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[Marks DTM Calib]

Basic theory of operation

The EGR allows the ECU to add an inert gas (in this case exhaust fumes because in theory there is no oxygen or fuel) into the inlet system. This in turn dilutes the air/fuel mix, this can be considered as almost a way of reducing the cc of the engine and as a result less heat is produced during the combustion cycle. This in turn reduces NOx.....

This valve only opens at part throttle and constant speed driving (its variable via PWM to allow different openings to be supported with feedback to the ECU on its position).

Its should not be open at all at idle or full throttle.

If it is open at idle it causes poor running because it dilutes the fuel/air mix to much...(there is very little any way at idle!).

So not it cant cause pinking as it does not impact on the fuel/air ratio and hence engine temps will not suddenly rocket

[Webby the Bear]

Thanks Mark,

With that cleared up i'll block the damn thing off

[Andy H]

The recirculation will allow hot air back into the combustion sequence.

Without EGR, the engine will be fuelled purely by fresh air, meaning cool air.

The EGR system is a waste of time and money in the UK as NOx (the gas it is intended to reduce) is not tested in the MOT.

I thought because the exhaust gas is inert means that it doesn't combust so therefore that's where the temperature is kept down (and power  )?

Exactly.
When the engine is cruising exhaust gases are recirculated back into the intake to reduce NOx. It doesn't help at idle or at full throttle so the valve must shut then presumably. It doesn't reduce maximum power (assuming the valve is working) and it should improve MPG at part throttle openings by increasing the pressure in the combustion chambers and dropping the temperature.

So you are for the EGR, Andy? 

Yes, when it works.

If it is working it does no harm & reduces your contribution to acid rain.

If it stops working then a cheap fix is to blank it off.