced to open and shut. The full cure, replace both the inlet manifold and EGR Valve which comes at quite a hefty cost for original parts.

Usually if the car is older and higher mileage, we tend to see the inlet manifold cleaned out and the EGR Valve replaced with an aftermarket item.

EGR Cooler from a MK2 Ford Focus 1.8TDCi

Diesels also tend to have an EGR cooler, which is a heat exchanger built in to the exhaust gas pathway that further reduces the temperature of the gases before entering the inlet manifold. These can wear internally allowing coolant to be burnt off, eventually leading to quite commonly, misdiagnosis of a head gasket failure as a generic ‘sniff test’ will indicate exhaust fumes in the coolant.

Both Petrol and Diesel engines can have EGR valves and with the ever increasing need to lower emissions, more and more have them. 

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