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EGR valve


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I removed the EGR valve yesterday, but I am puzzled as to what it does and when. Obviously I know it allows exhaust gas into the inlet manifold, but when? Under what conditions? And what does that actually achieve? The exhaust valve is controlled by the big diaphragm on the top.

There is also a butterfly in the unit, controlled by the smaller diaphragm on the side of the unit. This appears to default open, so when does it close and why? What does it do for me?

Both valves are apparently operated by vacuum.



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As far as i understand it's to control NOx emissions (NOx isn't tested as part of MOT on diesels) i think the valve closes (recirculates) on cruise and hot idle.

It's open (no recirculation) on acceleration & full throttle so no difference to performance.

EGRs have been known to clog up the inlet manifold with soot


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NOx emissions form at high temperatures in the engine. Advancing the injection gives higher temperatures, higher pressures and better efficiency. Exhaust gas has (among other interesting properties) a high specific heat capacity, but it displaces the fresh air charge in the cylinder, so the EGR valve opens under part-load operation to allow the engine to run more advance than it would otherwise be able to without producing high NOx emissions.

Disconnect it... :)

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It is already dumped, I was curious to know more about it. I am still in the dark as regards the butterfly valve, does this serve some function other than the EGR valve? Or is it simply a means of restricting the availability of fresh air to promote use of exhaust gasses?

Without the EGR present the turbo seems to come on boost sooner/from lower revs. I was not expecting that to happen so I am happy that it is not just in my mind.


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