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Exhaust Gas Re-circulation

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Andre/fellow tuners,

Can you all give me a proper explanation on the purpose of EGR in modern factory cars and its impact on performance of an engine?

I have physically blocked mine off from being able to send any exhaust gasses back to the intake, thinking this would reduce IAT's, allow for more fresh air to be drawn in to the cylinder and hence raising the knock threshold of the engine as well as promoting better performance. Is their any truth to these theories?

I have also noticed with the EGR blocked off on my car the AFR's have gone really rich. I checked the service manual for fail safe symptoms when the check engine is lit for this code and they did not list the ECU as going into any fail safe mode, yet I know for a fact with a wide band that the engine is running much ricer. Can you guys explain this ? If it goes richer then that means that there is more fuel now which kind of goes against what I am thinking from above, I would have thought it would have leaned out a bit drawing in more fresh air to fill the cylinders.


Hi Chris

Regarding the first question, purpose of EGR (reduction of NOx), here a wiki link:


In the earlier days (80's - mid 90's in European vehicles K-Jetronic, some KE-Jetronic and carburator), when many cars did not have overrun shutoff, the EGR was only active during overrun, nowadays during light loads. What you are seeing, going rich, is because the mixture is not being diluted with exhaust gases anymore, no additional fuel is injected but hence richer. The recirculated gases contain no oxygen (barely any) that is why you won't see leaning.

I would have thought that the self adaption in the OEM ECU will compensate for that. Modern OEM ECU's will detect a fault based on the change of airmass and Lambda. Depending on emission law requirements this fault detection capability may or may not be programmed into the ECU by the vehicle manufacturer.

EGR has a couple of purposes. The first being , as already mentioned, a reduction of NOx and the second is a reduction of combustion temps. The reason your AFR went rich is because the EGR dilutes the intake charge in effect leaning it, removing the EGR removes the dilution thus making the intake charge richer. Using an EGR also allowed fuel octane ratings to be lowered with the added benefit of helping engines stay out of the detonation zone. EGRs only work, or should only work, at part throttle so they have no effect on full throttle performance as such.

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