What effect does the EGR valve actually have on a car? When does it actually function. A diesel Passat in my example.
Just had mine replaced. Did my first long motorway run and mpg seems a lot better (48 insted of the usual 42)
Edited by Glaikit Wee Scunner {P} on 04/12/2007 at 12:45
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By introducing some very hot exhaust gas into the intake flow, emissions of certain elements (Nox, I think) are greatly reduced, but at the expense of economy. It normally only works at part throttle, being closed at idle and full throttle.
To improve from 42mpg to 48 under "identical?" conditions means that a major fault has been corrected - I didn't think a faulty EGR would make that much difference.
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I'm interested as to which engine this is, ie 1.9 or 2.0 TDi ? What were the symptoms that made them replace the EGR valve. And yes, I do think this could cause a significant drop in MPG.
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1.9 PD TDI 100ps. Was dribbling oil as per an earlier post.
Err the mpg has increased- not dropped. Any reason how/why this could occur by the operation of a new EGR valve?
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If an inert gas is introduced into the combustion chamber at lighter loads then NOx emissions are reduced. To save carrying cylinders of Argon around in the car and feeding that into the engine, engineers found that exhaust gas would do instead and is readily available, it doesn't have to be hot and works best if cooled. Nitrogen in the air reacts with Oxygen in the air when temperatures exceed 1370C such a very high temperature is found in the combustion chamber and produces NOx. What the addition of exhaust gas does is it reduces the temperature of the combustion to keep it below the threshold at which NOx is readily produced.
Edited by Hamsafar on 04/12/2007 at 23:30
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>>Any reason how/why this could occur by the operation of a new EGR valve?<<
Could it be that when the valve is working correctly and feeding burnt fumes back into the cylinder, the volume of fresh unburnt fuel vapour drawn into the cylinder is reduced, thereby you dont use as much fuel, create less emissions, and get higher mpg.
If your valve was sticking/stuck closed you would be burning a full cylinder of fuel all the time and therefore using more, and your mpg would drop?
Billy
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Err the mpg has increased- not dropped. Any reason how/why this could occur by the operation of a new EGR valve?
Sorry, that is what I meant, ie the MPG figure has increased.
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Presumably because it now opens and shuts-it was probably sticking open(slightly or completely)before.
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How do VW cool the re-circulated exhaust gas then? No obvious cooler on the Passat.
Or is it hidden under the top engine cover?
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GWS, cool is relative. Hot to us, but cool in comparison with the local temperatures during the combustion reactions in the cylinder.
Number_Cruncher
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Most VW TDis are water cooled EGR, some just use a long stainless steel pipe.
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