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My 2003 Jetta TDI check engine light recently came on, and a scan by Advance Auto revealed code P0401 -- EGR insufficient flow.

I took the vehicle to the local TDI guru, who gave me the option of removing the EGR from the system entirely, and replacing it with a race pipe (and a rechip to prevent the CEL from coming on). The FAQ at Dieselgeek explains that there is no disadvantage to removing the EGR, and it saves the vehicle from future clogging, and with the addition of a race pipe, it gives +3hp over 3000rpm.

This site explains that:

The main purpose of the EGR is to reduce NOx emissions produced during diesel or gasoline engine combustion.

... it lowers peak combustion temperatures and reduces NOx emissions anywhere from 50%-75% vs. a non-EGR system.

And this site says:

Disabling the EGR increases NOx emissions, but also reduces particulate emissions.

And this site says:

[Soot buildup inside the manifold] further reduces engine performance and airflow, which in turn reduces combustion efficiency and increases other unwanted emissions, principally Co2 and particulate emission.

So this seems to imply that particulate emissions (and Co2) go up only after a build-up of soot occurs, and engine performance is degraded, but not as a direct result of removing the EGR.

But then later says:

"All EGR systems reduce air flow, combustion efficiency and consequently engine power"

What would be the complete effect of removing the EGR from this vehicle?

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Not an answer, but as an alternative to messing with "chipping" to eliminate the CEL, you might see if there are simple ways to replace the sensors that would cause the CEL with simple dummy circuits. Sometimes just putting the appropriate resistor across the terminals can trick the computer well enough. –  R.. Jul 26 '12 at 6:04
    
@R..: There are people who have done that, but I'll be re-chipping anyway for some bigger injectors, so that's not really any extra hassle. It's been something on my to-do list for quite a while, this recent EGR failure is just an excuse to do it all at once now. –  Flimzy Jul 26 '12 at 6:23
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1 Answer

Disadvantage is that tampering with emissions systems makes a vehicle no longer street legal.

Other than that, it probably won't hurt anything at all.

I don't see how removing EGR will increase power though. EGR closes at high power settings anyways. The gain would be entirely from some other feature of the racepipe (perhaps it's bigger). The fact that there's no longer an EGR isn't going to make a difference.

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"makes a vehicle no longer street legal" - quite correct, but this is hard to find and then prove so you can be left uncaught for decades, trust me. –  Andrei Rinea Aug 27 '13 at 13:08
    
Indeed. When I bought my Eclipse I found that the catalytic converter was missing and the EGR was blocked off. It'd recently passed emissions in a nearby city that way (as those changes looked quite old) too! Apparently the visual was less than thorough and the exhaust passed the test despite missing those systems. First thing I did was to get a replacement cat! I didn't know about the EGR at first, but eventually I started getting CELs while decelerating off the expressway and a vacuum inspection revealed the blocked off EGR, which I then unblocked! –  Brian Knoblauch Aug 27 '13 at 14:28
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